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Introduction & Determining when to irrigate
Water budgeting for irrigation scheduling
Soil moisture indicators for irrigation scheduling
Irrigation water management
A. G. Smajstrla, B. J. Boman, G. A. Clark,
D. Z. Haman, F. T. Izuno, and F S. Zazueta
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences
University of Florida / John T. Woeste, Dean
'V!ERSITY OF FLORIDA LIBRARIES
Introduction. ............... ...... 1
Determining When to Irrigate . . . . . . . . 1
Crop Water Requirements . . . . . . . . 3
Field Water Balance ................... 4
Water Budgeting for Irrigation Scheduling . . . . . 6
Understanding Evapotranspiration . . . . . . 6
Estimating Evapotranspiration . . . . . . . 7
Soil Water Storage... ................ .. 8
Allowable Soil Water Depletion . . . . . .... 10
The Water Budget .................... 11
Soil Moisture Indicators for Irrigation Scheduling . . ... 14
Irrigation Water Management . . . . . . .... 15
Summary .................. ....... 16
References . . . . . . . . . . . . . 17
A.J. Smajstrla is Water Mgt. Specialist, Agr. Eng. Dept.,
Gainesville; B.J. Boman is Citrus Irr. Specialist, Agr. Res. and Ed.
Center, Ft. Pierce; G.A. Clark is Water Mgt. Specialist, Gulf Coast
Res. and Ed. Center, Bradenton; D.Z. Haman is Water Mgt;
Specialist, Agr. Eng. Dept., Gainesville; F.T. Izuno is Water Mgt.
Specialist, Everglades Res. and Ed. Center, Belle Glade; and F.S.
Zazueta is Water Mgt. Specialist, Agr. Eng. Dept., Gainesville:
respectively, Institute of Food and Agricultural Sciences, University
Proper irrigation scheduling is the application of water to crops
only when needed and only in the amounts needed; that is, deter-
mining when to irrigate and how much water to apply. With proper
irrigation scheduling, crop yields will not be limited by water stress
from droughts, and the waste of water and the energy used in
pumping will be minimized. Other benefits include reduced leaching
of nutrients from excess water applications, and reduced pollution of
groundwater or surface waters from the leaching of nutrients.
Irrigation is practiced to provide water when rainfall is not
sufficient or timely to meet water needs of a crop. For most
agricultural crops, yield or quality reductions result from water
stress. Therefore, if water is available and if it is relatively low in
cost, as is the case in Florida, irrigations are normally scheduled to
avoid plant water stress.
Despite Florida's relatively large average yearly rainfall of 52-60
inches, irrigation is practiced extensively. Irrigation is necessary
because of the nonuniform distribution of rainfall, the very limited
water-holding capacities of typical sandy soils, and the extreme
sensitivity of many specialty crops to water stress. These factors
and the economic implications of under- or over-irrigation require
that irrigations be scheduled as efficiently as possible.
This publication discusses irrigation scheduling for Florida crops
grown on soils where the water table is substantially below the crop
root zone so that it does not contribute significantly to crop water
use. Thus, irrigation events must periodically occur to replenish
water in the crop root zone. Water budgeting for water table
management (also called subirrigation or seepage irrigation) on
poorly drained soils in which irrigation occurs as water applications
to a high water table (immediately below the crop root zone), is
discussed in IFAS Extension Circular 769, "Water Budgeting for High
Water Table Soils", available from IFAS County Extension Offices.
Determining When to Irrigate
Because the objective of irrigation is to maintain a favorable
environment for crops, the plants themselves are the best indicators
of the need for irrigation. Instrumentation exists which could allow
an irrigator to measure plant water status and to anticipate water
stress. However, such instrumentation is expensive, requires special
training for use, and is generally applicable only for research
purposes. Field scale use of such instruments is impractical.
Another indicator of plant water stress is the visual appearance
of the plant. Unfortunately, however, yield reduction has already
occurred by the time most agricultural crops show wilt symptoms.
Growth processes cease in many crops before visual wilting occurs,
and yield reduction may have occurred for some time before wilting
Finally, there are time lags associated with applying irrigation
water. Because several zones might be irrigated from a single pump
or other limiting distribution characteristics may exist, many irriga-
tion systems cannot quickly replenish water in the crop root zone.
