Pollution in runoff from nonpoint sources

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Pollution in runoff from nonpoint sources
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Florida Water Resources Research Center Publication Number 42
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Campbell, K. L.
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University of Florida
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Gainesville, Fla.
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Notes

Abstract:
The increasing concern for the quality of our environment demonstrates the importance of gaining an increased understanding of the mechanisms and processes involved in nutrient movement from diffuse sources. The extent of the problem is not well defined in many areas. In this study, nitrogen and phosphorus loads were determined for two agricultural watersheds: one primarily in native forest cover and the other primarily in intensive crop production. Small plots were used to evaluate the effects of selected cultural and water management practices on nitrogen and phosphorus loads in surface runoff from sandy soils. In both of these locations, nitrogen and phosphorus losses in runoff were very small compared with amounts received by the land area in precipitation and commercial fertilizer. Techniques were developed to simulate nitrogen movement through agricultural watersheds. Simulation models are an effective tool to assist in gaining a better understanding of the complex processes and interactions that occur in a watershed system. They can help in identifying which processes are most important in controlling nitrogen movement within a watershed.

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Publication No. 42
POLLUTION IN RUNOFF FROM NONPOINT SOURCES


By


K.L. Campbell
(Principal Investigator)


Agricultural Engineering Department

University of Florida

Gainesville


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POLLUTION IN RUNOFF FROM NONPOINT SOURCES


K. L.
(Principal


Campbell
Investigator)


PUBLICATION NO. 42



FLORIDA WATER RESOURCES RESEARCH CENTER



RESEARCH PROJECT TECHNICAL COMPLETION REPORT



OWRT Project Number B-023-FLA



Matching Grant Agreement Number

14-31-0001-5064



Report Submitted: June 28, 1978



The work upon which this report is based was supported in part
by funds provided by the United States Department of the
Interior, Office of Water Research and Technology as
authorized under the Water Resources Research
Act of 1964 as amended.
















ACKNOWLEDGEMENTS


Appreciation is extended to the Office of Water Research and
Technology, United States Department of the Interior, for financial
support of this project, and to Dr. W. H. Morgan, Director of the
Florida Water Resources Research Center, for his administrative
assistance, and Ms. Mary Robinson for accounting assistance. Fi-
nancial support of the Center for Environmental Programs, Insti-
tute of Food and Agricultural Sciences, University of Florida also
is gratefully acknowledged.

The cooperation of Mr. Thomas Malphurs, Mr. Lamar Malphurs,
and Mr. James Swick in allowing portions of this study to be con-
ducted on their land and in providing related land use information
is gratefully acknowledged. The assistance of Mrs. York Malphurs
in data collection also was greatly appreciated. Mr. Walt
Auffenberg and Mr. John Ominski contributed invaluable field and
laboratory assistance without which this project would have been
impossible. The computer programming support of Mr. Bill
Lancaster also was greatly appreciated.












TABLE OF CONTENTS
Page

ACKNOWLEDGMENTS . . . . . i

ABSTRACT . . . . . . 1

INTRODUCTION . . . . . 2

CHAPTER

I. WATERSHED STUDIES . . . . 5

Methods and Procedures . . . 5

Results and Discussion . . . 7

Summary . . . . . 21

II. SMALL PLOT EXPERIMENTS . . . 23

Methods and Procedures . . . 23

Results and Discussion . . . 24

Summary . . . . . 30

III. SIMULATION MODEL DEVELOPMENT . . . 33

Modeling Approach . . . . 33

Hydrologic Model Calibration . . . 35

Nitrogen Model Development . . . 39

CONCLUSIONS AND RECOMMENDATIONS . . . 45

LITERATURE CITED . . . . . 47














ABSTRACT


The increasing concern for the quality of our environment
demonstrates the importance of gaining an increased understanding
of the mechanisms and processes involved in nutrient movement from
diffuse sources. The extent of the problem is not well defined in
many areas. In this study, nitrogen and phosphorus loads were
determined for two agricultural watersheds: one primarily in na-
tive forest cover and the other primarily in intensive crop pro-
duction. Small plots were used to evaluate the effects of selec-
ted cultural and water management practices on nitrogen and phos-
phorus loads in surface runoff from sandy soils. In both of these
locations, nitrogen and phosphorus losses in runoff were very
small compared with amounts received by the land area in precip-
itation and commercial fertilizer. Techniques were developed to
simulate nitrogen movement through agricultural watersheds.
Simulation models are an effective tool to assist in gaining a
better understanding of the complex processes and interactions
that occur in a watershed system. They can help in identifying
which processes are most important in controlling nitrogen move-
ment within a watershed.







2






INTRODUCTION


There has been increasing concern in recent years about the
quality of the water in our lakes and streams. This quality is
obviously influenced by the quality of water flowing into the
lakes and streams from the surrounding land areas. However,
because of the widely dispersed origin of this water as runoff
from the contributing watershed areas, data concerning its effects
on the quality of our bodies of water have been very sparse.
Good background data indicating water quality under native water-
shed conditions for comparative purposes are also very sparse.

The importance of minimizing nutrient losses from agricul-
tural land is quickly recognized when one considers the high costs
of commercial fertilizer. The development of management practices
to reduce nutrient losses may reduce fertilizer costs and also
result in improved quality of the water in lakes and streams. The
extent and value of this improvement in water quality is at best
very difficult to quantify. With the present concern for the
environment and the quality of water in our lakes, streams and
rivers, there is a need to determine the effects of our modern
agricultural production technology on the nutrient loads in
surface runoff. Because agricultural runoff is introduced to the
streams from a non-point source, it is more difficult to determine
the actual nutrient loads being introduced.

The soils and climate of Florida create a unique situation
for nutrient movement through watersheds. The sandy soils along
with high rainfall and warm temperatures create a condition con-
ducive to rapid nutrient movement under some conditions. The
tremendous variations in these sandy soils and their resulting
effects on water movement add considerable confusion to any
analysis of the potential for nutrient losses in runoff in various
locations. In some locations the sandy soils are deep with
corresponding water table depths of 9-12 m or more, while in
nearby flatwoods areas, spodic layers and clay layers in the
sandy soil may cause a shallow water table fluctuating from 2 m
deep up to the ground surface. These variable soil conditions
affect the volume and quality of surface runoff and the movement
of nutrients into the ground water.

The problem of pollution from non-point sources is a partic-
ular concern in Florida because of its many lakes and the close
intermingling of surface water and ground water. There is also







3




the important consideration that pollution from non-point sources
in Florida may quickly find its way into the estuarine zones which
are of great economic and aesthetic importance. With these facts
in mind, this research effort was undertaken to determine nutrient
loads in runoff from native areas and watersheds in agricultural
production in north central Florida (Figure 1).

The objectives of the study were (1) to determine the nitro-
gen and phosphorus loads in streamflow from agricultural water-
sheds with intensive cropping and with native vegetation, (2) to
determine the effects of selected cultural and water management
practices on nitrogen and phosphorus loads in agricultural runoff
from sandy soils, and (3) to develop techniques to simulate nitro-
gen movement through agricultural watersheds.














SITES


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Figure 1. Map of Florida showing the location of the study area.







5






CHAPTER I
WATERSHED STUDIES

Methods and Procedures


One watershed of about 437 ha (upper watershed) observed in
this study is primarily in native forest cover with some unimproved
pasture and a very small amount of crop land. The outlet of this
watershed is a small stream which flows continuously except during
extremely dry periods. Another watershed of about 208 ha located
immediately downstream, (lower watershed) is primarily in inten-
sive agricultural crop production with some improved pasture near
the stream. The soils in these watersheds are sandy with a clay
layer at a depth of 1-2 m in most areas creating a shallow water
table during wet periods. Average land slopes are 0-3 percent in
the upper watershed and 3-8 percent in the lower watershed. Pre-
dominant soil associations are Arredondo-Gainesville-Fort Meade,
Leon-Plummer-Rutledge, and Scranton-Ona.

Precipitation was measured by a small wedge-shaped gauge near
the edge of the watersheds and a tipping bucket recording rain
gauge near the center of the two watersheds. Samples were collec-
ted from the recording gauge to determine nutrient concentrations
in rainfall. Stage recorders were installed on the stream at the
outlet of each watershed to provide a continuous record of the
stream level (Figure 2). Manning's velocity formula (Chow, 1959)
was used to develop a stage-discharge relationship for use in
determining flow rates and volumes and nutrient loads from the
watersheds. Discharge measurements made on the stream by fluor-
ometry techniques (Replogle et al 1966, Wilson 1968) and current
meter measurements at a variety of stages verified this relation-
ship.

At each watershed outlet an automatic water sampler collected
streamflow samples every eight hours for later nitrogen and phos-
phorus analyses. During periods of low flow, these samples were
composite into a single daily sample. Nitrogen forms measured
were total Kjeldahl nitrogen (U.S. Environmental Protection Agency,
1974), ammonium nitrogen by the selective ion electrode (U.S.
Environmental Protection Agency, 1974) and nitrate nitrogen by the
chromotropic acid method (American Public Health Association, 1971).
Phosphorus forms measured were total phosphorus and orthophosphate
by the ascorbic acid method (American Public Health Association,
1971). Nutrient loads in the stream during periods of low flow


















































0

Figure 2. Map of upper and


WATERSHED BOUNDARY
STREAM CHANNEL
FLOW MEASUREMENT
AND WATER SAMPLER

RECORDING RAIN GAGE

lower watersheds studied in Alachua County, Florida.












provided an indication of the movement of nutrients through the
soil profile to the shallow ground water. All flow volumes and
nutrient loads for the lower watershed were determined by sub-
tracting the upper watershed measurements from those of the total
watershed. Land owners in the watersheds were interviewed to
determine cropping, livestock numbers, and fertilizer applied, for
use as components of nutrient balances for the watersheds.


Results and Discussion


Data were collected during the period from July 1975 to June
1977. Precipitation from 7/75 6/76 totalled 105 cm, about 25 cm
below average. From 7/76 6/77 precipitation totalled only 88 cm,
about 42 cm below average. According to the landowners, flow
levels in the stream were below normal as would be expected with
this lower than average rainfall.

Tables 1 and 2 show the nitrogen and phosphorus loads in the
streamflow from the upper and lower watersheds. The upper water-
shed is primarily in native forest cover and the lower watershed
is in intensive agricultural crop production, as indicated pre-
viously. The flow-weighted average nutrient concentrations, shown
in Table 3, are very similar for both the upper and lower water-
sheds for most nutrient forms during the same year. However, the
flow volume from the lower watershed is about four times greater
than that from the upper watershed during 1975-76 and somewhat
greater during 1976-77. Thus, it appears that most of the in-
creased nutrient load from the lower watershed, shown in Tables
1 and 2, can be attributed to the increased flow volume from that
watershed. Flow volumes are reported as a uniform depth over the
appropriate drainage area.

In Tables 4 and 5 the nutrient loads from the two watersheds
are broken down to show the loads of each nutrient form occurring
during storm flow and during low flow periods. Storm flow and low
flow volumes also are given for both watersheds. As mentioned
earlier, the nutrients in the streamflow during low flow periods
are contributed by baseflow from the surrounding shallow ground-
water, therefore this indicates the extent of movement of nutri-
ents through the soil profile into the groundwater. In the upper
watershed, the flow volume and nutrient loads are divided about
equally between storm and low flow periods during 1975-76 while,
in the lower watershed, about 80 percent of the flow volume and
nutrient loads occurred during storm periods (Table 4). These
















Table 1. Nitrogen load (kg/ha) in streamflow from July 1975 to
June 1976 and from July 1976 to June 1977.


Upper Watershed Lower Watershed
1975-76 1976-77 1975-76 1976-77
----------------------kg/ha-------------------


Organic N 1.21 1.49 5.30 1.92
Ammonium N 0.11 0.07 0.68 0.09
Nitrate N 0.12 0.09 0.37 0.09
Total N 1.43 1,65 6.36 2.10


Table 2. Phosphorus load (kg/ha) in streamflow from July 1975 to
June 1976 and from July 1976 to June 1977.


Upper Watershed Lower Watershed
1975-76 1976-77 1975-76 1976-77
-------------------kg/ha----------------


Orthophosphate P 0.30 0o52 1.21 0.63
Total P 0.33 0.68 1.34 0.86





















Table 3. Flow-weighted average nutrient concentrations (mg/1) and
flow volumes (cm) for the periods July 1975 to June 1976
and July 1976 to June 1977.


Upper Watershed Lower Watershed
1975-76 1976-77 1975-76 1976-77
-------------------mg/l-------- ---


Organic N 2.31 1.70 2.49 1.59
Ammonium N 0.21 0.07 0.32 0.07
Nitrate N 0.22 0.10 0.17 0.07
Total N 2.73 1.87 2.98 1.73

Orthophosphate P 0.58 0.59 0.57 0.52
Total P 0.63 0.77 0.63 0.71

Flow Volume, cm 5.25 8.79 21.3 12.1












Table 4. Nutrient loads (kg/ha) and flow volumes (cm) by type of
flow for each watershed from July 1975 to June 1976.


Upper Watershed Lower Watershed
Storm Flow Low Flow Storm Flow Low Flow
------------------- kg/ha-----------------


Organic N 0.61 0.60 4.30 1.00
Ammonium N 0.06 0.05 0.51 0.17
Nitrate N 0.07 0.05 0.30 0.07
Total N 0.73 0.70 5.12 1.24

Orthophosphate P 0ol13 0.17 0.94 0.27
Total P 0.15 0.18 1.04 0.30

Flow Volume, cm 2.57 2.67 17.4 3.95






Table 5. Nutrient loads (kg/ha) and flow volumes (cm) by type of
flow for each watershed from July 1976 to June 1977.


Upper Watershed Lower Watershed
Storm Flow Low Flow Storm Flow Low Flow
--------------------kg/ha-----------------


Organic N 0.59 0.90 1.23 0.69
Ammonium N 0o03 0.04 0.04 0.05
Nitrate N 0.03 0.06 0.05 0.04
Total N 0.65 0.99 1.31 0.78

Orthophosphate P 0.18 0.34 0.34 0.29
Total P 0.27 0.41 0.43 0.43

Flow Volume, cm 3.50 5.29 5.73 6.34












data indicate that most of the increased flow volume and nutrient
load from the lower watershed in 1975-76 occurred during storm
periods. Two probable reasons for this are the more intensive
land use with less ground cover and the somewhat greater land
slopes in the lower watershed. This watershed is typical of this
general farming area where much of the intensive agricultural
production is on the more sloping land which has better drainage.
Because of the very dry year of 1976-77, a much larger percentage
of the total streamflow occurred as low flow, especially in the
lower watershed. The nutrient loads still remained relatively
proportional to flow volume between storm and low flow periods,
with the exception of the organic nitrogen load in the lower
watershed (Table 5).

Flow-weighted average nutrient concentrations by type of flow
for each watershed showed no definite pattern for nitrogen forms
(Tables 6 and 7). The concentrations were very similar between
storm flow and low flow within a watershed for a given year except
the one case of organic nitrogen when the concentration was
doubled during storm periods. However, phosphorus concentrations
reduced consistently during storm flow periods in both watersheds
in 1975-76 (Table 6)o This trend was consistent for individual
storm flow periods as well as for the total year. This trend did
not continue during 1976-77, however, except for orthophosphate
in the upper watershed (Table 7).

Figures 3, 4 and 5 show the variations in monthly flow-
weighted average nutrient concentrations and monthly flow volume
throughout the period of record for the upper watershed. Similar
relationships for the total watershed (upper and lower combined)
are shown in Figures 6, 7 and 8. An examination of these data
for possible correlations may be very helpful in understanding
some of the processes taking place in the watersheds. This under-
standing is very important to the development of models for
simulation of nutrient movement through watersheds. Nitrate
concentrations were lowest during the fall and winter months in
the upper watershed (Figure 3) as might be expected due to cooler
temperatures during this period. A similar response occurred in
the total watershed initially, but nitrate concentrations remained
very low throughout the period of record after the first summer
(Figure 6). Ammonium concentrations in the upper watershed were
relatively low during most of the period of record (Figure 3).
This was also true for the total watershed except for some periods
in the late spring and summer when concentrations were considerably
higher (Figure 6). The increased ammonium concentrations may be
due to increased ammonification as the temperature increases.
They also could be influenced by fertilizer applied to the












Table 6. Flow-weighted average nutrient concentrations (mg/1) by
type of flow for each watershed from July 1975 to June
1976.


