Historic note
 Front Cover
 Back Cover

Group Title: CIrcular - University of Florida Institute of Food and Agricultural Sciences ; 431
Title: Irrigate by the accounting method
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00067092/00001
 Material Information
Title: Irrigate by the accounting method
Series Title: Circular
Physical Description: 8 p. : ill. ; 23 cm.
Language: English
Creator: Choate, Rush E ( Rush Edgar ), 1918-
Harrison, D. S ( Dalton Sidney ), 1920-
Florida Cooperative Extension Service
University of Florida -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1977
Subject: Sprinkler irrigation   ( lcsh )
Plants, Protection of -- Florida   ( lcsh )
Plants -- Frost protection -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: R.E. Choate and D.S. Harrison.
General Note: Cover title.
General Note: "May 1974."
Funding: Circular (Florida Cooperative Extension Service) ;
 Record Information
Bibliographic ID: UF00067092
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 51253913

Table of Contents
    Historic note
        Unnumbered ( 1 )
    Front Cover
        Front Cover 1
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Back Cover
        Back Cover
Full Text


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source

site maintained by the Florida
Cooperative Extension Service.

Copyright 2005, Board of Trustees, University
of Florida


Irrigate By The

Accounting Method
R. E. Choate and D. S. Harrison


Florida Cooperative Extension Service
University of Florida, Gainesville

Institute of Food and Agricultural Sciences
John T. Woeste, Dean for Extension

Circular 431


R. E. Choate and D. S. Harrison*

In any business, the function of management is to develop
the policies and objectives that give the firm purpose and direc-
tion. A manager's duties include planning, organizing, coordi-
nating and controlling the business to earn a satisfactory profit.
The purpose of this report is to discuss planning and manage-
ment of irrigation by the accounting method in relation to the
elements of management. It shall be assumed that management
has reviewed the alternatives and concluded that irrigation is
economically justified, the system has been selected, properly
designed and installed. Therefore, the next decision of manage-
ment is to provide the right amount of water at the right time.
This discussion will be presented around three subtopics,
namely: soil moisture and root zone; amount per application;
and frequency of irrigation, as related to soil moisture and crop
needs. In addition, present concepts of programming supple-
mental irrigation, which are a direct concern of the manager,
are discussed.
In planning and managing a soil moisture and root zone irri-
gation program, the soil root zone should be regarded as a water
storage reservoir. The reservoir extends in depth from the soil
surface to the lower reaches of the active roots. A generalized
moisture extraction pattern is shown in Figure 1. For citrus
grown on the ridge, the effective root zone may be five feet
whereas for field crops in north Florida or grown on the same
land, the active root zone depth may be only one to two feet.
Available water is stored in the capillary pore space of the
soil. The maximum available moisture capacity of the reservoir
is determined by the volume of the capillary pore space within
the root zone. The capillary pore space is not the same for all
soils. There are three forms of soil moisture, (1) gravitational,
(2) capillary, and (3) hygroscopic (Fig. 2). Almost all the water
used by the plants is the capillary form of soil moisture.
Frequency of Irrigation
Various methods have been devised to aid the irrigation man-
ager in recognizing when to irrigate. Such methods have vary-
ing degrees of practicability, due either to the physical meas-
urements required or to inadequate sensitivity of the tools used.
*Professors, Agricultural Engineering Department, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville.






Figure 1. Moisture extraction pattern showing percentage moisture ex-
tracted from root zone