Many hours or days may be required. Therefore, the need to
irrigate must be anticipated because of limitations of the irrigation
system. This problem is compounded in Florida by the low water-
holding capacities of most agricultural soils and by the shallow root
zones of many crops.
When to irrigate can also be determined by calendar methods (for
example every 5 days), by crop growth stage (for example, every 5
days during early vegetative growth stage, and every 3 days during
peak growth stage), or by similar methods based on long-term
average irrigation requirements. However, these methods fail to
consider the tremendous effect of climatic variability on daily crop
water use. Therefore, the use of long-term average values may not
be adequate during periods of hot, dry days, while it may result in
overirrigation during periods of cool, overcast days, especially if
rainfall is not considered. Day-to-day climatic conditions are highly
variable during much of the year in Florida because of cloud cover
and the random nature of rainfall occurrences.
Because of the these limitations, irrigations are most often
scheduled based on the soil water status. Three procedures may be
used: 1) a water balance procedure based on the estimated crop
water use rate and soil water storage, 2) a direct measurement
procedure based on instrumentation to measure the soil water status,
and 3) a combination of the above two methods in which soil water
status instrumentation is used with a water balance procedure.
These procedures require a knowledge of the crop water require-
ments, effective root-zone, soil water-holding capacity, and irriga-
tion system capabilities in order to schedule irrigations effectively.
Crop Water Requirements
Water is used in a cropped field in several ways: 1) assimilation
into the plant and plant fruit, 2) direct evaporation from the soil or
other surfaces, 3) transpiration, which is loss of water vapor from
plant leaves, and 4) other uses such as leaching of salts, crop
cooling, and freeze protection. Less than 1% of the water used in
crop production is assimilated into the plants. Other uses (category
4, above) may be significant, but they depend on factors other than
maintaining adequate soil water content, and they will not be
considered in this publication.
Most of the water applied to meet the water requirements of a
crop is used in evaporation and transpiration. Evaporation and
transpiration are important in cooling a crop, to maintain tempera-
tures in the range that permits photosynthetic activity and crop
growth to occur. Transpiration is also required to transport
nutrients into and through plants.
The combination of evaporation and transpiration is called
evapotranspiration (ET). Because the amount of water assimilated by
a plant is very small with respect to ET, ET is often considered to
be the crop water requirement -- the amount of water required by a
growing crop to avoid water stress.
Delivering water to a crop in the field results in losses which
increase the amount of water which must be pumped to supply the
crop water requirement. Losses may occur because of inefficiencies
in the conveyance system, evaporation and wind drift (if water is
sprayed through the air), surface run-off, or percolation below the
root zone. These losses can be minimized through good management
practices, but they are impossible to completely eliminate, and they
must be considered when determining the total (or gross) irrigation
The total irrigation water requirement is the total amount of
irrigation water which is required for crop production, including ET,
all losses incurred in delivering water to the crop, and other needs
such as leaching of salts, crop cooling, and freeze protection. In
humid areas such as Florida, a large part of the crop water require-
ment can be provided by rainfall. Effective rainfall, rainfall that is
stored in the root zone and available for crop use, proportionally
reduces the amount of water which must be pumped for irrigation.
Field Water Balance
The water balance of a field during and after irrigation is shown
in Fig. 1. In Florida, runoff losses are normally minimal because of
the high infiltration rates of the sandy soils. Conveyance losses can
be minimized by conveying water to the field in pipes rather than
Application losses, including evaporation and wind drift, can
occur during irrigation, especially from sprinkler irrigation systems.
These losses are, however, relatively small during periods of low
radiatipn, low wind velocities, and high humidities. Also, water which
evaporates during application, or which is intercepted and later
evaporates from soil, plant, or other surfaces is not entirely lost.
Rather, some evaporation during application compensates for ET by
reducing ET that would have occurred if the intercepted water had
not been evaporated.
Evaporation and wind drift losses can be minimized by irrigation
at night, early mornings, and late afternoons when climatic condi-
tions are not severe. However, cultural aspects such as disease must
be considered for crops in which wet foliage may promote bacterial
or other growths which could reduce yields.
Deep percolation losses from well-designed irrigation systems can
be minimized by good irrigation management. If water is applied
uniformly and the water-holding capacity of a soil is not exceeded,
water losses to deep percolation will be minimized. If saline water
is used for irrigation, it may be necessary to leach excess salts from
the crop root zone by adding water in excess of the soil water-
holding capacity. However, excess irrigation for leaching should be
required only during extended dry periods in Florida because rainfall
normally leaches salts.