Upper Watershed Lower Watershed
Storm Flow Low Flow Storm Flow Low Flow
--------------------mg/l------------------


Organic N 2.37 2.25 2.47 2.53
Ammonium N 0.22 0.19 0.30 0.43
Nitrate N 0.26 0.18 0.17 0.18
Total N 2.85 2o62 2o95 3.13

Orthophosphate P 0.51 0.64 0.54 0.68
Total P 0.58 0.68 0.60 0.77







Table 7. Flow-weighted average nutrient concentrations (mg/1) by
type of flow for each watershed from July 1976 to June
1977.


Upper Watershed Lower Watershed
Storm Flow Low Flow Storm Flow Low Flow
--------------------mg/1------------------


Organic N 1.69 1.71 2.15 1.08
Ammonium N 0.08 0.07 0.06 0.08
Nitrate N 0.09 0.11 0.08 0.07
Total N 1o86 1.88 2.29 1.23

Orthophosphate P 0.51 0.64 0.58 0.46
Total P 0.77 0.77 0o75 0.68













0







E
0



4 o


6-

6


J A S 0 N D J F M A M J J A S 0 N D J F M A


1975


1976


1977


Monthly flow-weighted average nitrate N and ammonium N concentrations and monthly flow
volume from the upper watershed.


06


0.4





02


Figure 3.










































J A S 0 N D J F M A M J J A S 0 N D J F M A


1975


1976


1977


Figure 4. Monthly flow-weighted average organic N concentration and monthly flow volume from the
upper watershed.














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1.6 -- Orthophosphate P 2 0
-- Total P


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S0.8 -6





0.4-
0-0








O I III I I I I III llI
JASON D J FM AM J J A S O N D J F M A
1975 1976 1977

Figure 5. Month-ly flow-weighted average orthophosphate P and total P concentrations and monthly
flow volume from the upper watershed.


















E
2

E

0

4





6
6


J A S 0 N D J F M A M J J A S 0 N D J F M A M J


1975


1976


1977


Monthly flow-weighted average nitrate N and ammonium N concentrations and monthly flow
volume from the total watershed.


0.6


0.4,





0.2


Figure 6.















6.68



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.- 3 4

2 6

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u
t.--.













J A S 0 N D J F M A M l J A S 0 N D J F M A M J

1975 1976 1977
Figure 7. Monthly flow-weighted average organic N concentration and monthly flow volume from the
total watershed.













2.0 I',I Iii" 0




I. E
1.6 Orthophosphate P -2
-----Total P




1.2 4






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0.4




0 I I I I i I I I I I I 1 1 I I I I I I I I
J A S 0 N D J F M A M J J A S 0 N D J F M A M J
1975 1976 1977
Figure 8. Monthly flow-weighted average orthophosphate P and total P concentrations and monthly
flow volume from the total watershed.












watershed since most of it is applied in March and April and in
the lower watershed. Figures 4 and 7 show several drops in
organic nitrogen concentration that correspond to the increased
ammonium concentrations as might be expected if ammonification
was increasing during these months. These changes may be con-
nected to the increase in microbial activity as warmer weather
begins

The bulk of the total phosphorus load in the stream is in
the orthophosphate form as is evident in Figures 5 and 8o Phos-
phorus concentrations tended to decrease in months with larger
flow volumes and increase in months with small flow volumes during
much of the period of record (Figures 5 and 8). This is consist-
ent with the observation made earlier about phosphorus concentra-
tions decreasing during individual storm flow periods. This trend
indicates that more than a flow-proportionate amount of the phos-
phorus load from the watersheds is delivered in the shallow
groundwater during low flow periods and storm flow has a partial
dilution effect on this phosphorus load. The soils in these
watersheds are naturally rather high in phosphorus content,
therefore these results were not entirely unexpected.

Components of nutrient balances were calculated for both
watersheds. Average values of nitrogen and phosphorus content
in the harvested crops were obtained from the literature (Carlile
and Phillips 1976, Pritchett and Gooding 1975, Pritchett and
Smith 1974, Thompson and Troeh 1973, USDA Soil Conservation
Service 1975). In addition to the nutrient inputs listed in
Tables 8 and 9, natural mineralization processes in the soil and
plant residues provide some nitrogen and phosphorus. Nitrogen
(N2) fixation also provides some available nitrogen. No attempt
was made to estimate the amounts provided by these processes On
the output side, leaching of nutrients to the deep groundwater
should be minimal because of the relatively impermeable clay
layers which underly this area. Water balance estimates for this
period also support this statement0 Denitrification, immobiliz-
ation and phosphorus fixation are additional sinks that may
account for much of the difference between inputs and outputs in
Tables 8 and 9. While the nutrient balances in Tables 8 and 9
are not complete, they do serve to show the relative magnitudes
of some of the individual components of the balances. For example,
nutrient losses in streamflow were considerably smaller than the
amount of nutrients added to the watersheds in precipitation during
this period. An exception to this was phosphorus losses in 1976-77
when precipitation input was very low and shallow groundwater flow
was greater than in 1975-76, resulting in phosphorus losses
greater than precipitation input. Nutrient losses in streamflow














Table 8. Components of the nitrogen balance for each watershed
for the periods July 1975 to June 1976 and July 1976
to June 1977.


Upper Watershed Lower Watershed
1975-76 1976-77 1975-76 1976-77
-------------------kg/ha----------------


Fertilizer 21.0 33.3 96.9 142.7
Animal Waste 5.7 7.2 24.2
Precipitation 18.9 16.7 18.9 16.7

Harvested Crops 19.9 19.2 93.6 55.1
Streamflow 1.43 1.65 6.36 2.10







Table 9. Components of the phosphorus balance for each watershed
for the periods July 1975 to June 1976 and July 1976
to June 1977o


Upper Watershed Lower Watershed
1975-76 1976-77 1975-76 1976-77
-------------------kg/ha----------------


Fertilizer 6o4 3.1 33.2 32.7
Animal Waste 1L5 1.9 6.5
Precipitation 1L57 0o44 1.o57 0.44

Harvested Crops 3.4 2.0 19.1 8.1
Streamflow 0.33 0.68 1o34 0.86












amounted to only a small percentage of the nutrients applied in
commercial fertilizer. The 1976-77 crop season was extremely dry
resulting in a total loss for many crops. This accounts for the
decrease in nutrients removed in harvested crops from 1975-76 to
1976-77.


Summary


Two agricultural watersheds were instrumented to determine
water quantity and quality measurements. The upper watershed of
437 ha was primarily in forest cover with some pasture and a small
amount of row crop. The lower watershed of 208 ha was mostly in
intensive agricultural crop production with some pasture. The
following results are from data collected during the two-year
period:

1. Nitrogen and phosphorus loads in streamflow were approx-
imately proportional to the flow volume in the two water-
sheds during each year.

2. The average total nitrogen concentration was about 10
percent greater in the lower watershed the first year,
and about 10 percent smaller the second year. The
average total phosphorus concentrations were the same in
both watersheds the first year, and about 10 percent
smaller in the lower watershed the second year.

3. The larger flow volume per hectare in the lower watershed
as compared to the upper watershed occurred mostly during
storm periods and was probably primarily due to land use
and topography differences.

4. Average concentration changes between storm and low flow
periods were small in both watersheds. About 50 percent
of the flow volume and nutrient load from the upper
watershed occurred during storm flow periods, while about
80 percent of the flow volume and nutrient load from the
lower watershed occurred during storm flow periods in
1975-76. During 1976-77 the fractions were about 40 and
50 percent, respectively.

5. Components of nutrient balances indicate that nutrient
losses in streamflow are a very small part of the total
nutrient flow system in these two watersheds. Total
nitrogen and phosphorus losses in streamflow were







22




equivalent to about five percent of the commercial
fertilizer applied in each watershed. Nutrient loads in
streamflow also were less than those contributed to the
watersheds in precipitation during the period with one
exception.















CHAPTER II
SMALL PLOT EXPERIMENTS


Small plot experiments were conducted during three crop years
to evaluate the effects of cultural practices, fertilizer applica-
tion methods, and water management on nutrient loads in surface
runoff from sandy soils. These practices were evaluated on plots
producing bell peppers and tomatoes. Practices evaluated the first
two seasons on bell peppers (1975 and 1976) included (1) plastic
mulch over the plant bed with all fertilizer applied beneath the
plastic at planting time, (2) no mulch over the plant bed with all
fertilizer applied at planting time, and (3) no mulch over the
plant bed with fertilizer applied in three equal applications
during the growing season. All treatments received sprinkler
irrigation. Practices evaluated the last season (1977) were
(1) drip irrigation under plastic mulch with fertilizer applied
through the irrigation system at weekly intervals on both tomatoes
and bell peppers, and (2) drip irrigation under plastic mulch with
all fertilizer banded along the plant row at planting time on bell
peppers.


Methods and Procedures


The small plot experiments were conducted at the University
of Florida Horticultural Unit near Gainesville. The experiments
for 1975 and 1976 used six plots (5.5m by 15m) containing three
beds of bell peppers each fertilized at a rate of 224 kg N/ha from
ammonium sulfate, 84 kg P/ha from superphosphate, and 140 kg K/ha
from potassium chloride. There were two replicates of each of the
three treatments enumerated above.

Beds were formed by a specially designed rototiller. Ferti-
lizer was applied to the raised beds and the beds were again
rototilled to mix the fertilizer. Plastic mulch was then applied
to the appropriate plots before peppers were transplanted from
greenhouse beds. Sprinkler irrigation was applied as required
throughout the season to provide adequate moisture.

Critical depth flumes and water stage recorders were in-
stalled at the end of each plot to measure surface runoff. Auto-
matic samplers were placed at each flume to collect a flow pro-
portional composite water sample from each runoff event.












Water samples were analyzed for organic N by the microkjeldahl
method (U.S. Environmental Protection Agency, 1974), ammonium N
by the selective ion electrode method (U.S. Environmental Protec-
tion Agency, 1974), nitrate N by the chromotropic acid method
(American Public Health Association, 1971), orthophosphate P by
the ascorbic acid method (American Public Health Assoication,
1971), and total P by the ascorbic acid method after persulfate
digestion (American Public Health Association, 1971). Sediment
concentrations in most samples were very low because of the sandy
soils and small slopes ( < 1 percent), therefore all analyses were
run on the unfiltered sample.

Nutrient loads from the plots were calculated by a specially
written flow and nutrient analysis computer program. The program
provided runoff volumes, nutrient loads, and average nutrient
concentrations on a storm event, monthly, and seasonal basis.
This analysis program also was used in the watershed studies of
Chapter I.

During 1977 the experiment used six plots. All plots were
drip irrigated under plastic mulch and received 40 kg N/ha,
120 kg P/ha, and 40 kg K/ha broadcast at planting time. In addi-
tion, the tomato treatment and one bell pepper treatment received
weekly applications of N and K fertilizer through the irrigation
system to total 135 kg N/ha and 168 kg K/ha by drip irrigation.
The other bell pepper treatment received an additional 135 kg N/ha
and 168 kg K/ha banded along the plant row at planting time with
none applied in the irrigation. Each of the above treatments had
two replicates. The methods and procedures used during this last
season were the same as the first two seasons except that sprinkler
irrigation was not used and all beds received plastic mulch.

These experiments were superimposed upon larger, more compre-
hensive studies being conducted simultaneously in cooperation with
other researchers. Some of their findings (to be available soon)
may contribute to an understanding of the results presented here.
(D. A. Graetz, personal communication. Soil Science Department,
University of Florida, Gainesville. 1978).


Results and Discussion


Treatments were randomly assigned to plot locations within
the experimental area each year. All results presented are
averaged over the two replicates of each treatment. During 1975,












surface runoff amounts were 6.32 cm from the plastic mulch,
5.49 cm from the no mulch, and 3.84 cm from the split fertilizer
treatment. Difficulties with the automatic water samplers pre-
vented obtaining enough samples to calculate nutrient loads for
the entire season. Grab samples were collected from a runoff
event in May that produced about one-fourth of the total seasonal
runoff from most plots. Nutrient concentrations in these samples
were very similar to the average concentrations measured during
1976 (Table 11). In 1975 there was less leaching of fertilizer
nitrogen in plots with plastic mulch (D. A. Graetz, personal
communication). Split fertilizer application also resulted in
more efficient use of nitrogen than in the unmulched single appli-
cation. Near the end of the crop season there did not appear to
be adequate nitrogen for good plant growth in either of the
unmulched treatments, while the mulched treatment was adequate.

Surface runoff volumes and nutrient loads from the three
treatments during 1976 are shown in Table 10. Somewhat greater
runoff was expected from the mulched plots because the mulch
prevented infiltration on the plant bed to a great extent.
Differences in surface runoff volume between the treatments were
not consistent during the two years. This inconsistency probably
resulted from the great amount of variation between individual
plots even with the same treatment. Precipitation during the
period of record was 81 cm and 77 cm for 1975 and 1976, respec-
tively. Runoff volumes were relatively low, as expected. Nutri-
ent loads from all treatments also were relatively low, especially
when compared with the amount of fertilizer applied. Nutrient
loads in runoff from the plastic mulch and no mulch treatments
during 1976 were similar for all nitrogen and phosphorus forms
(Table 10). However, nutrient loads were considerably higher
from the split fertilizer treatment for all forms. This was
partially because of a greater runoff volume from this treatment.
Table 11 shows, however, that concentrations also were somewhat
greater from the split fertilizer treatment. This was especially
true for the ammonium N and nitrate N forms. The timing of fer-
tilizer applications relative to rainfall was a very important
factor in causing this treatment difference. When runoff occurs
soon after fertilizer is applied, as happened during this exper-
iment with the split applications, the fertilizer is subject to
washoff in the runoff water. Therefore, while the split appli-
cations resulted in more efficient use of nitrogen, runoff losses
were increased in this particular year because of the timing
problem. Runoff losses were still very minimal, however.

Fertilizer leaching losses from the unmulched treatment in
1976 were much greater than from the mulched and split fertilizer












Table 10o


Nutrient loads (kg/ha) and surface runoff volume (cm)
from bell peppers with plastic mulch, no mulch, and no
mulch with split fertilizer applications during 1976o


Plastic No Split
Mulch Mulch Fertilizer
----------------kg/ha----------------


Organic N 0.93 0.94 1.35
Ammonium N 0o10 0.06 0.19
Nitrate N 0ol12 0.12 0.72
Total N 1.15 1.12 2.26

Orthophosphate P 0.13 0.21 0.37
Total P 0.18 0.28 0.46

Runoff Volume, cm 4.78 5.31 7.49





Table 11. Flow-weighted nutrient concentrations (mg/1) in surface
runoff from bell peppers with plastic mulch, no mulch,
and no mulch with split fertilizer applications during
1976.


Plastic No Split
Mulch Mulch Fertilizer
-----------------mg/1----------------


Organic N 1.93 2.06 1.78
Ammonium N 0.17 0.12 0.26
Nitrate N 0,36 0O22 0.94
Total N 2.46 2.40 2.98

Orthophosphate P 0.28 0.40 0.48
Total P 0.38 0.56 0.61












application treatments (D. A. Graetz, personal communication).
This effect was more prominent than during 1975 as a result of
heavy rainfall occurring early in the growing season. Fruit yields
were much lower from the unmulched treatment as a result of these
leaching losses. Leaching losses and fruit yields were very
similar for the mulched and the split fertilizer treatments (D. A.
Graetz, personal communication).

In 1977 both tomatoes and bell peppers were grown on the
experimental plots. All plots with bell peppers were planted to
sweet corn as soon as the peppers were finished producing fruit;
while the tomato plots were left idle after the crop was finished.
Therefore, the direct comparison of the two fertilizer application
methods was limited to the immediate growing season from April to
July, 1977. Precipitation during this period totalled 30 cm.
This is normally a rather dry time of year in north central
Florida, resulting in the relatively low surface runoff volumes
shown in Table 12. Because of the very small amount of runoff,
most of the nutrient loads are correspondingly small. More varia-
tion among treatments can be observed from the nutrient concen-
trations in runoff water shown in Table 13o Because of the rela-
tively large amount of variability between plots of the same
treatment, it is difficult to attribute the differences in nutri-
ent loads and concentrations for this short period to actual
treatment effects. Information on leaching losses is not yet
available from these treatments (D. A. Graetz, personal
communication)

On July 29, 1977 all plots with bell peppers were planted to
sweet corn. These plots received 42 kg N/ha, 56 kg P/ha, and
56 kg K/ha at planting. The plots were sidedressed with 40 kg N/ha
from ammonium nitrate on September 1 and again on September 13.
The tomato plots were left idle after production finished as
indicated previously and received no more fertilizer. Nutrient
loads and runoff volumes for the complete season from April, 1977
to January, 1978 are shown in Table 14 for all treatments. Precip-
itation during this period was 83 cm. Nutrient loads from drip
fertilized tomatoes and drip fertilized peppers followed by sweet
corn were similar. Additional nutrient losses from the production
of the sweet corn crop were not observed. The slightly larger
nutrient loads from the pepper and sweet corn treatment were more
than accounted for by the somewhat larger runoff volume from that
treatment. It follows that most of the nutrient concentrations
in runoff water (Table 15) were smaller for the drip fertilized
peppers and sweet corn than for the tomatoes.