-- --I--L

Figure 2. Diagram of soil moisture forms as related to moisture content


During the past 20 years, increasing thought and research have
been directed toward the evaluation of climatic factors and cor-
relating them to soil moisture depletion rates for the various
crops. Indications are that this climatological approach holds
considerable promise as an adequately sensitive means of de-
termining when to irrigate. It also is easy to understand and
simple to use.
This approach is called the "water account system" which is
operated like an ordinary bank account. It is based on the con-
cept that the moisture in the soil at any given time is a balance
between available moisture which is deposited as rainfall or irri-
gation and that which is withdrawn through evapotranspira-
tion (ET).
The daily water requirement for a specific type of vegetation
is very closely correlated to the daily mean temperature, day
length and relative humidity. Mean temperatures and day length
values are readily available for specific areas and latitudes.
Average daily moisture requirements for several Florida crops
are shown in Table 1. Plant growth as a function of soil mois-
ture is shown in Figure 3.
The usable soil moisture capacity (in inches), divided by the
high average daily requirement (in inches), will give the design
irrigation frequency (in days). In the accounting system, daily
use represents withdrawals while rainfall and irrigation repre-
sent deposits. Irrigation would be applied as necessary to return
the soil root zone to field capacity before economic growth re-
tardation occurs. Management must make this decision very
carefully. Too large a water application is undesirable, not only
from the standpoint of leaching plant foods such as nitrogen,
Table 1. Potential daily moisture requirements (ET) of certain crops
grown under Florida conditions

Crop Maximum ET* Minimum ET

Inches Per Day
Citrus 0.20 0.08
Corn 0.27 0.06
Grasses 0.25 0.06
Melons 0.17 0.06
Ornamentals 0.25 0.10
Tobacco 0.25 0.06
Vegetables 0.20 0.10
*Design water use rate, in inches of water per day.




Figure 3. Rate of plant growth as a function of soil moisture


10090 80 70 60 50 40 30 0 100 10 20 30 40 50 60 70 80 90 100
Figure 4. Wetted depths from a sprinkler for a given amount of water,
as applied to different soil types

potash, magnesium and boron, but also from the added irriga-
tion cost.

Usable Soil Moisture (in.)
Irrig. Freq. (in days) = High Daily ET (in.)
High Daily ET (in.)

Soil is wet by rainfall or supplemental irrigation progressively
from the surface downward. The depth wetted by a given irriga-
tion application is determined by the moisture holding capacity of
the soil. For example, on a sand a one-inch application may wet
to a depth of four feet, whereas, the same amount of clay soil
may wet to a depth of only one foot (Fig. 4). Therefore, when
rainfall does not occur in a sufficient amount to return the soil
root zone to field capacity, there would be a nonuniform moisture
condition in the root zone subsequent to such rains. That is,
the top portion would be at field capacity and the moisture in
the lower portion would not be changed appreciably by the rain-
fall. If we further assume that the rain in question fell immedi-
ately prior to a scheduled irrigation, irrigation should not be
applied just as planned. The amount applied should be only that
required to make-up the deficiency and return the total root
zone to field capacity. Light rains (0.10 to 0.20-inch) do not
generally allow scheduled irrigation to be deferred but they do
decrease the required depth of water per irrigation.

Available Soil Moisture
The capillary pore space of the soil constitutes a reservoir
which contains the moisture which is available to the plants.
Field capacity is the upper limit of the reservoir (Fig. 3);
whereas, the lower limit or point of depletion is known as the
"wilting point." It is customary to express this range of avail-
able soil moisture in terms of the equivalent depth of water in
inches. Estimated available moisture capacities of some typical
Florida soils are indicated in Table 2.

Usable Soil Moisture
The dryer the soil becomes the more force required to remove
the water from the soil particles. Therefore, as soil moisture is
gradually depleted through transpiration and evaporation, there
is an associated increased stress on the plant. The rate of crop
growth as compared to soil moisture depletion is depicted in
Figure 3.

Table 2. Approximate available moisture capacity of some Florida soils*

a b c d
Inches water Inches water Usable moisture (in.)
Soil type per ft. soil per 5 ft. soil (50% depletion)

Blanton f.s. 0.4 2.0 1.0
Eustis f.s. 0.8 4.0 2.0
Fort Meade f.s. 0.7 3.5 1.8
Lakeland f.s. 0.8 4.0 2.0
Lakeland I.f.s. 1.0 5.0 2.5
Lakewood f.s. 0.4 2.0 1.0
Norfolk I.s. 1.2 6.0 3.0
Orlando f.s. 0.8 4.0 2.0
Orangeburg I.s. 1.3 6.5 3.3

Adamsville f.s. 0.5 2.5 1.3
Ft. Meade f.s. 0.8 3.8 1.9
Immokalee f.s. 0.7 3.5 1.8
Keri f.s. 0.5 2.5 1.3
Leon f.s. 0.8 4.0 2.0
Pomello f.s. 0.3 1.2 0.6
Ona f.s. 0.7 3.5 1.8
Seranton I.f.s. 0.8 3.8 1.9

*From Irrigation Guide, Soil Conservation & Service, Cooperatively with
Fla. Agr. Exp. Sta., and Fla. Coop. Ext. Service.