If the losses shown in Fig. 1 are kept to a minimum, most of the
irrigation water applied will evaporate or transpire in proportion to
the climatic demand. Unfortunately, rainfall is relatively unpre-
dictable and its occurrence immediately following an irrigation
reduces rainfall effectiveness. Irrigation can be minimized by
THE WATER BALANCE OF A FIELD
AT THE FARM
1. Water balance components of an irrigated field.
W = CHANGES IN SOIL
BOTTOM OF ROOT ZONE
anticipating rainfall and providing soil storage capacity (that is,
irrigating to less than field capacity to leave room for rainfall
storage) to increase rainfall effectiveness.
Water Budgeting for Irrigation Scheduling
Two questions must be answered in order to schedule irrigations:
1) When to irrigate, and 2) How much water to apply? A water-
budget procedure can be used to answer both questions.
From Fig. 1, the crop root zone can be visualized as a reservoir
where water is temporarily stored for use by the crop. Inputs to
that reservoir occur from both rainfall and irrigation. If the capacity
of the soil-water reservoir (the volume of water stored in the crop
root zone) and the daily rates of ET extraction from that reservoir
are known, the date of the next irrigation and the amount of water
to be applied can be determined. Thus, ET and soil-water storage in
the plant root zone are the basic information needed to use the
water-budget method for irrigation scheduling.
Evaporation involves the change of state of water from a liquid
to a vapor. Energy is required for evaporation to occur. If field
surfaces, such as the leaves of well-watered plants or wet soils, are
moist, the amount of water vaporizing and moving into the atmos-
phere in a humid region such as Florida is mainly determined by the
energy available from solar radiation. Thus, the solar radiation level
is the main climatic factor that determines the ET rate, although air
temperature, humidity, and wind also affect ET rates. For these
reasons, ET rates are higher in summer when daily solar radiation
levels and temperatures are high.
Exceptionally low relative humidity and high winds will increase
ET rates above normal. Hot dry winds may raise the ET rates of
isolated irrigated fields by 25 percent or more above the normal,
although such periods are usually brief.
The most significant crop factors that affect ET from a well-
watered crop are the crop species, the stage of growth, and the
plant size or leaf area with respect to the ground surface on which
radiation is incident. Methods of expressing plant size and leaf area
include the degree of ground cover or percent canopy coverage. ET
rates are greatest when the entire soil surface is covered by the
Many crops do not totally shade the ground, especially during
their early stages of growth, and evaporation from the dry soil
surface between plants is low. This is especially true of sandy soils
which act as a mulch to greatly reduce evaporation when the
When the crop canopy is not complete, the ET rate is strongly
influenced by the area of leaf surface that is intercepting sunlight,
that is, the percent of soil surface shaded by the crop. For this
reason, ET for row crops during early growth stages and that of
many orchards and vineyards is considerably less than the ET that
would occur from a complete canopy. As growth increases, ET
reaches its maximum at nearly complete ground cover. ET measure-
ments indicate that when the percent of ground covered by the
canopy is above 60-70 percent, full ground cover and full ET rates
can be assumed.
Immediately after an irrigation, evaporation from the wet soil
occurs at approximately the same rate as full cover ET, but as the
soil dries, rates of evaporation are quickly reduced. Thus, frequency
of irrigation plays an important role in determining evaporation
losses from the soil, especially when the entire soil surface is
wetted. There are both positive and negative aspects to evaporation
from sandy soils -- the soils are self-mulching and evaporation rates
are quickly reduced when the soil surfaces dry, but, because of their
low water-holding capacities, the surfaces must be wetted more
frequently than those of heavier-textured soils because more
frequent irrigations are required.
Because climatic conditions largely determine ET, various methods
based on meteorological factors have been developed to estimate ET
rates. A summary and discussion of several ET equations and their
modifications for Florida conditions were presented by a committee
of IFAS researchers (Jones et al., 1984). The ET estimation equations
which can be applied on a daily basis for irrigation scheduling
require inputs of measured or estimated solar radiation. The Penman
equation, which is believed to be the most accurate for Florida
conditions, is also mathematically complex and difficult to use
manually. For this reason, computer software which calculates ET
from climatic and crop factors was developed for IBM-compatible
microcomputers (Zazueta et al., 1987) and is available from the
University of Florida.