The primary nutrient forms, if any, expected to be affected
by fertilizer applied for crop production are ammonium N and
nitrate N. The band fertilized peppers followed by sweet corn had












Table 12.


Nutrient loads (kg/ha) and surface runoff volume (cm)
from drip fertilized tomatoes, drip fertilized bell
peppers, and band fertilized bell peppers during the
growing season April to July, 1977.


Drip Fertilized Drip Fertilized Band Fertilized
Tomatoes Peppers Peppers
----------------------kg/ha-------------------


Organic N 0.10 0.36 0.59
Ammonium N 0.04 0.04 0.02
Nitrate N 0.03 0.04 0.08
Total N 0.17 0.44 0.69

Orthophosphate P 0.02 0.01
Total P 0.04 0.01

Runoff Volume, cm 0.17 0.40 0.26

*less than 0.005




Table 13. Flow-weighted nutrient concentrations (mg/1) in surface
runoff from drip fertilized tomatoes, drip fertilized
bell peppers, and band fertilized bell peppers during
the growing season April to July, 1977.


Drip Fertilized Drip Fertilized Band Fertilized
Tomatoes Peppers Peppers
----------------------- mg/l-------------------


Organic N 2.28 3.86 6.68
Ammonium N 0.85 0.39 0.40
Nitrate N 0.80 0.45 1.14
Total N 3.93 4.70 8.22

Orthophosphate P 0.04 0.19 0.07
Total P 0.08 0.34 0o12












Table 14.


Nutrient loads (kg/ha) and surface runoff volume (cm)
from drip fertilized tomatoes, drip fertilized bell
peppers, and band fertilized bell peppers during the
period April, 1977 to January, 1978.


Drip Fertilized Drip Fertilized Band Fertilized
Tomatoes Peppers Peppers1
------------------kg/ha-------------------


Organic N 1.17 1.21 1.86
Ammonium N 0o10 0.13 1.48
Nitrate N Oo10 0.15 0.36
Total N 1.37 1.49 3.70

Orthophosphate P 0.24 0.26 0.36
Total P 0.43 0.55 0.46

Runoff Volume, cm 3.86 5.21 6.16

1Sweet corn followed peppers on July 29, 1977



Table 15. Flow-weighted nutrient concentrations (mg/1) in surface
runoff from drip fertilized tomatoes, drip fertilized
bell peppers, and band fertilized bell peppers during
the period April, 1977 to January, 1978.


Drip Fertilized Drip Fertil zed Band Fertil zed
Tomatoes Peppers Peppers1
-----------------------mg/l----------------------


Organic N 3.22 2.12 2.40
Ammonium N 0.25 0.24 2.06
Nitrate N 0.27 0.27 0.50
Total N 3.74 2.63 4.96

Orthophosphate P 0o64 0.56 0.60
Total P 1.13 1.19 0.78

1Sweet corn followed peppers on July 29, 1977













increased losses of both these nitrogen forms compared with the
other treatments (Table 14). The increased losses were greater
than that accounted for by the slightly larger runoff volume from
this treatment. This is reflected by the higher ammonium N and
nitrate N concentrations in Table 15. These increased losses
occurred primarily during August, which was a relatively wet month
(21 cm of rain). Most of the increased ammonium N loss occurred
from only one of the two plots with this treatment. As referred
to earlier, the relatively large variability between plots in a
given treatment makes it difficult to determine whether it is a
real treatment difference or a result of the natural heterogeneity
in the many factors which interact to affect the overall nutrient
losses. Since it was not feasible to have enough plots for a
valid statistical analysis this question cannot be answered from
this experiment. The increased nitrate N loss, however, was
relatively uniform in both plots. This gives more indication of
a real treatment difference. The most important result of these
1977 plot studies was that the total nutrient loads in runoff
from all treatments, even with a double crop on some treatments,
were very small compared to the fertilizer applied and the natural
contributions from rainfall (Table 16). One exception to this
was that the total phosphorus loads in runoff were nearly the
same as the contribution in rainfall. Table 16 indicates that
these relationships also were true during 1976.


Summary


The small plot experiments evaluated the effects of manage-
ment practices including use of mulch, fertilizer application
methods and timing, and double cropping on nitrogen and phosphorus
losses in surface runoff. In all but one case, nitrogen and phos-
phorus losses in surface runoff were less than one percent of the
amount applied in fertilizer. In all treatments total nitrogen
losses in surface runoff were less than 25 percent of the contri-
bution of rainfall. Total phosphorus losses in surface runoff
were less than or equal to the contribution in rainfall except in
one case when the runoff loss was about 30 percent greater than
the contribution in rainfall. Phosphorus contributions in rain-
fall, however, were very low.

Ammonium N and nitrate N losses in runoff were greater from
the split fertilizer application than from a single application
because of the timing of runoff events which happened to occur
soon after the split applications were made. No changes in
nutrient losses by runoff were observed from use of plastic mulch.





















Table 16,


Nitrogen and phosphorus contributions from fertilizer
and rainfall compared with the largest surface runoff
losses from any treatment during a given year.


Fertilizer Rainfall Surface Runoff
Applied Contribution Losses (Largest)
--------------kg/ha----------------------


1976 Peppers
Total N
Total P

1977 Tomatoes
Total N
Total P

1977 Peppers
and Corn
Total N
Total P


224
84


175
120



297
176


15.8
0.46


14.5
0.42



14.5
0.42


2.26
0.46


1.37
0,43



3.70
0.55







32




No increased nutrient losses in runoff were measured from double
cropping which had extra fertilizer applied, except for the
treatment where all of the first crop (peppers) fertilizer was
applied at planting time. In this case, ammonium N and nitrate
N losses were greater with double cropping, however there was a
large variation between plots with the same treatment so this may
not have been a real treatment effect.















CHAPTER III
SIMULATION MODEL DEVELOPMENT

Modeling Approach


Movement of nitrogen through agricultural watersheds involves
many complex processes and interactions within the watershed.
These processes are illustrated in Figure 9, adapted from Stewart
(1976). In order to simulate nitrogen movement through a water-
shed into streamflow the potential amount of nitrogen available
for transport must be known. This nitrogen can come from many
sources including precipitation, fertilizer, animal wastes, and
soil organic nitrogen reserves. Movement of this nitrogen in
water through the watershed depends upon its form. Therefore,
transformation processes and rates must be simulated. These are
dependent upon watershed conditions including soil temperature,
moisture content, soil type, pH, aeration, agricultural practices,
and organic matter content (Porter 1975, Duffy and Franklin 1972,
Hagin and Amberger 1974, Mehran and Tanji 1974). There are
numerous sinks for nitrogen within the watershed including uptake
by crops, immobilization of nitrate, and denitrification. These
sinks reduce the amount of nitrogen available for movement from
the watershed in streamflow.

Transport of nitrogen through a watershed also depends very
heavily upon the hydrology of the watershed. Therefore, a good
hydrologic simulation of the watershed is very important for the
simulation of nitrogen movement. The hydrologic model should
simulate the quantity of water moving through the watershed, its
rate and direction for both overland and subsurface flow. This
requires a comprehensive deterministic hydrologic model. It is
particularly important that the hydrologic model predict the
quantity of runoff and subsurface flow that results from different
land-use areas and surface covers within the watershed.

The modeling approach used in this study was (1) to select a
hydrologic model, meeting the above criteria, that was already
developed and had been tested in a number of areas, (2) develop a
model to simulate the nitrogen sources, sinks, and transformation
processes within a watershed, and (3) couple the above two models
together to obtain a simulation of the water and nitrogen movement
through a watershed.






AIR
N 2 NH3 DUST


-rio


HARVEST ADSORPTION
PRODUCTS


ANIMALS I
PLANTS VOLATILIZATION


/ RESIDUES\


I-I-

N03 IMMOBILIZATION SOIL ORGANIC AMMONIFICATI(
3 MATTER
SIMMOBILIZATII


LEACHING NITRIFICATION

-INi ^^


tfi


CLAY AND
ORGANIC COMPLEXES


Figure 9. The nitrogen cycle in agriculture.


I I


RUNOFF
S 0TnN


ON NH4
ON













Hydrologic Model Calibration


The USDAHL-74 revised model of watershed hydrology (Holtan
et al, 1975) was chosen as the hydrologic model for this study.
This model is a deterministic, semi-empirical lumped hydrologic
model. It was developed by the USDA Hydrograph Laboratory to be
used as a practical tool for predicting runoff and infiltration
in relatively small watersheds under natural rainfall conditions.
The model utilizes some well known mathematical descriptions of
the major hydrologic processes within a watershed. The model is
written in Fortran IV computer language. Input requirements are
relatively large. About 72 different parameters are required as
input in addition to the historical climatic data. Computation
time requirements are relatively low since no numerical solutions
are involved in the computation processes. The model has been
evaluated in a number of locations around the United States in
the past few years (Nicks et al 1977, Hanson 1977, Crow 1977,
Perrier et al 1977, Molnau and Yoo 1977, James et al 1977). It
divides the watershed into hydrologic response zones based upon
soil and watershed conditions. It also provides for response
differences due to land use. Daily moisture status in the soil
profile, soil water movement in both the vertical and horizontal
directions, and other pertinent variables are readily available
for use in a nutrient movement model. The USDAHL model is designed
with each major hydrologic process in a separate subroutine. This
makes understanding, modification, and improvement of the model
easier. These are all advantages of this model for use with a
nutrient transport model. Because of its advantages the USDAHL-74
model was chosen to serve as the hydrologic part of a larger model
to simulate nitrogen movement through an agricultural watershed.

The hydrologic model was calibrated using data from the
agricultural watersheds described in Chapter I. The USDAHL model
requires four types of input parameters in addition to climatic
data: watershed, soils, land use, and hydraulic (Holtan et al,
1975). Tables 17, 18, and 19 list the input parameter values used
in the model to obtain the best simulation of streamflow for the
calibration period of January to June, 1976. This combination of
input parameters was selected after considerable trial and error
selection of values for certain parameters as explained in the
following discussion. The watershed was divided into three zones
based upon hydrologic response. Zone 1 is the very flat upper
part of the watershed, zone 2 is the hillside portion with more
slope, and zone 3 is the alluvium portion of the watershed along
the stream channel. The watershed parameters and some land use












Table 17. Watershed input parameters for USDAHL-74 model.


WATERSHED PARAMETERS


Size: 645 ha Number of Zones: 3
Deep Groundwater Recharge: 1.27 mm/hr


ZONE PARAMETERS


Watershed
Zone Area,
__ percent


Number of Crops: 45


Final
Infiltration
Length, Slope, Capacity,
m percent mm/hr


274
305
122


0.4
2.0
0.8


1.27
7.62
1.27


Topsoil
Depth,
cm

38
38
64


Table 18. Soil input parameters for USDAHL-74 model.


TOPSOIL


Total
Zone Porosity,
__ percent


Field
Capacity,
percent

20
20
40


Wilting
Point,
percent

7
7
15


Antecedent
Soil Water,
percent

20
20
40


SOIL PROFILE BELOW TOPSOIL


Total
Zone Porosity,
__ percent


Field
Capacity,
percent

20
15
30


Wilting
Point,
percent

10
4
17


Antecedent
Soil Water,
percent

30
30
45


Total
Soil
Depth,
cm

127
127
127


Cracking,
percent

0
0
0


Cracking,
percent

0
0
0












Table 19.


Routing and land use input parameters for USDAHL-74
model.


ROUTING PARAMETERS

Number of Routing Coefficients: 3 Channel Routing, At: 0.2 hr
Channel Coefficient: 1.0 hr Initial Channel Flow: 0.0025 mm/hr
Subsurface Routing: Regime Q-max, Coefficient,
mm/hr hr
1 0.13 22.0
2 0.05 90.0


Cascading:


Zone

1
2
3


To Next Zone,
percent
80
90


Rest Goes To

Alluvium
Channel
Channel


LAND USE PARAMETERS

Crop
A Value
Crop Vd, mm
ET/EP
Root Depth, cm
Upper Temp., C
Lower Temp., C
Zone Area, 70:
1
2
3


Row Crop
0.20
1.27
1.60
51
30
7

25
40
0


Smal Grain
0.30
2.54
1.4
51
27
4

15
10
0


Forest
1.00
2.54
2.0
254
27
4

45
40
80


Grass
0.30
2.54
1.4
76
27
4

15
10
20












parameters were determined from U.S. Geological Survey quadrangle
maps, judgements from direct observations of the watershed, and
aerial photos of the watershed. The remaining land use parameters
and the routing parameters were determined using the procedures
described in Holtan et al (1975). Soil input parameters were
determined from data on Florida soils in Stewart et al (1963).

The value for deep groundwater recharge was initially deter-
mined as 0.02 mm/hr by estimating average annual precipitation,
ET, and streamflow yield in the area as suggested by Holtan et al
(1975). This resulted in excessive streamflow and essentially no
deep recharge occurring during the six month calibration period.
Free water must be present in the bottom layer of the soil pro-
file for deep recharge to occur in the model. This condition was
present in the model for only a very short time. For this reason,
the deep groundwater recharge value was increased to 1.27 mm/hr,
equal to the final infiltration capacity of the soil. This change
resulted in a better simulation of streamflow volume and 11.3 mm
of deep recharge during the calibration period. This amount was
still relatively low for deep recharge, however, precipitation
was below normal during this calibration period.

Weekly average pan evaporation and air temperature data were
obtained from the nearest observation station a few miles away
from the watershed. Rainfall was measured with a recording gauge
and a small wedge-shaped gauge on the watershed. Break-point
rainfall data from the recording gauge for model input were poor
during portions of the calibration period because of instrument
malfunctions. Estimated data were used in these cases.

In this region, soil and watershed conditions are such that
much of the runoff occurs by shallow lateral return flow. Over-
land flow also occurs during and immediately after heavy rainfall
periods. Under these soil and watershed conditions, the sub-
surface flow components of a hydrologic model become very important
to a good streamflow simulation. Considerable difficulties were
encountered during calibration in obtaining a good simulation of
the storm hydrograph shape and timing. A large portion of the
problem was in the empirical equation used in the USDAHL model to
determine the recession curve coefficients and maximum subsurface
flow rates for each zone. In the model these are a function of
the watershed length, watershed slope, watershed area, final in-
filtration capacity, and free water capacity. This approach does
not appear to adequately represent the subsurface flow character-
istics under the conditions encountered in this study. Holtan et
al (1975) also suggest this as an area for further research.













In calibration of the USDAHL model, the output parameters of
primary interest were the total flow volume, the monthly flow
volume, and the daily flow volume and streamflow hydrograph for a
selected period. Observed and simulated monthly flow volumes from
the watershed for the calibration period are shown in Table 20
along with monthly rainfall. Simulated runoff volume for the
total period compared very well with the observed runoff volume.
The monthly distribution was less accurate, however. Runoff volume
was underestimated in the lower rainfall months and overestimated
in the one month of higher rainfall. Table 21 gives a more de-
tailed look at a portion of the wet month of May. Most of the
runoff occurred during a six-day period that was preceded by re-
latively dry conditions. Runoff volume for the rainy period was
overestimated because the model generated sustained high flows
during most of the period (Figure 10). The time of the simulated
peak discharge was delayed, but was very close to the magnitude
of the observed peak discharge. Subsurface flow was not adequately
simulated to provide appropriate recession characteristics on the
discharge hydrograph. This resulted in an overestimated flow
volume for the period. Detailed study of the model output data
indicated that the model generated excessive lateral flows through
the top soil layer to the stream during storms before downward
percolation filled the lower soil layers with moisture. This
contributed to the poor simulation of the hydrograph recession
curve.