Irrigation should be applied before the soil moisture content
has been reduced to the point at which an undue stress is placed
on the plant. Undue stress may be defined as the point at which
the stress is sufficient to cause an economic retardation in either
volume and quality in terms of yield. This stress will be en-
countered at higher soil moisture content on fine textured soils
than on coarse textured soils. Research has shown that certain
varieties of citrus grown on Lakeland fine sand should be irri-
gated when only one-third of the available moisture has been
depleted in the 5-foot zone, from January through June, and
when two-thirds depleted from July through December. For
vegetable and field crops, irrigation is commenced usually when
about 50 to 60 percent of moisture is depleted in the root zone.

Mathematically speaking, the maximum depth of water to
apply at each irrigation under a properly managed program,
would be computed by multiplying the available soil moisture
capacity for the respective root zone depth (expressed in inches)
by the percent depletion factor. Based upon a 50 percent deple-
tion factor, the amount of water (in inches of equivalent depth)
which may be used between irrigations from certain selected
soils is shown in Column "d" of Table 2.

The Account Method
The use of the accounting method to determine when to irri-
gate is illustrated in Table 3. The daily accounting must be
started when the soil moisture is at field capacity. This condition
may be attained by natural rainfall or through supplemental
Soil moisture is allowed to deplete at the average daily rate
until the selected lower limit is reached. Soil is wetted to field
capacity progressively from the surface downward. Therefore,
it is to be pointed out that should a given rain occur in an in-
sufficient amount to return the soil to field capacity, there would
be a non-uniform moisture condition in the root zone subsequent
to such rains.
Assuming that the rain in question fell on the day that irri-
gation was normally scheduled, irrigation should be applied as
planned. However, it should be applied only in an amount to
make up the deficiency.
If the rainfall should exceed the deficiency, the excess would
be gravitational water. It would not be considered available to
Following such a rain, the figure representing maximum
usable moisture would be entered in the usable moisture column.
Then the accounting system would be started again, as shown
in Table 3.

When irrigation is undertaken within the concepts stated
above, the consumption of a highly effective and efficient irriga-
tion program is easily attainable. In the planning stage certain
decisions must be made. These decisions will include depth to
wet the soil, available moisture capacity of the soil, amount to
apply, design frequency of irrigation, present and projected
areas to be irrigated.

Table 3. Example of accounting method
sand for the month of June

for citrus grown on Eustis fine

1 2 3 4 5
Elapse Usable Potential
time moisture daily use Rainfall Irrigation
days Inches inches inches inches

0 2.00 .17
1 1.83 .17
2 1.66 .17
3 1.49 .17
4 1.32 .17 .40
5 1.15 .17
6 .98 .17
7 .81 .17
8 .64 .17
9 .47 .17
10 .30 .17
11 2.00 .17 1.47
1 1.83 .17
2 2.00 .17 1.20
1 1.83 .17

Irrigate the correct amount, no more or less. Irrigate accord-
ing to a pre-planned schedule. Space and operate all equipment
according to specifications. Failure to manage irrigation properly
usually results in poor economy.
Irrigation should be planned, programmed and managed with
a high degree of astuteness similar to that exercised in other
cultural and production practices.

This publication was promulgated at a cost
of $640.30, or 6.4 cents per copy, to inform
growers and production managers on how
and when to irrigate.

Single copies are free to residents of Florida and may be obtained
from the County Extension Qffice. Bulk rates are available upon
request. Please submit details of the request to C.M. Hinton, Publi-
cation Distribution Center, IFAS Building 664, University of
Florida, Gainesville, Florida 32611.


(Acts of May 8 and June 30, 1914)
Cooperative Extension Service, IFAS, University of Florida
and United States Department of Agriculture, Cooperating
K. R. Tefertiller, Director

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