One of the simpler methods of estimating daily ET in the field is
by measuring evaporation from a standardized free-water surface,
since a correlation exists between crop ET and evaporation from
free water. The standard water surface commonly used is the
National Weather Service Class A evaporation pan located in an
irrigated grassed area. The ratio between potential ET (ET for a
well-watered short green grass crop) and evaporation from a
well-maintained evaporation pan is typically assumed to be about 0.7.
Then crop ET can be estimated by multiplying potential ET by water
use coefficients (Kc) for specific crops, growth stages, and manage-
ment factors. Kc values for many crops that are grown in Florida
have been published by Doorenbos and Pruitt (1977), Jones et al.
(1984), and SCS (1970).
When a complete crop canopy exists, the daily ET can be es-
timated by multiplying the measured pan evaporation by 0.7. This
procedure can be used as a "rule of thumb" if other specific crop
coefficient data are not available.
During irrigation, water infiltrates (penetrates) the soil surface.
It is then distributed in the soil by gravity and soil capillary forces
(attraction for water). As the soil becomes wetter, gravitational
forces dominate and water drains downward through the soil.
Drainage is rapid at first, but after one to several days (depending
on soil type, layering, etc.) it decreases to a very small rate, so
that for practical purposes it may be neglected. At this point, soil
moisture in the root zone may be considered to be in storage; it can
be depleted primarily by plant transpiration or evaporation from the
soil surface. This upper limit of water storage in soil is called
"field capacity" (FC). Field capacity in sandy soils in Florida
commonly occurs within one or two days after an intense rainfall or
maximum irrigation because of the rapid movement of water in sandy
A practical lower limit of soil water may be defined as the
soil-water content below which severe crop water stress and perma-
nent wilting occurs. This lower limit has been defined as the
permanent wilting point (PWP). While plants may remove some
water below this level, such extraction has little or no significance
in irrigated agriculture, although it may be crucial for plant sur-
vival. In fact, yield reduction typically occurs long before PWP is
The difference between FC and PWP is called the available water
(AW). Table 1 presents typical values of AW for various soil types.
Most of the major irrigated soils in Florida are in the top category
(Sands and fine sands) in Table 1. Local soil surveys and irrigation
guides available from the Soil Conservation Service (SCS) provide
information on specific Florida soil types. Available water may also
be estimated in the field by applying a known limited amount of
water to the soil when the profile water content is near PWP,
observing the volume of soil wetted, and calculating the volume of
water stored per unit volume of soil.
Table 1. Available Water for Various Soil Types
Available Water (AW)
Type of Soil (inches/ft) (inches/ft)
Sands and fine sands 0.4 to 1.00 0.75
sandy loams and fine 1.00 to 1.50 1.25
very fine sandy loams 1.25 to 1.75 1.50
to silty clay loam
Fine and very fine texture- 1.50 to 2.50 2.00
silty clay to clay
Peats and mucks 2.00 to 3.00 2.50
Once AW is known, the total depth of water available, TAW (and
thus the capacity of the soil-water reservoir), can be obtained by
multiplying AW by the crop effective root zone depth. For layered
soils, TAW is calculated by adding the multiples of AW and depths
of all soil layers contained in the crop root zone.
The effective root depths of Florida agricultural crops can be
estimated from SCS irrigation guides, but local conditions may
affect root depths. The best way to determine effective root zone
depths is by digging and observing where most of the roots are
located. The effective root zone is that where most of the roots
actively involved in water uptake are located -- this is normally the
upper 1 to 3 ft of the soil profile, depending on the crop being
grown. In a humid area such as Florida, irrigations should be
concentrated in this upper portion of the crop root zone where the
great majority of the crop roots are located.
Allowable Soil Water Depletion
The allowable soil water depletion is the fraction of the available
soil water that will be'used to meet ET demands. As ET occurs, the
soil water reservoir begins to be depleted. As the soil dries, the
remaining water is bound more tightly to the soil, making it more
difficult for the plant to extract it. For this reason ET will start to
decrease long before the PWP is reached. This lower ET generally
does not increase water-use efficiency because it also reduces yield.