In summary, calibration of the USDAHL-74 hydrologic model to
the research watershed resulted in an acceptable simulation of
total water yield for the period. Simulation of daily and monthly
flows was not as good as desired. Components of the model needing
modification to improve the hydrologic simulation of this watershed
were identified.


Nitrogen Model Development


The goal in developing the nitrogen model was to simulate the
nitrogen concentrations and loads in streamflow from a watershed.
The first requirement was to adequately account for the sources of
nitrogen in the watershed. Precipitation is a significant source
of nitrogen and needs to be accounted for as an input of nitrogen
to the watershed (see Tables 8 and 16). Nitrogen concentrations
in rainfall are highly variable with both time of year and loca-
tion, making it difficult to use average values for input to a
model (Allen and Kramer, 1972). Fertilizer and animal wastes ap-
plied to a watershed are other sources of nitrogen that need to be












Observed and simulated monthly flow volume
rainfall for the research watershed during
tion period in 1976.


and observed
the calibra-


Observed Simulated
Month Rainfall, Runoff, Runoff, Error,
mm mm mm percent

January 45.0 8.51 8.00 -6.0
February 42.9 7.84 3.89 -50.4
March 39.9 1.84 0.00 -100.0
April 24.9 0.17 0.00 -100.0
May 191.8 4.63 8.84 90.9
June 37.1 0.51 0.00 -100.0

Total 381.6 23.50 20.73 -11.8





Table 21. Observed and simulated daily flow volume and observed
rainfall for the research watershed from May 23 to
May 28, 1976.

Observed Simulated
Day Rainfall, Runoff, Runoff, Error,
mm mm mm percent

23 69.1 1.03 0.91 -11.6
24 8.4 0.80 2.97 271.2
25 15.5 0.89 2.24 151.7
26 0.0 0.52 2.13 309.6
27 7.9 0.30 0.51 70.0
28 4.1 0.18 0.05 -72.2

Total 105.0 3.72 8.81 136.8


Table 20.


















/ \
I \


/ \
",

I-


Observed
Simulated


$- --.
/ '..-
/
/




1%


20 40 60 80


Time, hours


Figure 10. Observed and simulated discharge hydrographs for the period May 23-28, 1976.


0.25





0.20


CA
Cnr)
E


C-
Cn
r-
t/1


0.15





0.10


0.05




0












accounted for by a model. Organic decomposition of crop residues,
leaf litter, and organic matter accumulations on the watershed
surface may contribute nitrogen. The effect of this nitrogen
source is likely to be a function of season, moisture, and temper-
ature. In particular, at the end of the growing season when
temperatures are still relatively high there may be large influxes
of organic and rapid decomposition with release of soluble forms
of nitrogen (M. D. Smolen, personal communication. Southern
Piedmont Research and Continuing Education Center, Virginia Poly-
technic Institute and State University, Blackstone. 1977). Under
Florida conditions this may be a significant source of the soluble
organic nitrogen occurring in streamflow. While erosion is a
large source of nitrogen in streamflow in many areas, it is not
a significant source in much of Florida. The soil organic nitro-
gen pool is another source of nitrogen in a watershed. This
nitrogen becomes available for movement through the watershed
slowly by the natural mineralization process. Mineralization is
primarily a function of temperature and moisture.

Nitrogen may also be removed from the system in a watershed
through several sinks. The largest nitrogen sink is uptake by
crops. It is a function of transpiration and nitrate concentra-
tion in the root zone. Mass flow is the predominate mechanism
for moving nitrate through the soil to the plant roots (Barber,
1962). Therefore, nitrogen uptake should be closely connected
to transpiration. The amount of transpiration also is related to
the amount of adsorbing root surface and reflects the growth rate
of the plant (Frere et al, 1975). Other nitrogen sinks are
volatilization of ammonia, immobilization, and denitrification.
Volatilization of ammonia occurs only from the soil surface or the
upper soil layer and is probably not significant except from appli-
cation of ammonium fertilizers or animal wastes. Immobilization
is the conversion of inorganic nitrogen forms to organic forms.
It depends upon the amount of nitrogen in the soil and the carbon-
nitrogen (C:N) ratio. Denitrification usually occurs when poor
aeration limits the amount of free oxygen in the soil. It is
dependent on several factors including organic matter content,
moisture, temperature, oxygen concentration, and pH. Denitrifica-
tion is rapid if conditions are favorable. Appreciable losses of
nitrogen as nitrogen gas can occur even when conditions favorable
to denitrification exist for only a day or less. Estimates of
total losses by denitrification on cropped lands average 10 to 20
percent of all nitrates formed or added as fertilizers and can be
as much as 40 to 60 percent of added nitrate nitrogen (Donahue et
al, 1977). Patrick et al (1976) showed that denitrification was
even significant in well-drained agricultural soil in the absence
of excessive organic matter.













After the nitrogen sources and sinks have been provided for
in a model, the transformation processes and rates regulating
changes in nitrogen forms must be considered in order to simulate
the nitrogen losses in streamflow. This is important because of
the different reactions that take place in the soil-water-air-
plant system for different nitrogen forms. In model formulation
of these processes, it appears that first-order rate equations
are adequate (Frere 1975, Rao et al 1976). Mineralization rates,
or decomposition of organic nitrogen to ammonium, and nitrifica-
tion rates of ammonium to nitrate are dependent on several factors
including soil temperature, moisture content, soil type, pH,
aeration, agricultural practices, C:N ratio, and organic matter
content. Models can be developed to consider one or all of these
factors with varying degrees of sophistication (Frere et al 1975,
Duffy and Franklin 1972, Mehran and Tanji 1974, Hagin and Amberger
1974, Donigian and Crawford 1976, Donigian et al 1977). Beek and
Frissel (1973) simulated heat flow in the soil to determine the
soil temperatures for use in nitrogen transformation calculations.
This required inputs of air temperature, soil moisture, soil heat
conductivity and soil heat capacity.

The approach chosen for this study was to select relatively
simple expressions for the most important parts of the nitrogen
cycle and develop a simple model to interface with the USDAHL-74
hydrologic model. Other expressions could then be added to this
model to include other nitrogen forms and transformation processes
to obtain a better simulation as the model is tested and further
developed. Based on this approach, the nitrate option of the
ACTMO model (Frere et al, 1975) was selected for use as a basic
nitrogen model. It has the advantage of being designed to be
interfaced with the USDAHL-74 model, however it was not available
in this form. The basic framework of this nitrate model was
developed as an option of the ACTMO model, but it was never opera-
tional (M. H. Frere, personal communication. Southern Great Plains
Research Watershed, USDA-SEA, Chickasha, Oklahoma. 1977).

The ACTMO nitrate model considers only the soil organic
nitrogen and fertilizer applied as nitrogen sources. Organic
nitrogen is mineralized to nitrate according to a first order rate
equation. The rate coefficient is sensitive to temperature and
moisture. The watershed is separated into zones as in the USDAHL-
74 model, and fertilizer can be applied by zone in one or two
applications. The only nitrogen sink considered by the model is
plant uptake. Nitrate uptake is a function of the amount of
nitrate available in the soil, the amount of evapotranspiration
from each soil layer weighted for the distribution of the nitrate













within layers, and the amount of water available in the soil.
Vertical and lateral water flow through each soil layer, calculated
in the USDAHL-74 hydrologic model, is used in the ACTMO nitrate
model to calculate the nitrate movement through the soil profile
to the stream. These calculations are all made independently for
each zone of the watershed to provide the total watershed output.
The ACTMO model has been changed from a storm basis to operate
on a daily basis with daily input parameters being supplied from
the USDAHL-74 hydrologic model. The ACTMO model considers only
nitrate and does not simulate the amount or movement of any other
nitrogen forms, with the exception of the amount of soil organic
nitrogen remaining to be mineralized. Nitrate is assumed to move
only by subsurface flow, therefore none is allowed to move in
surface runoff. This assumes that all surface applied fertilizer
is dissolved and moves into the soil with the initial infiltration
before overland flow begins.

The ACTMO nitrate model was first cleared of errors and oper-
ated as an independent model on our Amdahl 470-V6 computer. It
was then converted to run on a daily basis instead of its original
storm basis. The next step was to interface it with the USDAHL-74
hydrologic model. This involved locating the appropriate para-
meters in the USDAHL-74 model and writing them in the correct
sequence on a magnetic tape during its operation. This tape was
then used to provide the input parameters to the ACTMO nitrate
model. The nitrate model requires some additional direct input
parameters related to the initial nitrogen status of the watershed
and fertilizer applied during the period of simulation.

Modeling nitrogen movement through a watershed is a very
difficult and complex problem. This model is only the first step
in the process of developing a model to satisfactorily simulate
movement of nitrate, ammonium, and soluble organic nitrogen forms
through agricultural watersheds. The model has not as yet been
tested, however this will be done very soon. This research is
being continued to include simulation of organic and ammonium
nitrate forms. Precipitation and organic matter decomposition
will be included as nitrogen sources. The concentration of soluble
organic nitrogen in surface runoff will be assumed to be a function
of seasonal variables, land use and cover. Denitrification will
be included as a nitrogen sink based on a first order rate equation.
The rate coefficient will be a function of organic nitrogen content
in the soil profile, temperature, and moisture content, as a
representation of aeration in the soil.














CONCLUSIONS AND RECOMMENDATIONS


Nitrogen and phosphorus loads were determined for two agri-
cultural watersheds: one primarily in native forest cover and
the other primarily in intensive crop production. Nitrogen and
phosphorus concentration differences between the two watersheds
were very minimal over the period of record. Nutrient concentra-
tion changes also were minimal between storm and low flow periods
in both watersheds. This leads to the conclusion that nutrient
loads were relatively proportional to streamflow volume. Nitrogen
and phosphorus losses in streamflow were minor compared with
amounts received by the watersheds in precipitation and commercial
fertilizer.

Small plots were used to evaluate the effects of selected
cultural and water management practices on nitrogen and phosphorus
loads in surface runoff from sandy soils. Again, on these small
plots nitrogen and phosphorus losses in surface runoff were small
compared with the contributions to the plots in rainfall and
commercial fertilizer. Since the magnitude of nutrient losses
from all treatments was small, it was difficult to determine
whether differences among treatments were the result of the treat-
ments. Because of the natural heterogeneity, even on this small
scale, there were relatively large differences in both runoff
amounts and nutrient concentrations between plots with the same
treatment. Therefore, conclusive differences among treatments
could not be determined.

Techniques were developed to simulate nitrogen movement
through agricultural watersheds. The USDAHL-74 model of watershed
hydrology has several advantages that make it a good choice to
provide the hydrologic information required to model nitrogen
movement. Calibration of the model to the research watersheds was
adequate, but not as good as expected. This was in part the result
of the poor quality of some of the rainfall input data during the
calibration period. The model should be modified in its subsurface
and return flow components to better simulate the conditions of
high lateral return flows with a shallow watertable, as encountered
in this study. The ACTMO nitrate model provides a good framework
for development of a more complete model of nitrogen transformations
and movement. Simulating nitrogen transformations and movement
through a watershed is a very difficult and complex problem. This
nitrate model needs to be modified to include other nitrogen forms.
Precipitation and organic matter decomposition are important








46



nitrogen sources and denitrification is an important sink. These
factors need to be included if the model is to simulate conditions
similar to those encountered in this study. Simulation models are
an effective tool to assist in gaining a better understanding of
the complex processes and interactions that occur in a watershed
system. They can help in identifying which processes are most
important in controlling nitrogen movement within a watershed.















LITERATURE CITED


Allen, H. E. and J. R. Kramer. 1972. Nutrients in natural waters.
John Wiley and Sons, New York.

American Public Health Association. 1971. Standard methods for
the examination of water and wastewater. 13th edition. APHA.
Washington, D.C.

Barber, S. A. 1962. A diffusion and mass flow concept of nutri-
ent availability. Soil Science 93:39-49.

Beek, J. and M. J. Frissel. 1973. Simulation of nitrogen
behaviour in soils. Center for Agricultural Publishing and
Documentation, Wageningen, Netherlands.

Carlile, B. L. and J. A. Phillips. 1976. Evaluation of soil
systems for land disposal of industrial and municipal effluents.
Report No. 118. Water Resources Research Institute of the
University of North Carolina, Raleigh.

Chow, V. T. 1959. Open-channel hydraulics. McGraw-Hill Book Co.
New York.

Donahue, R. L., R. W. Miller, and J. C. Shickluna. 1977. Soils:
an introduction to soils and plant growth. 4th edition. Prentice-
Hall, Inc., Englewood Cliffs, New Jersey.

Donigian, Jr., A. S., D. C. Beyerlein, H. H. Davis, Jr., and
N. H. Crawford. 1977. Agricultural runoff management model -
version II: refinement and testing. EPA-600/3-77-098.

Donigian, Jr., A. S., and N. H. Crawford. 1976. Modeling pesti-
cides and nutrients on agricultural lands. EPA-600/2-76-043.

Duffy, J. and M. Franklin. 1972. On mathematical models of
nitrification in soils. Summer Computer Simulation Conference
Proc. 2:997-1006.

Frere, M. H. 1975. Integrating chemical factors with water and
sediment transport from a watershed. Journal of Environmental
Quality 4:12-17.













Frere, M. H., C. A. Onstad, and H. N. Holtan. 1975. ACTMO, an
agricultural chemical transport model. U.S. Dept. of Agr.,
ARS-H-3.

Hagin, J. and A. Amberger. 1974. Contribution of fertilizers
and manures to the N- and P- load of waters A computer simula-
tion. Technion-Israel Institute of Technology, Haifa, Israel.

Hanson, C. L. 1977. Evaluation of the components of the
USDAHL-74 model of watershed hydrology. American Society of
Agricultural Engineers Paper No. 77-2533.

Holtan, H. N., G. J. Stiltner, W. H. Henson, and N. C. Lopez.
1975. USDAHL-74 revised model of watershed hydrology. USDA-ARS
Tech. Bul. No. 1518.

James, L. G., M. F. Walter, and R. E. Muck. 1977. Evaluation of
several levels of hydrologic models on small watersheds. American
Society of Agricultural Engineers Paper No. 77-2050.

Mehran, M. and K. K. Tanji. 1974. Computer modeling of nitrogen
transformations in soils. Journal of Environmental Quality
3:391-396.

Molnau, M. and K. H. Yoo. 1977. Application of runoff models to
a Palouse watershed. American Society of Agricultural Engineers
Paper No. 77-2048.

Nicks, A. D., G. A. Gander, and M. H. Frere. 1977. Evaluation of
the USDAHL hydrologic model on watersheds in the Southern Great
Plains. American Society of Agricultural Engineers Paper No.
77-2049.

Patrick, W. H., R. D. Delaune, R. M. Engler, and S. Gotoh. 1976.
Nitrate removal at the water-mud interface in wetlands.
EPA-600/3-76-042.

Perrier, E. R., J. Harris, and W. B. Ford, III. 1977. A compari-
son of deterministic mathematical watershed models. American
Society of Agricultural Engineers Paper No. 77-2047.

Porter, K. S. 1975. Nitrogen and phosphorus food production,
waste and the environment. Ann Arbor Science, Ann Arbor, Michigan.

Pritchett, W. L. and J. W. Gooding. 1975. Fertilizer recommenda-
tions for pines in the Southeastern Coastal Plain of the United
States. Univ. of Fla. Agr. Exp. Sta. Tech. Bul. 774.












Pritchett, W. L. and W. H. Smith. 1974.
savanna forest soils for pine production.
Sta. Tech. Bul. 762.


Management of wet
Univ. of Fla. Agr. Exp.


Rao, P. S. C., H. M. Selim, J. M. Davidson, and D. A. Graetz.
1976. Simulation of transformations, ion-exchange and transport
of selected nitrogen species in soils. Soil and Crop Science
Society of Florida 35:161-164.


Replogle, J. A., L. E. Myers, and K. J.
measurements with fluorescent tracers.
Engineers Proceedings 92(HY5):1-14.


Stewart, B. A. 1976.
Vol. II, an overview.


Brust. 1966. Flow
American Society of Civil


Control of water pollution from cropland:
U.S. Dept. of Agr., ARS-H-5-2.


Stewart, E. H., D. P. Powell, and L. C. Hammond. 1963.
characteristics of some representative soils of Florida.
Dept. of Agr., ARS 41-63.


Thompson, L. M. and F. R. Troeh. 1973.
3rd edition. McGraw-Hill Book Company.


Soils and soil fertility.
New York, N.Y.