For this reason, growers should irrigate before the root zone water
content reaches a level that restricts ET.
The critical soil water depletion level depends on several factors:
crop factors (rooting density and developmental stage), soil factors
(AWC and effective root depth), and atmospheric factors (current ET
rate). Therefore, no single level can be recommended for all
Allowable depletions of 1/3 to 2/3 of the available soil water are
commonly used in scheduling irrigations. The smaller allowable
depletions are commonly used for sensitive crops at critical stages
of crop growth. The greater depletions are allowed for less sensitive
crops and at less-critical growth stages. As a "rule of thumb", an
allowable water depletion of 1/2 of AW should be used if other
specific data are not available.
The Water Budget
The water-budget procedure is also called a water balance or
bookkeeping procedure. It is similar to keeping a bank account
balance. If the balance on a starting date and the dates and
amounts of deposits and withdrawals are known, the balance can be
calculated at any time. Most importantly, the time when all funds
(or water) would be withdrawn can be determined so that an over-
draft is avoided (or an irrigation can be scheduled).
The water budget equation for irrigation scheduling on a daily
basis can be written as follows:
AS = R + I-ET-(D + RO) (1)
A S = change in available soil water (inches),
R = rainfall measured at the field site (inches),
I = irrigation applied (inches),
ET evapotranspirationn estimated from pan evaporation or
other method (inches),
D + RO = drainage and runoff, calculated as rainfall in excess of
that which can be stored in the soil profile to field
The soil water content on any day (i) can be calculated in terms
of the water storage on the previous day (i-1), plus the rain and
irrigation, and minus the ET, drainage, and runoff that occurred
since the previous day as:
S(i) = S(i-1) + R + I ET (D + RO) (2)
The starting point for irrigation scheduling is often after a
thorough wetting of the soil by irrigation or rainfall. This brings
the soil reservoir to full capacity and S(i) to TAW. If this does not
occur, the initial available soil water must be determined by direct
observation (measurement or estimation).
Daily measurements or estimates of ET are subtracted from the
available soil water until the soil water has been reduced to the
allowable depletion level. At that point an irrigation should be
applied with a net amount equivalent to the accumulated ET losses
since the last irrigation. The soil reservoir is thus recharged to full
capacity, and the depletion cycle begins again. Fig. 2 shows a
sample of a water budget for a Florida sandy soil with a total
available water of 1.5 inches in the plant root zone. It was assumed
that a management decision was made to irrigate when 2/3 of the
available soil water (1.00 inch) was depleted. In this example, that
level of depletion occurred after 4 days. At that time, an irrigation
should be scheduled to replenish the 1-inch of soil water depleted.
The water budget procedure also accounts for rainfall. Rainfall
is entered into Fig. 2 in the same way that an irrigation application
would be. That is, it refills the soil profile and raises the soil
water content. If large rainfalls occur, only that portion required
to restore the soil water content to field capacity will be effective.
Greater amounts of rain will either run off of the soil surface or
drain below the plant root zone.
The management decision concerning the level of allowable water
depletion (AWD) is one that will need to be made by each irrigation
manager. It will vary depending upon soil, crop, and climatic
factors. Commonly it will vary during the growing season. For
example, AWD may be set at 2/3 during non-critical crop growth
stages, but it may be decreased to 1/3 during critical growth stages
such as during fruit set. Decreasing AWD increases the frequency
of irrigation (but decreases the amount per irrigation) to provide a
more favorable crop root environment to reduce water stress during
critical growth stages. Decreasing AWD will generally result in
greater irrigation requirements because the soil will be maintained
wetter and thus rainfall will be less effective. More frequent
irrigations will also promote increased evaporation from the soil
The capacity of the root zone reservoir and allowable depletion
levels can be estimated before the start of a growing season. For
annual crops the capacity will change as the season progresses and
as their roots develop. For mature perennial crops such as citrus,
the root zone may be considered to be a constant for a given set of
The soil depth to be managed for irrigation must be refined by
field experience. For example, experience in many parts of the
world has shown that the citrus root zone to be irrigated should be
much less than the 5 to 8 ft depths where some plant roots exist.
THE WATER BUDGET METHOD OF IRRIGATION SCHEDULING
AVAILABLE = 1.0 IN.
= 1.5 IN.
1___ ________ ___
- 0.28 4
IRRIGATE I. WHEN- -
2. HOW MUCH *
* AFTER 4 DAYS
* APPLY 1.0 INCHES OF WATER
*ALLOWABLE DEPLETION= 2/ AVAILABLE SOIL WATER (MANAGEMENT DECISION)
2. Illustration of the water budget method of irrigation scheduling.
Rather, the irrigated zone should be the upper 2 to 3 ft of the root
zone where the majority of the roots are located. This practice
also has the advantage of allowing some soil capacity for rainfall
when it occurs.
Daily ET values for specific water use periods should be es-
timated from pan evaporation or ET equations. If current daily ET
estimates are not available, the use of soil moisture measurement
instrumentation or the installation of evaporation pans should be
considered. The use of long-term average ET data (Smajstrla et al.,
1984) will result in scheduling errors because day-to-day ET rates
are highly variable. Long-term average ET data can be used as a
guide for daily ET estimates, but they will need to be modified for
climatic variabilities. That is, they will need to be increased during
long-term hot, dry periods, and decreased during mild weather
Soil-Moisture Indicators for Irrigation Scheduling
Devices for monitoring soil moisture have been available for more
than 20 years. Among them, tensiometers are the instruments most
commonly used for scheduling irrigations. Gypsum blocks are also
being used on a limited basis. These devices register the status of
water in the soil, in terms of soil-water tension, at the depth at
which the device is placed. They have the advantage of providing a
measurement of the soil water status rather than relying upon
estimates of ET to calculate the soil water content. When placed in
the plant root zone they indicate the soil water status that the
plants are experiencing. Disadvantages of soil moisture sensors
include their cost, labor requirements for reading and servicing, and
need for periodic calibration. They also make point rather than
field scale measurements, thus many instruments may need to be
installed to accurately represent a given field.
Details of the use, cost, advantages, and disadvantages of these
and other devices which can be used for soil moisture measurement
are given in IFAS Extension Circular 532, "Measurement of Soil
Water for Irrigation Management". Details of the use of tensio-
meters are given in IFAS Extension Circular 487, "Tensiometers for
Soil Moisture Measurement and Irrigation Scheduling", available from
IFAS County Extension Offices.
No single soil-water tension level can be recommended as indica-
ting the need for irrigation when using tensiometers. For the same
reasons that allowable soil water depletion is not constant for all
crops and conditions, critical soil water tension also varies with soil
and crop conditions and management objectives. The level also
varies with depth of placement of the tensiometer. However, crop
water stress is normally avoided when irrigations are scheduled in
the range of 15-30 centibars (cb) in the upper portion of the crop
root zone where most of the roots actively involved in soil water
extraction are located. Lower readings should be used for crops
that are more sensitive to water stress. Field experience is required
to refine the interpretation of instrument readings for a given crop
and management system.
Tensiometers or any other soil-moisture monitoring device are
most effectively used in combination with ET data. The device is
read to determine when to irrigate, and the ET data are used to
calculate the volume of water lost since the last irrigation. From
this, the volume to be replaced can be determined.
Irrigation Water Management
Good on-farm water management practices include not only
precise irrigation scheduling, but also knowing (or being able to
accurately measure) the volume of water applied to each field. For
example, if the field associated with the irrigation scheduling
example in Fig. 2 was 40 acres of citrus, irrigated with an overhead
sprinkler system in 4 sets of 10 acres each, and if the application
efficiency for the overhead system was 75% (25% of the water
applied is assumed to be lost to evaporation, wind drift, and
nonuniform application during sprinkling), the depth of water to be
pumped at each irrigation would be 1.0"/0.75 = 1.33 inches. The
volume of water required for each 10 acre set would be (1.33 in.)
(10 acres) = 13.3 acre-inches or approximately 362,000 gal.
Flow meters can accurately measure irrigation water to verify
that the correct amount is applied. They are available with registers
in units of either gallons or acre-inches. Flow meters can easily
pay for themselves with savings in fuel costs for irrigation pumping.
More information on irrigation flow measurement is available in
IFAS Extension Bulletin 207, "Agricultural Water Measurement",
available through IFAS County Extension Offices.
Good farm irrigation management requires that an irrigation
system be capable of applying water in sufficient quantities to meet
the crop's water requirements and with high uniformity to minimize
waste. Nonuniform irrigation will cause excess water to be applied
in some areas while other areas will not get enough.