U.S. Department of Agriculture, Soil Conservation Service. 1975.
Agricultural waste management field manual. U.S. Government
Printing Office, Washington, D.C.

U.S. Environmental Protection Agency. 1974. Methods for chemical
analysis of water and wastes. U.S. Government Printing Office,
Washington, D.C.

Wilson, J. F., Jr. 1968. Fluorometric procedures for dye tracing.
Techniques for Water-Resources Investigations of the United States
Geological Survey, Book 3, Chapter A12, U.S. Government Printing
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Moisture
U.S.




Full Text

PAGE 1

WATER IiRESOURCES researc center Publication No. 42 POLLUTION IN RUNOFF FROM NONPOINT SOURCES By K. L. Campbell (Principal Investigator) Agricultural Engineering Department University of Florida GainesviUe UNIVERSITY OF FLORIDA

PAGE 2

POLLUTION IN RUNOFF FROM NONPOINT SOURCES By K. L. Campbell (Principal Investigator) PUBLICATION NO. 42 FLORIDA WNIER RESOURCES RESEARCH CENTER RESEARCH PROJECT 'IECHNICAL COMPIETION REPORT OWRT Project Number B-023-FLA Matching Grant Agreement Number 14-31-0001-5064 Report Submitted: June 28, 1978 The work upon which this report is based was supported in part by funds provided by the United States Department of the Interior, Office of Water Research and Technology as authorized under the Water Resources Research Act of 1964 as amended.

PAGE 3

i ACKNOWLEDGEMENTS Appreciation is extended to the Office of Water Research and Technology, United States Department of the Interior, for financial support of this project, and to Dr. W. H. Morgan, Director of the Florida Water Resources Research Center, for his administrative assistance, and Ms. Mary Robinson for accounting assistance. Fi nancial support of the Center for Environmental Programs, Institute of Food and Agricultural Sciences, University of Florida also is gratefully acknowledged. The cooperation of Mr. Thomas Ma1phurs, Mr. Lamar Ma1phurs, and Mr. James Swick in allowing portions of this study to be con ducted on their land and in providing related land use information is gratefully acknowledged. The assistance of Mrs. York Ma1phurs in data collection also was greatly appreciated. Mr. Walt Auffenberg and Mr. John Ominski contributed invaluable field and laboratory assistance without which this project would have been impossible. The computer programming support of Mr. Bill Lancaster also was greatly appreciated.

PAGE 4

ABSTRACT .. INTRODUCTION CHAPTER TABLE OF CONTENTS I. WATERSHED STUDIES Methods and Procedures Results and Discussion Summary II. SMALL PLOT EXPERIMENTS Methods and Procedures Results and Discussion Summary ...... III. SIMULATION MODEL DEVELOPMENT. Modeling Approach ..... Hydrologic Model Calibration Nitrogen Model Development CONCLUSIONS AND RECOMMENDATIONS LITERATURE CITED . Page i 1 2 5 5 7 21 23 23 24 30 33 33 35 39 45 47

PAGE 5

ABSTRACT The increasing concern for the quality of our environment demonstrates the importance of gaining an increased understanding of the mechanisms and processes involved in nutrient movement from diffuse sources. The extent of the problem is not well defined in many areas. In this study, nitrogen and phosphorus loads were determined for two agricultural watersheds: one primarily in native forest cover and the other primarily in intensive crop pro duction. Small plots were used to evaluate the effects of selec ted cultural and water management practices on nitrogen and phos..., phorus loads in surface runoff from sandy soils. In both of these locations, nitrogen and phosphorus losses in runoff were very small compared with amounts received by the land area in precipitation and commercial fertilizer. Techniques were developed to simulate nitrogen movement through agricultural watersheds. Simulation models are an effective tool to assist in gaining a better understanding of the complex processes and interactions that occur in a watershed system. They can help in identifying which processes are most important in controlling nitrogen movement within a watershed.

PAGE 6

2 INTRODUCTION There has been increasing concern in recent years about the quality of the water in our lakes and streams. This quality is obviously influenced by the quality of water flowing into the lakes and streams from the surrounding land areas. However, because of the widely dispersed origin of this water as runoff from the contributing watershed areas, data concerning its effects on the quality of our bodies of water have been very sparse. Good background data indicating water quality under native water shed conditions for comparative purposes are also very sparse. The importance of minimizing nutrient losses from agricultural land is quickly recognized when one considers the high costs of commercial fertilizer. The development of management practices to reduce nutrient losses may reduce fertilizer costs and also result in improved quality of the water in lakes and streams. The extent and value of this improvement in water quality is at best very di ffi cult to quanti fy. Wi th the present concern for the environment and the quality of water in our lakes, streams and rivers, there is a need to determine the effects of our modern agricultural production technology on the nutrient loads in surface runoff. Because agricultural runoff is introduced to the streams from a non-point source, it is more difficult to determine the actual nutrient loads being introduced. The soils and climate of Florida create a unique situation for nutrient movement through watersheds. The sandy soils along with high rainfall and warm temperatures create a condition con ducive to rapid nutrient movement under some conditions. The tremendous variations in these sandy soils and their resulting effects on water movement add considerable confusion to any analysis of the potential for nutrient losses in runoff in various locations. In some locations the sandy soils are deep with corresponding water table depths of 9-12 m or more, while in nearby flatwoods areas, spodic layers and clay layers in the sandy soil may cause a shallow water table fluctuating from 2 m deep up to the ground surface. These variable soil conditions affect the volume and quality of surface runoff and the movement of nutrients into the ground water. The problem of pollution from non-point sources is a particular concern in Florida because of its many lakes and the close intermingling of surface water and ground water. There is also

PAGE 7

3 the important consideration that pollution from non-point sources in Florida may quickly find its way into the estuarine zones which are of great economic and aesthetic importance. With these facts in mind, this research effort was undertaken to determine nutrient loads in runoff from native areas and watersheds in agricultural production in north central Florida (Figure 1). The objectives of the study were (1) to determine the nitrogen and phosphorus loads in streamflow from agricultural water sheds with intensive cropping and with native vegetation, (2) to determine the effects of selected cultural and water management practices on nitrogen and phosphorus loads in agricultural runoff from sandy soils, and (3) to develop techniques to simulate nitrogen movement through watersheds.

PAGE 8

() RESEARCH SITES Figure 1. Map of Florida showing the location of the study area. 4

PAGE 9

CHAPTER I WATERSHED STUDIES Methods and Procedures 5 One watershed of about 437 ha (upper watershed) observed in this study is primarily in native forest cover with some unimproved pasture and a very small amount of crop land. The outlet of this watershed is a small stream which flows continuously except during extremely dry periods. Another watershed of about 208 ha located immediately downstream, (lower watershed) is primarily in inten sive agricultural crop production with some improved pasture near the stream. The soils in these watersheds are s.andy with a clay layer at a depth of 1-2 m in most areas creating a shallow water table during wet periods. Average land slopes are 0-3 percent in the upper watershed and 3-8 percent in the lower watershed. Predominant soil associations are Arredondo-Gainesville-Fort Meade, Leon-Plummer-Rutledge, and Scranton-Dna. Precipitation was measured by a small wedge-shaped gauge near the edge of the watersheds and a tipping bucket recording rain gauge near the center of the two watersheds. Samples were collec ted from the recording gauge to determine nutrient concentrations in rainfall. Stage recorders were installed on the stream at the outlet of each watershed to provide a continuous record of the stream level (Figure 2). Manning's velocity formula (Chow, 1959) was used to develop a stage-discharge relationship for use in determining flow rates and volumes and nutrient loads from the watersheds. Discharge measurements made on the stream by fluorometry techniques (Replogle et al 1966, Wilson 1968) and current meter measurements at a variety-of stages verified this relationship. At each watershed outlet an automatic water sampler collected streamflow samples every eight hours for later nitrogen and phos phorus analyses. During periods of low flow, these samples were composited into a single daily sample. Nitrogen forms measured were total Kjeldahl nitrogen (U.S. Environmental Protection Agency, 1974), ammonium nitrogen by the selective ion electrode (U.S. Environmental Protection Agency, 1974) and nitrate nitrogen by the chromotropic acid method (American Public Health Association, 1971). Phosphorus forms measured were total phosphorus and orthophosphate by the ascorbic acid method (American Public Health Association, 1971). Nutrient loads in the stream during per.iods of low flow

PAGE 10

o '\ / J f'. ........ /' I r"" j \. I / J" \ f ./' I ) I I I WATERSHED BOUNDARY STREAM CHANNEL FLOW MEASUREMENT AND WATER SAMPLER RECORDING RAIN GAGE 6 } } J Figure 2. of upper and lower watersheds studied ,in Alachua County, Florida.

PAGE 11

7 provided an indication of the movement of nutrients through the soil profile to the shallow ground water. All flow volumes and nutrient loads for the lower watershed were determined by subtracting the upper watershed measurements from those of the total watershed. Land owners in the watersheds were interviewed to determine cropping, livestock numbers, and fertilizer applied, for use as components of nutrient balances for the watersheds. Results and Discussion Data were collected during the period from July 1975 to June 1977. Precipitation from 7/75 -6/76 totalled 105 cm, about 25 cm below average. From 7/76 6/77 precipitation totalled only 88 em, about 42 cm below average. According to the landowners, flow levels in the stream were below normal as would be expected with this lower than average rainfall. Tables 1 and 2 show the nitrogen and phosphorus loads in the streamflow from the upper and lower watersheds. The upper watershed is primarily in native forest cover and the lower watershed is in intensive agricultural crop production, as indicated pre viously. The flow-weighted average nutrient concentrations, shown in Table 3, are very similar for both the upper and lower water sheds for most nutrient forms during the same year. However, the flow volume from the lower watershed is about four times greater than that from the upper watershed during 1975-76 and somewhat greater during 1976-77. Thus, it appears that most of the in creased nutrient load from the lower watershed, shown in Tables 1 and 2, can be attributed to the increased flow volume from that watershed. Flow volumes are reported as a uniform depth over the appropriate drainage area. In Tables 4 and 5 the nutrient loads from the two watersheds are broken down to show the loads of each nutrient form occurring during storm flow and during low flow periods. Storm flow and low flow volumes also are given for both watersheds. As mentioned earlier, the nutrients in the streamflow during low flow periods are contributed by baseflow from the surrounding shallow ground water, therefore this indicates the extent of movement of nutrients through the soi 1 profi 1 e into the groundwater. In the upper watershed, the flow volume and. nutrient loads are divided about equally between storm and low flow periods during 1975-76 while, in the lower watershed, about 80 percent of the flow volume and nutrient loads occurred during storm periods (Table 4). These

PAGE 12

Table 1. Nitrogen load (kg/ha) in streamflow from July 1975 to June 1976 and from July 1976 to June 1977. Organic N Ammonium N Nitrate N Total N Upper Watershed Lower Watershed 1975-76 1976-77 1975-76 1976-77 ----------------------kg/ha----------------------1.21 0.11 0.12 1.43 1049 0.07 0.09 1065 5.30 0.68 0.37 6.36 1. 92 0.09 0.09 2.10 Table 2. Phosphorus load (kg/ha) in streamflow from July 1975 to June 1976 and from July 1976 to June 1977. Orthophosphate P Total P Upper Watershed Lower Watershed 1975-76 1976-77 1975-76 1976-77 -------------------kg/ha------------------0.30 0.33 1.21 1.34 0.63 0.86 8

PAGE 13

9 Table 3. Flow-weighted average nutrient concentrations (mg/l) and flow volumes (cm) for the periods July 1975 to June 1976 and July 1976 to June 1977. UEEer Watershed Lower Watershed 1975-76 1976-77 1975-76 1976-77 Organic N 2.31 1. 70 2.49 1.59 Ammonium N 0.21 0.07 0.32 0.07 Nitrate N 0.22 0.10 0.17 0.07 Total N 2.73 1.87 2.98 1. 73 Orthophosphate P 0.58 0.59 0.57 0.52 Total P 0.63 0.77 0.63 0.71 Flow Vol ume, cm 5.25 8.79 21.3 12.1

PAGE 14

10 Table 4. Nutrient loads (kg/ha) and flow volumes (cm) by type of flow for each watershed from July 1975 to June 1976. Upper Watershed Lower Watershed Storm Flow Low Flow Storm Fl ow Low Flow --------------------kg/ha-------------------Organic N 0.61 0.60 4.30 1.00 Ammonium N 0.06 0.05 0.51 0.17 Nitrate N 0.07 0.05 0.30 0.07 Total N 0.73 0.70 5.12 1.24 Orthophosphate P 0013 0.17 0.94 0.27 Total P 0.15 0.18 1.04 0.30 Flow Volume, cm 2.57 2.67 17.4 3.95 Table 5. Nutrient loads (kg/ha) and flow volumes (cm) by type of flow for each watershed from July 1976 to June 1977. Upper Watershed Lower Watershed Storm Flow Low Flow Storm Flow Low Flow Organic N 0.59 0.90 1.23 0.69 Ammonium N 0003 0.04 0.04 0.05 Nitrate N 0.03 0.06 0.05 0.04 Total N 0.65 0.99 1. 31 0.78 Orthophosphate P 0.18 0.34 0.34 0.29 Total P 0.27 0.41 0043 0043 Flow Volume, cm 3.50 5.29 5.73 6.34

PAGE 15

11 data indicate that most of the increased flow volume and nutrient load from the lower watershed in 1975-76 occurred during storm periodso Two probable reasons for this are the more intensive land use with less ground cover and the somewhat greater land slopes in the lower watershedo This watershed is typical of this general farming area where much of the intensive agricultural production is on the more sloping land which has better drainageo Because of the very dry year of 1976-77, a much larger percentage of the total streamflow occurred as low flow, especially in the lower watershed. The nutrient loads still remained relatively proportional to flow volume between storm and low flow periods, with the exception of the organic nitrogen load in the lower watershed (Table 5)0 Flow-weighted average nutrient concentrations by type of flow for each watershed showed no definite pattern for nitrogen forms (Tables 6 and 7). The concentrations were very similar between storm flow and low flow within a watershed for a given year except the one case of organic nitrogen when the concentration was doubled during storm periods. However, phosphorus concentrations reduced during storm flow periods in both watersheds in 1975-76 (Table 6)0 This trend was consistent for individual storm flow periods as well as for the total year. This trend did not continue during 1976-77, however, except for orthophosphate in the upper watershed (Table 7)0 Figures 3, 4 and 5 show the variations in monthly flow weighted average nutrient concentrations and monthly flow volume throughout the period of record for the upper watershed. Similar relationships for the total watershed (upper and lower combined) are shown in Figures 6, 7 and 8. An examination of these data for possible correlations may be very helpful in understanding some of the processes taking place in the watersheds. This under standing is very important to the development of models for simulation of nutrient movement through watershedso Nitrate concentrations were lowest during the fall and winter months in the upper watershed (Figure 3) as might be expected due to cooler temperatures during this period. A similar response occurred in the total watershed initially, but nitrate concentrations remained very low throughout the period of record after the first summer (Figure 6). Ammonium concentrations in the upper watershed were relatively low during most of the period of record (Figure 3)0 This was also true for the total watershed except for some periods in the late spring and summer when concentrations were considerably higher (Figure 6). The increased ammonium concentrations may be due to increased ammonification as the temperature increaseso They also could be influenced by fertilizer applied to the

PAGE 16

12 Table 6. Flow-we;ghted averagenutr;ent concentrations (mg/l) by type of flow for each watershed from July 1975 to June 1976. Upper Watershed Lower Watershed Storm Flow Low Flow Storm Flow Low Flow --------------------mg/l--------------------Organi c N 2.37 2.25 2.47 2.53 AmmoniumN 0.22 0.19 0.30 0.43 Nitrate N 0.26 0.18 0.17 0.18 Total N 2.85 2062 2 095 3.13 Orthophosphate P 0.51 0.64 0.54 0.68 Total P 0.58 0.68 0.60 0.77 Table 7. Flow-weighted average nutrient concentrations (mg/l) by type of flow for each watershed from July 1976 to June 1977 Upper Watershed Lower Watershed Storm Flow Low Flow Storm Flow Low Flow --------------------mg/l--------------------Organic N 1.69 1.71 2.15 1.08 Ammonium N 0.08 0.07 0.06 0.08 Nitrate N 0.09 0.11 0.08 0.07 Total N 1086 1.88 2.29 1.23 Orthophosphate P 0.51 0.64 0058 0.46 Total P 0.77 0.77 0075 0.68

PAGE 17

i E Nitrate N i2 u Ammonium N Q) E :;, ..-0 > 4 ): 0 r-:" I.L. ..-......... 0'1 E 0.4 !:: 0 ..... +' ct! s... +' !:: Q) 002 u !:: 0 U I'. I "-... I '-......... ../ '... ... ... ....... ,_ ...... 0' '. J A SO N D J F M A M J J A SO N D J F M A 1975 1976 1977 Figure 3. Monthly flow-weighted average nitrate N andammoniumN concentrations and monthly flow volume from the upper watershed. 1-' W

PAGE 18

5 II I i 0 E u ? 4 t 0 > /\ i4 o 3 S-4J e OJ u e 0 U t:::r\ A. \ I 16 z u 'r-e ro 01 S-o 1 o I __ ____ -h ____ __ __ __ __ __ __ __ __ __ ________________ J A SON 0 J F M A. M J J A SON D J F M A 1975 1976 1977 Figure 4. Monthly flow-weighted average organic N concentration and monthly flow volume from the upper watershed. ....... ..j:::=.

PAGE 19

200, I .0 s 1.61-Orthophosphate P 12 u -----To ta 1 P .. OJ S :::5 ,..... ,..... 1.2L ......... 0) S t: oJ I 0 I .r-I ....., I n:l I ....., ... I t: ,,-I'K'" ,J -I 6 OJ I .. ,., U t: 0 U 4 J A SON 0 J F M A M J J A SON 0 J F M A 1975 1976 1977 Figure 5. flow-weighted average orthophosphate P and total P concentrations and monthly flow volume from the upper watershed. ....... U1

PAGE 20

-NitrateN ----Ammo n i urn N 0.6 ,\ ,I \ I \ r, \ ........ \ A I 0.4 1\ I \ c: I \ 1\ o I \ A 'r-I \ j' +-l I \ l \ I \ \ I \ / \ +-l \' I ,\ c: \ I \ I O. 2 \ .,.. .. \ / \ c: "', / \/ \ o ',' \, A \ u "', \ ,.... ,. ."...,.-" ... ,. ',1 --r ..... I f f I I I J A SON 0 J F M A J.J A SON 0 J F M A M J 1975 1976 1977 2 4 6 E u OJ E ::s r-0 :::-?; 0 r-4. Figure 6. Monthly flow-weighted average nitrateN and ammonium N concentrations and monthly flow vol ume f.rom the total watershed. I-' O"l

PAGE 21

5 i i 0 6.68 E 4 I I ._ u ..... L / \ I I J r0o / \ I ) e OJ u e 0 u 2r\ I \ A I ..,6 z: u "r-e ro O'l s-o __ __ __ __ __ __ ______ J A SON 0 J F M A.M J J A SON 0 J F M A 1975 1976 1977 Figure 7. Monthly flow-weighted average organic N concentration and monthly flow volume from the total watershed. ....... -....J

PAGE 22

20 a I I I I I I I I I ,fa I I ( I I E 1.6}-I -Orthophosphate P I -;2 u -----Total P I Q) I E I ::s I I 0 > E12 I 43: 0 t, I o I \ \ f I ", I \ I s, \ f J r"'''' \' ., OJ O. 8 r-"v.\ ,,/, \ ,/ / -I 6 U I \ I' \ / ... '" I \ I' .. o .... ..... .. \ I \ /' ", -----...-""..\ \ / u ... I \ 01 I I I I I I I I I I I I I I I I I I I I I I J A SON D J F M A ,M J JA SON 0 J -.. -.. -1975 1976 1977 Figure 8. Monthly flow-weighted average orthophosphate P and total P concentrations and monthly flow volume from the total watershed. J-l co

PAGE 23

watershed since most of it is applied in March and April and in the lnwer watershed. Figures 4 and 7 show several drops in organic nitrogen concentration that correspond to the increased ammonium concentrations as might be expected if ammonification was increasing during these months. These changes may be con nected to the increase in microbial activity as warmer weather begins. The bulk of the total phosphorus load in the stream is in 19 the orthophosphate form as is evident in Figures 5 and 8. Phos phorus concentrations tended to decrease in months with larger flow volumes and increase in months with small flow volumes during much of the period of record (Figures 5 and 8). This is consistent with the observation made earlier about phosphorus concentrations decreasing during individual storm flow periods. This trend indicates that more than a flow-proportionate amount of the phos phorus load from the watersheds is delivered in the shallow groundwater during low flow periods and storm flow has a partial di 1 ut i on effect on thi s phosphorus load. The soil sin these watersheds are naturally rather high in phosphorus content, therefore these results were not entirely unexpected. Components of nutrient balances were calculated for both watersheds. Average values of nitrogen and phosphorus content in the harvested crops were obtai ned from the 1 iterature (Carl il e and Phillips 1976, Pritchett and Gooding 1975, Pritchett and Smith 1974, Thompson and Troeh 1973, USDA Soil Conservation Service 1975). In addition to the nutrient inputs listed in Tables 8 and 9, natural mineralization processes in the soil and plant residues provide some nitrogen and phosphorus. Nitrogen (N2) fixation also provides some available nitrogen. No attempt was made to estimate the amounts provided by these processes. On the output side, leaching of nutrients to the deep groundwater should be minimal because of the relatively impermeable clay layers which underly this area. Water balance estimates for this period also support this statement. Denitrification, immobiliz ation and phosphorus fixation are additional sinks that may account for much of the difference between inputs and outputs in Tables 8 and 9. While the nutrient balances in Tables 8 and 9 are not complete, they do serve to show the relative magnitudes of some of the individual components of the balances. For example, nutrient losses in streamflow were considerably smaller than the amount of nutrients added to the watersheds in precipitation during this period. An exception to this was phosphorus losses in 1976-77 when precipitation input was very low and shallow groundwater flow was greater than in 1975-76, resulting in phosphorus losses greater than precipitation input. Nutrient losses in streamflow

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Table 8. Components of the nitrogen balance for each watershed for the periods July 1975 to June 1976 and July 1976 to June 1977. Upper Watershed Lower Watershed 1975-76 1976-77 1975-76 1976-77 -------------------kg/ha-------------------Fertilizer 21.0 33.3 96.9 142.7 Animal Waste 5.7 7.2 24.2 Precipitation 18.9 16.7 18.9 16.7 Harvested Crops 19.9 19.2 93.6 55.1 Streamflow 1.43 1.65 6.36 2.10 20 Table 9. Components of the phosphorus balance for each watershed for the periods July 1975 to June 1976 and July 1976 to June 1977. Upper Watershed Lower Watershed 1975-76 1976-77 1975-76 1976-77 -------------------kg/ha-------------------Ferti 1 i zer 6.4 3.1 33.2 32.7 Animal Waste 1.5 1.9 6.5 Precipitation 1.57 0044 1.57 0.44 Harvested Crops 3.4 2.0 19.1 8.1 Streamflow 0.33 0.68 1.34 0.86

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21 amounted to only a small percentage of the nutrients applied in commercial fertilizer. The 1976-77 crop season was extremely dry resulting in a total loss for many crops. This accounts for the decrease in nutrients removed in harvested crops from 1975-76 to 1976-77 Summary Two agricultural watersheds were instrumented to determine water quantity and quality measurements. The upper watershed of 437 ha was primarily in forest cover with some pasture and a small amount of row crop. The lower watershed of 208 ha was mostly in intensive agricultural crop production with some pasture. The following results are from data collected during the two-year period: 1. Nitrogen and phosphorus loads in streamflow were approxima tely proporti ona 1 to the flow volume in the two wa tersheds during each year. 2. The average total nitrogen concentration was about 10 percent greater in the lower watershed the first year, and about 10 percent smaller the second year. The average total phosphorus concentrations were the same in both watersheds the first year, and about 10 percent smaller in the lower watershed the second year. 3. The larger flow volume per hectare in the lower watershed as compared to the upper watershed occurred mostly during storm periods and was probably primarily due to land use and topography differences. 4. Average concentration changes between storm and low flow periods were small in both watersheds. About 50 percent of the flow volume and nutrient load from the upper watershed occurred during storm flow periods, while about 80 percent of the flow volume and nutrient load from the lower watershed occurred during storm flow periods in 1975-76. During 1976-77 the fractions were about 40 and 50 percent, respectively. 5. Components of nutrient balances indicate that nutrient losses in streamflow are a very small part of the total nutrient flow system in these two watersheds. Total nitrogen and phosphorus losses in streamflow were

PAGE 26

22 equivalent to about five percent of the commercial fertilizer applied in each watershed. Nutrient loads in streamflow also were less than those contributed to the watersheds in precipitation during the period with one exception.

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23 CHAPTER II SMALL PLOT EXPERIMENTS Sma 11 plot experiments were conducted duri ng three crop years to evaluate the effects of cultural practices, fertilizer applica tion methods, and watermanagement on nutrient loads in surface runoff from sandy soils. These practices were evaluated on plots producing bell peppers and tomatoes. Practices evaluated the first two seasons on bell peppers (1975 and 1976) included (1) plastic mulch over the plant bed with all fertilizer applied beneath the plastic at planting time, (2) no mulch over the plant bed with all fertilizer applied at planting time, and (3) no mulch over the plant bed with fertilizer applied in three equal applications during the growing season. All treatments received sprinkler irrigation. Practices evaluated the last season (1977) were (1) drip irrigation under plastic mulch with fertilizer applied through the irrigation system at weekly intervals on both tomatoes and bell peppers, and (2) drip irrigation under plastic mulch with all fertilizer banded along the plant row at planting time on bell peppers. Methods and Procedures The small plot experiments were conducted at the University of Florida Horticultural Unit near Gainesville. The experiments for 1975 and 1976 used six plots (5.5m by 15m) containing three beds of bell peppers each fertilized at a rate of 224 kg N/ha from ammonium sulfate, 84 kg P/ha from superphosphate, and 140 kg K/ha from potassium chloride. There were two replicates of each of the three treatments enumerated above. Beds were formed by a specially designed rototiller. Fertilizer was applied to the raised beds and the beds were again rototi11ed to mix the fef1tilizer. Plastic mulch was then applied to the appropriate plots before peppers were transplanted from greenhouse beds. Sprinkler irrigation was applied as required throughout the season to provide adequate moisture. Critical depth flumes and water stage recorders were installed at the end of each plot to measure surface runoff. Automati c sarnp 1 ers were placed at each fl ume to collect a flow pro portional composite water sample from each runoff event.

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24 Water samples were analyzed for organic N by the microkjeldahl method (U.S. Environmental Protection Agency, 1974), ammonium N by the selective ion electrode method (U.S. Environmental Protec tion Agency, 1974), nitrate N by the chromotropic acid method (American Public Health Association, 1971), orthophosphate P by the ascorbic acid method (American Public Health Assoication, 1971), and total P by the ascorbic acid method after persulfate digestion (American Public Health Association, 1971). Sediment concentrations in most samples were very low because of the sandy soils and small slopes ( < 1 percent), therefore all analyses were run on the unfi 1 tered sample .. Nutrient loads from the plots were calculated by a specially written flow and nutrient analysis computer program. The program provided runoff volumes, nutrient loads, and average nutrient concentrations on a storm event, monthly, and seasonal basis. This analysis program also was used in the watershed studies of Chapter I. During 1977 the experiment used six plots. All plots were drip irrigated under plastic mulch and received 40 kg N/ha, 120 kg P/ha, and 40 kg K/ha broadcast at planting time. In addi tion, the tomato treatment and one bell pepper treatment received weekly applications of Nand K fertilizer through the irrigation system to total 135 kg N/ha and 168 kg K/ha by drip irrigation. The other bell pepper treatment received an additional 135 kg N/ha and 168 kg K/ha banded along the plant row at planting time with none applied in the irrigation. Each of the above treatments had two replicates. The methods and procedures used during this last season were the same as the first two seasons except that sprinkler irrigation was not used and all beds received plastic mulch. These experiments were superimposed upon larger, more comprehensive studies being conducted simultaneously in cooperation with other researchers. Some of their findings (to be available soon) may contribute to an understanding of the results presented here. (D. A. Graetz, personal communication. Soil Science Department, University of Florida, Gainesville. 1978). Results and Discussion Treatments were randomly assigned to plot locations within the experimental area each year. All resul ts presented are averaged over the two replicates of each treatment. During 1975,

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25 surface runoff amounts were 6.32 cm from the plastic mulch, 5.49 cm from the no mulch, and 3.84 cm from the split fertilizer treatment. Difficulties with the automatic water samplers pre vented obtaining enough samples to calculate nutrient loads for the entire season. Grab samples were collected from a runoff event in May that produced about one-fourth of the total seasonal runoff from most plots. Nutrient concentrations in these samples were very similar to the average concentrations measured during 1976 (Table 11). In 1975 there was less leaching of fertilizer nitrogen in plots with plastic mulch (D. A. Graetz, personal communication). Split fertilizer application also resulted in more efficient use of nitrogen than, in the unmulched single application. Near the end of the crop season there did not appear to be adequate nitrogen for good plant growth in either of the unmulched treatments, while the mulched treatment was adequate. Surface runoff volumes and nutrient loads from the three treatments during 1976 are shown in Table 10. Somewhat greater runoff was expected from the mulched plots because the mulch prevented infiltration on the plant bed to a great extent. Differences in surface runoff volume between the treatments were not consistent during the two years. This inconsistency probably resulted from the great amount of variation between individual plots even with the same treatment. Precipitation during the period of record was 81 cm and 77 cm for 1975 and 1976, respectively. Runoff volumes were relatively low, as expected. Nutri ent loads from all treatments also were relatively low, especially when compared with the amount of fertilizer applied. Nutrient loads in runoff from the plastic mulch and no mulch treatments during 1976 were similar for all nitrogen and phosphorus forms (Table 10). However, nutrient loads were considerably higher from the split fertilizer treatment for all forms. This was partially because of a greater runoff volume from this treatment. Table 11 shows, however, that concentrations also were somewhat greater from the split fertilizer treatment. This was especially true for the ammonium N and nitrate N forms. The timing of ferti 1 i zer app 1 i cati ons re 1 ati ve to rai nfa 11 was a very important factor in causing this treatment difference. When runoff occurs soon after fertil i zer is appl i ed, as happened duri ng thi s exper iment with the split applications, the fertilizer is subject to washoff in the runoff water. Therefore, while the split applications resulted in more efficient use of nitrogen, runoff losses were increased in this particular year because of the timing problem. Runoff losses were still very minimal, however. Fertilizer leaching losses from the unmulched treatment in 1976 were much greater than from the mulched and split fertilizer

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26 Table 100 Nutrient loads (kg/ha) and surface runoff volume (cm) from bell peppers with plastic mulch, no mulch, and no mulch with split fertilizer applications during 19760 Plastic No Split Mulch Mulch Fertilizer ----------------kg/ha-------------------Organic N 0.93 0.94 1.35 Ammonium N 0010 0.06 0.19 Nitrate N 0012 0.12 0.72 Total N 1.15 1.12 2.26 Orthophosphate P 0.13 0.21 0.37 Total P 0018 0028 0.46 Runoff Volume, cm 4.78 5.31 7.49 Table 11. Flow-weighted nutrient concentrations (mg/1) in surface runoff from bell peppers with plastic mulch, no mulch, and no mulch with split fertilizer applications during 1976. Plastic No Split Mulch Mulch Fertilizer Organic N 1093 2.06 1.78 Ammonium N 0.17 0.12 0.26 Nitrate N 0036 0022 0.94 Total N 2046 2.40 2.98 Orthophosphate P 0.28 0040 0048 Total P 0.38 0.56 0.61