Irrigation systems are more expensive if they are designed for a
high degree of uniformity. Thus, there is a temptation to sacrifice
uniformity when systems are purchased on the basis of competitive
bids. The system manager should recognize that operating costs will
be greater or yield losses will result when systems which apply
water and chemicals nonuniformly are operated. A lower initial
system cost which sacrifices uniformity of water application may be
false economy. A technique for field evaluation of the uniformity of
water application by trickle irrigation systems is available as IFAS
Extension Bulletin 197, "Field Evaluation of Trickle Irrigation
Systems: Uniformity of Water Application", and as computer software
from the University of Florida.
Proper irrigation scheduling will help to assure efficient use of
water and energy in crop production. Irrigation scheduling methods
that are currently applicable in Florida are 1) a water budget
method requiring estimation of daily ET and soil water content, and
2) the use of soil moisture measurement instrumentation. Techniques
for estimating ET, determining soil water storage, determining
allowable water depletions, and water budgeting were described.
When properly used and combined with efficient methods of water
application, these techniques should also result in increased produc-
tion and profits.
Doorenbos, J. and W.O. Pruitt. 1977. Crop water requirements.
FAO Irrigation and Drainage Paper No. 24. Food and Agric.
Organiz. of the U.N. Rome.
Fereres, E., D.W. Henderson, W.O. Pruitt, W.F. Richardson and R.S.
Ayres. 1981. Basic irrigation scheduling. Leaflet 21199. Div. of
Agric. Sci., Univ. Calif.
Izuno, F.T. 1987. Water budgeting for high water table soils. Ext.
Cir. 769. IFAS, Univ. Fla.
Izuno, F.T. and D.Z. Haman. 1987. Basic irrigation terminology.
Agric. Engr. Dept. Fact Sheet AE-66. IFAS, Univ. Fla.
Jones, J.W., L.H. Allen, S.F. Shih, J.S. Rogers, L.C. Hammond, A.G.
Smajstrla and J.D. Martsolf. 1984. Estimated and measured
evapotranspiration for Florida climate, crops, and soils. Bul. 840
(Tech.) IFAS, Univ. Fla.
SCS Technical Staff. 1970. Irrigation water requirements. Tech.
Release 21. U.S. Dept. of Agric., Soil Conservation Service.
Smajstrla, A.G., G.A. Clark, S.F. Shih, F.S. Zazueta and D.S.
Harrison. 1984. Potential evapotranspiration probabilities and
distributions in Florida. Ext. Bul. 205. IFAS, Univ. Fla.
Smajstrla, A.G. and D.S. Harrison. 1984. Measurement of soil water
for irrigation management. Ext. Cir. 532. IFAS, Univ. Fla.
Smajstrla, A.G., D.S. Harrison and F.X. Duran. 1985. Tensiometers
for soil moisture measurement and irrigation scheduling. Ext. Cir.
437. IFAS, Univ. Fla.
Smajstrla, A.G., D.S. Harrison and F.S. Zazueta. 1985. Field
evaluation of trickle irrigation systems: Uniformity of water
application. Ext. Bul. 195. IFAS, Univ. Fla.
Smajstrla, A.G., D.S. Harrison and F.S. Zazueta. 1985. Agricultural
water measurement. Ext. Bul. 207. IFAS, Univ. Fla.
Zazueta, F.S., A.G. Smajstrla and D.Z. Haman. 1987. Evapotran-
spiration estimation utilities. Cir. 744, Computer Series. IFAS,
Zazueta, F.S., A.G. Smajstrla and D.S. Harrison. 1984. Glossary of
trickle irrigation terms. Agric. Engr. Dept. Fact Sheet AE-45.
IFAS, Univ. Fla.
MARSTON SCIENCE LIBRARY
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE -
OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste, Director, in coopera-
tion with the United States Department of Agriculture, publishes this information to
further the purpose of the May 8 and June 30,1914 Acts of Congress; and is authorized
to provide research, educational information and other services only to individuals and
institutions that function without regard to race, color, sex, age, handicap or national
origin. Single copies of extension publications (excluding 4-H and youth publications)
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bulk rates or copies for out-of-state purchasers is available from C.M. Hinton, Publications Distribution
Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing this
publication, editors should contact this address to determine availability. Printed 2/93.