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27 application treatments (D. A. personal communication). This effect was more prominent than during 1975 as a result of heavy rainfall occurring early in the growing seasono Fruit yields were much lower from the unmulched treatment as a result of these leaching losses. Leaching losses and fruit yields were very similar for the mulched and the split fertilizer treatments (D. A. Graetz, personal communication). In 1977 both tomatoes and bell peppers were grown on the experimental plotso All plots with bell peppers were planted to sweet corn as soon as the peppers were finished producing fruit; while the tomato plots were left idle after the crop was finishedo Therefore, the direct comparison of the two fertilizer application methods was limited to the immediate growing season from April to July, 19770 Precipitation during this period totalled 30 cm. This is normally a rather dry time of year in north central Florida, resulting in the relatively low surface runoff volumes shown in Table 12. Because of the very small amount of runoff, most of the nutrient loads are correspondingly small. More variation among treatments can be observed from the nutrient concentrations in runoff water shown in Table 130 Because of the relativelylarge amount of variability between plots of the same treatment, it is difficult to attribute the differences in nutrient loads and concentrations for this short period to actual treatment effects. Information on leaching losses is not yet available from these treatments (D. A. personal communication) 0 On July 29, 1977 all plots with bell peppers were planted to sweet corn. These plots received 42 kg N/ha, 56 kg P/ha, and 56 kg K/ha at planting. The plots were sidedressed with 40 kg N/ha from ammonium nitrate on September 1 and again on September 13. The tomato plots were left idle after production finished as indicated previously and received no more fertilizer. Nutrient loads and runoff volumes for the complete season from April, 1977 to January, 1978 are shown in Table 14 for all treatments. Precip itation during this period was 83 cm. Nutrient loads from drip fertilized tomatoes and drip fertilized peppers followed by sweet corn were similar. Additional nutrient losses from the production of the sweet corn crop were not observed. The slightly larger nutrient loads from the pepper and sweet corn treatment were more than accounted for by the somewhat larger runoff volume from that treatment. It follows that most of the nutrient concentrations in runoff water (Table 15) were smaller for the drip fertilized peppers and sweet corn than for the tomatoes. The primary nutrient forms, if any, expected to be affected by fertilizer applied for crop production are ammonium Nand nitrate N. The band fertilized peppers followed by sweet corn had

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Table 12. Nutrient loads (kg/ha) and surface runoff volume (cm) from drip fertilized tomatoes, drip fertilized bell peppers, and band fertilized bell'peppers during the growing season April to July, 1977. 28 Drip Fertilized Drip Fertilized Band Fertilized Tomatoes Peppers Peppers ----------------------kg/ha---------------------Organic N 0.10 0.36 0.59 Ammonium N 0.04 0.04 0.02 Nitrate N 0.03 0.04 0.08 Total N 0017 0.44 0.69 Orthophosphate P 0.02 0.01 Total P 0.04 0.01 Runoff Volume, cm 0.17 0040 0.26 *less than 00005 Table 13. Flow-weighted nutrient concentrations (mg/l) in surface runoff from drip fertilized tomatoes, drip fertilized be 11 peppers, and band ferti 1 i zed bell peppers duri ng the growing season April to July, 1977. Drip Fertilized Drip Fertilized Band Fertilized Tomatoes Peppers Peppers Organic N 2.28 3.86 6.68 Ammonium N 0.85 0.39 0.40 Nitrate N 0080 0.45 1.14 Total N 3.93 4.70 8.22 Orthophosphate P 0.04 0.19 0.07 Total P 0.08 0.34 0012

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Table 140 Nutrient loads (kg/ha) and surface runoff volume (cm) from drip fertilized tomatoes, drip fertilized bell peppers, and band fertilized bell peppers during the period April, 1977 to January, 1978. 29 Drip Fertilized Drip Fertill"zed Band Fertilized Tomatoes Peppers Peppers 1 Organic N 1.17 1.21 1.86 Ammonium N 0010 0.13 1.48 Nitrate N 0010 0.15 0.36 Total N 1.37 1.49 3.70 Orthophosphate P 0.24 0.26 0.36 Total P 0043 0.55 0.46 Runoff Volume, cm 3.86 5.21 6.16 lSweet corn followed peppers on July 29, 1977 Table 15. Flow-weighted nutrient concentrations (mg/l) in surface runoff from drip fertilized tomatoes, drip fertilized bell peppers, and bandferti 1 i zed bell peppers during the period April, 1977 to January, 1978. Drip Fertilized Drip FertilIzed Band Fertill"zed Tomatoes Peppers Peppers -----------------------mg/l---------------------Organic N 3.22 2012 2040 Arrmonium N 0.25 0.24 2.06 Nitrate N 0027 0.27 0.50 Total N 3.74 2.63 4.96 Orthophosphate P 0064 0.56 0.60 Total P 1.13 1019 0.78 lSweet corn followed peppers on July 29, 1977

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30 increased losses of both these nitrogen forms compared \iii th the other treatments (Table 14). The increased losses were greater than that accounted for by the s 1 i ghtly 1 arger runoff volume from this treatment. This is reflected by the higher ammonium Nand nitrate N concentrations in Table 15. These increased losses occurred primarily during August, which was a relatively wet month (21 cm of rain). Most of the increased ammonium N loss occurred from only one of the two plots with this treatment. As referred to earlier, the relatively large variability between plots in a given treatment makes it difficult to determine whether it is a real treatment difference or a result of the natural heterogeneity in the many factors which interact to affect the overall nutrient losses. Since it was not feasible to have enough plots for a valid statistical analysis this question cannot be answered from this experiment. The increased nitrate N loss, however, was relatively uniform in both plots. This gives more indication of a real treatment difference. The most important result of these 1977 plot studies was that the total nutrient loads in runoff from all treatments, even with a double crop on some treatments, were very small compared to the fertilizer applied and the natural contributions from rainfall (Table 16). One exception to this was that the total phosphorus loads in runoff were nearly the same as the contribution in rainfall. Table 16 indicates that these relationships also were true during 1976. Summary The small plot experiments evaluated the effects of management practices including use of mulch, fertilizer application methods and timing, and double cropping on nitrogen and phosphorus losses in surface runoff. In all but one case, nitrogen and phos phorus losses in surface runoff were less than one percent of the amount applied in fertilizer. In all treatments total nitrogen losses in surface runoff were less than 25 percent of the contribution of rainfall. Total phosphorus losses in surface runoff were less than or equal to the contribution in rainfall except in one caSe when the runoff loss was about 30 percent greater than the contribution in rainfall. Phosphorus contributions in rainfall, however, were very low. Ammonium N and nitrate N losses in runoff were greater from the split fertilizer application than from a single application because of the timing of runoff events which happened to occur soon after the split applications were made. No changes in nutrient losses by runoff were observed from use of plastic mulch.

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Table 160 Nitrogen and phosphorus contributions from fertilizer and rainfall compared with the largest surface runoff losses from any.treatment during a given year. Fertilizer Rainfall Surface Runoff 31 Applied Contribution Losses (Largest) 1976 Peppers Total N Total P 1977 Tomatoes Total N Total P 1977 Peppers and Corn Total N Total P ------------------kg/ha-------------------------224 84 175 120 297 176 15.8 0.46 14.5 0.42 14.5 0.42 2.26 0.46 1.37 0043 3.70 0.55

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32 No increased nutrient losses in runoff were measured from double cropping which had extra fertilizer except for the treatment where all of the first crop (peppers) fertilizer was applied at planting time. In this case, ammonium N and nitrate N losses were greater with double cropping, however there was a large variation between plots with the same treatment so this may not have been a real treatment effect.

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CHAPTER III SIMULATION MODEL DEVELOPMENT Modeling Approach 33 Movement of nitrogen through agricultural watersheds involves many complex processes and interactions within the watershed. These processes are illustrated in Figure 9, adapted from Stewart (1976). In order to s imul ate nitrogen movement through a water shed into streamflow the potential amount of nitrogen available for transport must be known. This nitrogen can come from many sources including precipitation, fertilizer, animal wastes, and soil organic nitrogen reserves. Movement of thi s nitrogen in water through the watershed depends upon its form. Therefore, transformation processes and rates must be simulated. These are dependent upon watershed conditions including soil temperature, moisture content, soil type, pH, aeration, agricultural practices, and organic matter content (Porter 1975, Duffy and Franklin 1972, Hagin and Amberger 1974, Mehran and Tanji 1974). There are numerous sinks for nitrogen within the watershed including uptake by crops, immobilization of nitrate, and denitrification. These sinks reduce the amount of nitrogen available for movement from the watershed in streamflow. Transport of nitrogen through a watershed also depends very heavily upon the hydrology of the watershed. Therefore, a good hydrologic simulation of the watershed is very important for the simulation of nitrogen movement. The hydrologic model should simulate the quantity of water moving through the watershed, its rate and direction for both overland and subsurface flow. This requires a comprehensive deterministic hydrologic model. It is particularly important that the hydrologic model predict the quantity of runoff and subsurface flow that results from different land-use areas and surface covers within the watershed. The modeling approach used in this study was (1) to select a hydrologic model, meet.ing the above criteria, that was already developed and had been tested in a number of areas, (2) develop a model to simulate the nitrogen sources, sinks, and transformation processes within a watershed, and (3) couple the above two models together to obtain a simulation of the water and nitrogen movement through a watershed.

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" m ...... r ...... t:::j S2 ...... ;a ...... ...... E ...... o z to ...... "0 ;<5 oS; zr HARVEST PRODUCTS AIR DUST N / '<: .,-_ .-_--, \ I m r \ ;0 U'> I fvVv10B I LI ZA T ION LEACHING SO I L ORGAN I C I AMvDN I FI CATION .. I i I"'" MATTER ..... L...____ IfviVOBILIZATION Q. Fi gure 9. The ni trogen cycl e in agri culture. I 1"-., CLAY AND ORGANIC COMPLEXES w .j:::o

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35 Hydrologic Model Calibration The USDAHL-74 revised model of watershed hydrology (Holtan et al, 1975) was chosen as the hydrologic model for this study. This model is a deterministic, semi-empirical lumped hydrologic model. It was developed by the USDA Hydrograph Laboratory to be used as a practical tool for predicting runoff and infiltration in relatively small watersheds under natural rainfall conditions. The model utilizes some well known mathematical descriptions of the major hydrologic processes within a watershed. The model is written in Fortran IV computer language. Input requirements are relatively large. About 72 different parameters are required as input in addition to the historical climatic data. Computation time requirements are relatively low since no numerical solutions are involved in the computation processes. The model has been evaluated in a number of locations around the United States in the past few years (Nicks et al 1977, Hanson 1977, Crow 1977, Perrier et al 1977, Molnau and Yoo 1977, James et al 1977). It divides the watershed into hydrologic response zones based upon soil and watershed conditions. It also provides for response differences due to land use. Daily moisture status in the soil profile, soil water movement in both the vertical and horizontal directions, and other pertinent variables are readily available for use in a nutrient movement model. The USDAHL model is designed with each major hydrologic process in a separate subroutine. This makes understanding, modification, and improvement of the model easier. These are all advantages of this model for use with a nutrient transport model. Because of its advantages the USDAHL-74 model was chosen to serve as the hydrologic part of a larger model to simulate nitrogen movement through an agricultural watershed. The hydrologic model was calibrated using data from the agricultural watersheds described in Chapter I. The USDAHL model requires four types of input parameters in addition:: to climatic data: watershed, soils, land use, and hydraulic (Holtan et al, 1975). Tables 17, 18, and 19 list the input parameter values used in the model to obtain the best simulation of streamflow for the calibration period of January to June, 1976. This combination of input parameters was selected after considerable trial and error selection of values for certain parameters as explained in the following discussion. The watershed was divided into three zones based upon hydrologic response. Zone 1 is the very flat upper part of the watershed, zone 2 is the hillside portion with more slope, and zone 3 is the alluvium portion of the watershed along the stream channel. The watershed parameters and some land use

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36 Table 17. Watershed input parameters for USDAHL-74 model. WATERSHED PARAMETERS Size: 645ha Number of Zones: 3 Number of Crops: 45 Deep Groundwater Recharge: 1.27 mm/hr ZDNE PARAMETERS Final Watershed 'Infiltration Topsoil Total Soil Depth, cm Zone Area, Length, percent m 1 75 274 2 15 30.5 3 10. 122 Slope, Capacity, percent mm/hr 0..4 1.27 2.0. 7.62 0..8 1.27 Depth, cm 38 38 64 127 127 127 Table 18. Soil input parameters for USDAHL-74 model. TDPSDIL Total Field Hilting Antecedent Zone Porosity, Capacity, Point, Soil Water, Cracking, percent percent percent percent percent 1 35 20. 7 20. 0. 2 35 20. 7 20. 0. 3 56 40. 15 40. 0. SDIL PRDFILE BELDW TDPSOIL Total Field Wilti ng Antec.edent Zone Porosity, Capacity, Point, Soil Water, Cracking, percent percent percent percent percent 1 32 20. 10. 3D 0. 2 32 15 4 3D 0. 3 45 3D 17 45 0.

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37 Table 19. Routing and land use input parameters for USDAHL-74 model. ROUTING PARAMETERS Number of Routing Coeffici.ents: 3 Channel Routing, At: 0 .. 2 hr Channel Coefficient: 1.0 hr Initial Channel Flow: 0.0025 mm/hr Subsurface Routing: Regime Q-max, Coefficient, Cascading: lone -1 2 3 LAND USE PARAMETERS Crop A Value Crop Vd, mm ET/EP Root Depth, cm Upper Temp., C Lower Temp., C Zone Area, 70: 1 2 3 mm/hr hr 1 0.13 22.D 2 0.05 90.0 To Next Zone, percent 80 90 Rest Goes To All uvi urn Channel Channel Row Crop Small Grain Forest 0.20 0.30 1.00 1.27 2.54 2.54 1.60 1.4 2.0 51 51 254 30 27 27 7 4 4. 25 15 45 40 10 40 0 0 80 Grass 0.30 2.54 1.4 76 27 4 15 10 20

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38 parameters were determined from U.S. Geological Survey quadrangle maps, judgements from direct observations of the watershed, and aerial photos of the watershed. The remaining land use parameters and the routing parameters were determined using the procedures described in Holtan et al (1975). Soil input parameters were determined from data on Florida soils in Stewart et al (1963). The value for deep groundwater recharge was initially determined as 0.02 mm/hr by estimating average annual precipitation, ET, and streamflow yield in the area as suggested by Holtan et al (1975). This resulted in excessive streamflow and essentially no deep recharge occurring during the six month calibration period. Free water must be present in the bottom layer of the soil profile for deep recharge to occur in the model. This condition was present in the model for only a very short time. For this reason, the deep groundwater recharge value was increased to 1.27 mm/hr, equal to the final infiltration capacity of the soil. This change resulted in a better simulation of streamflow volume and 11.3 mm of de.ep recharge during the calibration period. This amount was still relatively low for deep recharge, however, precipitation was below normal during this calibration period. Weekly average pan evaporation and air temperature data were obtained from the nearest observation station a few miles away from the watershed. Rainfall was measured with a recording gauge and a small wedge-shaped gauge on the watershed. Break-point rainfall data from the recording gauge for model input were poor during portions of the calibration period because of instrument malfunctions. Estimated data were used in these cases. In this region, soil and watershed conditions are such that much of the runoff occurs by shallow lateral return flow. Over land flow also occurs during and immediately after heavy rainfall peri ods. Unde.r tbe.se soi 1 and watershed condi ti ons, the s ubsurface flow components of a hydrologic model become very important to a good streamflow simulation. Considerable difficulties were encountered during calibration in obtaining a good simulation of the storm hydrograph shape and timing. A large portion of the problem was in the empirical equation used in the USDAHL model to determine the recession curve coefficients and maximum subsurface flow rates for each zone. In the model these are a function of the watershed length, watershed slope, watershed area, final infiltration capacity, and free water capacity. This approach does not appear to adequately represent the subsurface flow characteristics under the conditions encountered in this study. Holtan et al (1975) also suggest this as an area for further research.

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39 In cal ibration of the USDAHL model,. the output parameters of primary interest were the total flow vol ume, the monthly flow volume, and the daily flow volume and streamflow hydrograph for a selected period. Observed and simulated monthly flow volumes from the watershed for the calibration period are shown in Table 20 along with monthly rainfall. Simulated runoff volume for the total period compared very well with the observed runoff volume. The monthly distribution was less accurate, however. Runoff volume was underestimated in the lower rainfall months and overestimated in the one month of higher rainfall. Table 21 gives a more detailed look at a portion of the wet month of May. Most of the runoff occurred during a six-day period that was preceded by re-1 ati ve ly dry conditi ons. Runoff vol ume for the rainy peri od was overestimated because the model generated sustained high flows during most of the period (Figure 10). The time of the simulated peak discharge was delayed, but was very close to the magnitude of the observed peak discharge. Subsurface flow was not adequately simulated to provide appropriate recession characteristics on the discharge hydrograph. This resulted in an overestimated flow volume for. the period. Detailed study of the model output data indicated that the model generated excessive lateral flows through the top soil layer to the stream during storms before downward percolation filled the lower soil layers with moisture. This contributed to the poor simulation of the hydrograph recession In summary, calibration of the USDAHL-74 hydrologic model to the research watershed resulted in an acceptable simulation of total water yield for the period. Simulation of daily and monthly flows was not as good as desired. Components of the model needing modification to improve the hydrologic simulation of this watershed were indentified. Nitrogen Model Development The goal in developing the nitrogen model was to simulate the nitrogen concentrations and loads in streamflow from a watershed. The first requirement was to adequately account for the sources of nitrogen in the watershed. Precipitation is a significant source of nitrogen and needs to be accounted for as an input of nitrogen to the watershed (see Tables 8 and 16). Nitrogen concentrations in rainfall are highly variable with both time of year and location, making it difficult to use average values for input to a model (Allen and Kramer, 1972). Fertilizer and animal wastes appl ied to a watershed are other sources of nitrogen that need to be

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40 Table 20. Observed and simulated monthly flow volume and observed rainfall for the research watershed during the calibra-tion period in 1976. Observed Simulated Month Rainfall, Runoff, Runoff, Error, mm mm mm percent January 45.0 8.51 8.00 -6.0 February 42.9 7.84 3.89 -50.4 March 39.9 1.84 0.00 -100.0 April 24.9 0.17 0.00 -100.0 May 191.8 4.63 8.84 90.9 June 37.1 0.51 0.00 -100.0 Total 381.6 23.50 20.73 -11.8 Table 21. Observed and simulated daily flow vtilume and observed rainfall for the research watershed from May 23 to May 28, 1976. Observed Simulated Day Rainfall, Runoff, Runoff, Error, mm mm mm percent 23 69.1 1.03 0.9] -11. 6 24 8.4 0.80 2.97 271.2 25 15.5 0.89 2.24 151.7 26 0.0 0.52 2.13 309.6 27 7.9 0.30 0.51 70.0 28 4.1 0.18 0.05 -72.2 Total 105.0 3.72 8.81 136.8

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til ......... (V) E ClJ O"l Sottl ..c: U til '.-Cl 0.25 0.20 0.15 0.10 .05 o o I I { I I i I I I I I I I I I /.'\ / \ I \ I \ \ \ I \ / \ r --"'-.. __ I \ I '_, \ I \ I \ I \ \ 1 \ Observed Simulated \ \ \ \ \ v \ 20 40 60 80 Time, hours \ \ \ \ \ 100 \ 120 140 Figure 10. Observed and simulated discharge hydrographs for the period May 23-28, 1976. I-'

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42 accounted for by a model. Organic decomposition of crop residues, leaf litter, and organic matter accumulations on the watershed surface may contribute nitrogen. The effect of this nitrogen source is likely to be a function of season, moisture, and temperature. In particular, at the end of the growing season when temperatures are still relatively high there may be large influxes of organics and rapid decomposition with release of soluble forms of nitrogen (M. D. Smolen, personal communication. Southern Piedmont Research and Continuing Education Center, Virginia Poly technic Institute and State University, Blackstone. 1977). Under Florida conditions this may be a significant source of the soluble organic nitrogen occurring in streamflow. While erosion is a large source of nitrogen in streamflow in many areas, it is not a significant source in much of Florida. The soil organic nitrogen pool is another source of nitrogen in a watershed. This nitrogen becomes available for movement through the watershed slowly by the natural mineralization process. Mineralization is primarily a function of temperature and moisture. Nitrogen may also be removed from the system in a watershed through several sinks. The largest nitrogen sink is uptake by crops. It is a function of transpiration and nitrate concentra tion in the root zone. Mass flow is the predominate mechanism for moving nitrate through the soil to the plant roots (Barber, 1962). Therefore, nitrogen uptake should be closely connected to transpiration. The amount of transpiration also is related to the amount of adsorbing root surface and refl ects the growth rate of the plant (Frere et al, 1975). Other nitrogen sinks are volatilization of ammonia, immobilization, and denitrification. Volatilization of ammonia occurs only from the soil surface or the upper soil layer and is probably not significant except from application of ammonium fertilizers or animal wastes. Immobilization is the conversion of inorganic nitrogen forms to organic forms. It depends upon the amount of nitrogen in the soil and the carbon nitrogen (C:N) ratto. Denitrification usually occurs when poor aeration limits the amount of free oxygen in the soil. It is dependent on several factors including organic matter content, moisture, temperature, oxygen concentration, and pH. Denitrification is rapid if conditions are favorable. Appreciable losses of nitrogen as nitrogen gas can occur even when conditions favorable to denitrification exist for only a day or less. Estimates of total losses by denitrification on cropped lands average 10 to 20 percent of all nitrates formed or added as fertilizers and can be as much as 40 to 60 percent of added nitrate nitrogen (Donahue et al, 1977). Patrick et al (1976) showed that denitrification was even significant in well-drained agricultural soil in the absence of excessive organic matter.

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43 After the nitrogen sources and sinks have been provided for in a model, the transformation processes and rates regulating changes in nitrogen forms must be considered in order to simulate the nitrogen losses in streamflow. This is important because of the different reactions that take place in the soil-water-airplant system for different nitrogen forms. In model formulation of these processes, it appears that first-order rate equations are adequate (Frere 1975, Rao et al 1976). Mineralization rates, or decomposition of organic nitrogen to ammonium, and nitrification rates of ammonium to nitrate are dependent on several factors including soil temperature, moisture content, soil type, pH, aeration, agricultural practices, C:N ratio, and organic matter content. Models can be developed to consider one or all of these factors with varying degrees of sophistication (Frere et al 1975, Duffy and Franklin 1972, Mehran and Tanji 1974, Hagin and Amberger 1974, Donigian and Crawford 1976, Donigian et al 1977). Beek and Frissel (1973) simulated heat flow in the soil to determine the soil temperatures for use in nitrogen transformation calculations. This required inputs of air temperature, soil moisture, soil heat conductivity and soil heat capacity. The approach chosen for this study was to select relatively simple expressions for the most important parts of the nitrogen cycle and develop a simple model to interface with the USDAHL-74 hydrologic model. Other expressions could Jhen be added to this model to incl ude other nitrogen forms and transformati on processes to obtain a better simulation as the model is tested and further developed. Based on this approach, the nitrate option of the ACTMO model (Frere et al, 1975) was selected for use as a basic nitrogen model. It has the advantage of being designed to be interfaced with the USDAHL-74 model, however it was not available in this form. The basic framework of this nitrate model was developed as an option of the ACTMO model, but it was never opera tional (M. H. Frere, personal communication. Southern Great Plains Research Watershed, USDA-SEA, Chickasha, Oklahoma. 1977). The ACTMO nitrate model considers only the soil organic nitrogen and fertilizer applied as nitrogen sources. Organic nitrogen is mineralized to nitrate according to a first order rate equation. The rate coefficient is sensitive to temperature and moisture. The watershed is separated into zones as in the USDAHL74 model, and fertilizer can be applied by zone in one or two applications. The only nitrogen sink considered by the model is plant uptake. Nitrate uptake is a function of the amount of nitrate available in the soil, the amount of evapotranspiration from each soil layer weighted for the distribution of the nitrate

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44 within and the amount of water available in the soil. Vertical and lateral water flow through each soil layer, calculated in the USDAHL.,.74 hydrologic is used in the ACTMO nitrate model to c&lculate the nitrate movement through the soil profile to the stream. These calculations are all made independently for each zone of the watershed to provide the total watershed output. The ACTMO model has been changed from a storm basis to operate on a daily basis with daily input parameters being supplied from the USDAHL-74 hydrologic model. The ACTMO model considers only nitrate and does not simulate the amount or movement of any other nitrogen forms, with the exception of the amount of soil organic nitrogen remaining to be mineralized. Nitrate is assumed to move only by subsurface flow, therefore none is allowed to move in surface runoff. This assumes that all surface applied fertilizer is dissolved and moves into the soil with the initial infiltration before overland flow begins. The ACTMO nitrate model was first cleared of errors and oper ated as an independent model on our Amdahl 470-V6 computer. It was then converted to run on a daily basis instead of its original storm basis. The next step was to interface it with the USDAHL-74 hydrologic model. This involved locating the appropriate para meters in the USDAHL-74 model and writing them in the correct sequence on a magnetic tape during its operation. This tape was then used to provide the input parameters to the ACTMO nitrate model. The nitrate model requires some additional direct input parameters related to the initial nitrogen status of the watershed and fertilizer applied during the period of simulation. Modeling nitrogen movement through a watershed is a very difficult and complex problem. This model is only the first step in the process of developing a model to satisfactorily simUlate movement of nitrate, and soluble organic nitrogen forms through agricultural watersheds. The model has not as yet been however thi s wi 11 be done very soon. Thi s research is being continued to include simulation of organic and ammonium nitrate forms. Precipitation and organic matter decomposition will be included as nitrogen sources. The concentration of soluble organic nitrogen in surface runoff will be assumed to be a function of seasonal land use and cover. Denitrification will be included as a .nitrogen sink based on a first order rate equation. The rate coefficient will be a function of organic nitrogen content in the soil profile, temperature, and moisture as a representation of aeration in the soil.

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45 CONCLUSIONS AND RECOMMENDATIONS Nitrogen and phosphorus loads were determined for two agricultural watersheds: one primarily in native forest cover and the other primarily in intensive crop production. Nitrogen and phosphorus concentration differences between the two watersheds were very minimal over the period of record. Nutrient concentration changes also were minimal between storm and low flow periods in both watersheds. This leads to the conclusion that nutrient loads were relatively proportional to streamflow volume. Nitrogen and phosphorus losses in streamflow were minor compared with amounts received by the watersheds in precipitation and commercial fertilizer. Small plots were used to evaluate the effects of selected cultural and water management practices on nitrogen and phosphorus loads in surface runoff from sandy soils. Again, on these small plots nitrogen and phosphorus losses in surface runoff were small compared with the contributions to the plots in rainfall and commercial fertilizer. Since the magnitude of nutrient losses from a 11 treatments was small, it was di ffi cult to determi ne whether differences among treatments were the result of the treatments. Because of the natural heterogenei ty, even on thi s sma 11 scale, there were relatively large differences in both runoff amounts and nutrient concentrations between plots with the same treatment. Therefore, conclusive differences among treatments coul d not be determined. Techniques were developed to simulate nitrogen movement through agricultural watersheds. The USDAHL-74 model of watershed hydrology has several advantages that make it a good choice to provide the hydrologic information required to model nitrogen movement. Calibration of the model to the research watersheds was adequate, but not as good as expected. This was in part the result of the poor qual i ty of some of the ra i nfa 11 input data duri ng the calibration period. The model should be modified in its subsurface and return flow components to better simulate the conditions of high lateral return flows with a shallow watertable, as encountered in this study. The ACTMO nitrate model provides a good framework for development of a more complete model of nitrogen transformations and movement. Simulating nitrogen transformations and movement through a watershed is a very difficult and complex problem. This nitrate model needs to be modified to include other nitrogen forms. Precipitation and organic matter decomposition are important

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46 nitrogen sources and denitrification is an important sink. These factors need to be.included if the model is to simulate conditions similar to those encountered in this study. Simulation models are an effective tool to assist in gaining a better understanding of the complex processes and interactions that occur in a watershed system. They can help in identifying which processes are most important in controlling nitrogen movement within a watershed.

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47 LITERATURE CITED Allen, H. E. and J. R. Kramer. 1972. Nutrients in natural waters. John Wiley and Sons, New York. American Public Health Association. 1971. Standard methods for the examination of water and wastewater. 13th edition. APHA. Washington, D.C. Barber, S. A. 1962. A diffusion and mass flow concept of nutrient availability. Soil Science 93:39-49. Beek, J. and M. J. Frissel. 1973. Simulation of nitrogen behaviour in soils. Center for Agricultural Publishing and Documentation, Wageningen, Netherlands. Carlile, B. L. and J. A. Phillips. 1976. Evaluation of soil systems for land disposal of industrial and municipal effluents. Report No. 118. Water Resources Research Institute of the University of North Carolina, Raleigh. Chow, V. T. 1959. Open-channel hydraulics. McGraw-Hill Book Co. New York. Donahue, R. L., R. W. Miller, and J. C. Shickluna. 1977. Soils: an introduction to soils and plant growth. 4th edition. Prentice Hall, Inc., Englewood Cliffs, New Jersey. Donigian, Jr., A. S., D. C. Beyerlein, H. H. Davis, Jr., and N. H. Crawford. 1977. Agricultural runoff management model version II: refinement and testing. EPA-600/3-77-098. Donigian, Jr., A. S., and N. H. Crawford. cides and nutrients on agricultural lands. 1976. Modeling pesti EPA-600/2-76-043. Duffy, J. and M. Franklin. 1972. On mathematical models of nitrification in soils. Summer Computer Simulation Conference Proc. 2:997-1006. Frere, M. H. 1975. Integrating chemical factors with water and sediment transport from a watershed. Journal of Environmental Qual ity 4: 12-17.

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Frere, M. H., C. A. Onstad, and H. N. Holtan. 1975. ACTMO, an agricultural chemical transport model. U.S. Dept. of Agr., ARS-H-3. 48 Hagin, J. and A. Amberger. 1974. Contribution of fertilizers and manures to the N-and P-load of waters -A computer simulation. Technion-Israel Institute of Technology, Haifa, Israel. Hanson, C. L. 1977. Evaluation of the components of the USDAHL-74 model of watershed hydrology. American Society of Agricultural Engineers Paper No. 77-2533. Holtan, H. N.,.G. J. Stiltner, W. H. Henson, and N. C. Lopez. 1975. USDAHL-74 revised model of watershed hydrology. USDA-ARS Tech. Bul. 1518. James, L. G., M. F. Walter, and R. E. Muck. 1977. Evaluation of several levels of hydrologic models on small watersheds. American Society of Agricultural Engineers Paper No. 77-2050. Mehran, M. and K. K. Tanji. 1974. Computer modeling of nitrogen transformations in soils. Journal of Environmental Quality 3:391-396. Molnau, M. and K. H. Yoo. 1977. Application of runoff models to a Palouse watershed. American Society of Agricultural Engineers Paper No. 77-2048. Nicks, A. D., G. A. Gander, and M. H. Frere. 1977. Evaluation of the USDAHL hydrologic model on watersheds in the Southern Great Plains. American Society of Agricultural Engineers Paper No. 77-2049. Patrick, W. H., R. D. Delaune, R. M. Engler, and S. Gotoh. 1976. Nitrate removal at the water-mud interface in wetlands. EPA-600/3-76-042. Perrier, E. R., J. Harris, and W. B. Ford, III. 1977. A comparison of deterministic mathematical watershed models. American Society of Agricultural Engineers Paper No. 77-2047. Porter, K. S. 1975. Nitrogen and phosphorus -food production, waste and the environment. Ann Arbor Science, Ann Arbor, Michigan. Pritchett, W. L. and J. W. Gooding. 1975. Fertilizer recommendations for pines in the Southeastern Coastal Plain. of the United States. Univ. of Fla. Agr. Exp. Sta. Tech. Bul. 774.

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Pritchett, W. L. and W. H. Smith. 1974. savanna forest soils for pine production. Sta. Tech. Bul. 762. 49 Management of wet Univ. of Fla. Agr. Exp. Rao, P. S. C., H. M. Selim, J. M. Davidson, and D. A. Graetz. 1976. Simulation of transformations, ion-exchange and transport of selected nitrogen species in soils. Soil and Crop Science Society of Florida. 35:161-164. Replogle, J. A., L. E. Myers, and K. J. Brust. 1966. Flow measurements with fluorescent tracers. American Society of Civil Engineers Proceedings 92(HY5):1-14. Stewart, B. A. 1976. Vol. II, an overview. Control of water pollution from cropland: U.S. Dept. of Agr., ARS-H-5-2. Stewart, E. H., D. P. Powell, and L. C. Hammond. 1963. Moisture characteristics of some representative soils of Florida. U.S. Dept. of Agr., ARS 41-63. Thompson, L. M. and F. R. Troeh. 1973. 3rd edition. McGraw-Hill Book Company. Soils and soil fertility. New York, N.Y. U.S. Department of Agriculture, Soil Conservation Service. 1975. Agricultural waste management field manual. U.S. Government Printing Office, Washington, D.C. U.S. Environmental Protection Agency. 1974. Methods for chemical analysis of water and wastes. U.S. Government Printing Office, Washington, D.C. Wilson, J. F., Jr. 1968. Fluorometric procedures for dye tracing. Techniques for Water-Resources Investigations of the United States Geological Survey, Book 3, Chapter A12, U.S. Government Printing Office, Washington, D.C.