Water Use and
Of Agronomic Crops
J. M. Bennett, D. S. Harrison,
and A. G. Smajstria
Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
John T. Woeste, Dean for Extension
WATER USE AND IRRIGATION MANAGEMENT
OF AGRONOMIC CROPS
J. M. Bennett, D. S. Harrison, and A. G. Smajstrla*
To obtain maximum yields from agronomic crops requires that the
crop remain relatively free of water stress during its growth.
Although different crops may vary in their response to water deficits,
the amount of water used by a crop is generally closely associated
with final vegetative and grain yield of the crop. This occurs because
loss of water from plant leaves occurs through the same pores
stomataa) that allow carbon dioxide to move into the leaf where it
is fixed by photosynthesis. Thus, crop growth (photosynthesis) is link-
ed to crop water use (transpiration).
Evapotranspiration (ET) is a term used to describe the water loss
from land on which vegetation is growing. The evaporation compo-
nent (evapo) of ET is the process whereby water in the soil is chang-
ed to vapor and lost to the atmosphere. This is the same evapora-
tion process associated with water loss from the surface of a lake
or an ocean.
The second component of ET (transpiration) refers to the
vaporization and loss of water from plant leaves through the small
pores or stomata in the leaf. Although this is also an evaporation
process, it is termed transpiration because the evaporated water
comes from the crop's leaves.
If the amount of water evaporated from a given soil surface area
(the evapo part) is added to the amount of water transpired from
the leaves (the transpiration part), the result is the total water loss,
or ET. Thus, ET is composed of evaporation from the soil plus
transpiration from plant leaves. Values of ET for a crop are usually
expressed as the amount of water lost (inches, cm, mm) per unit of
time (hour, day, week, month, season, or year) over any unit of area.
At planting time, ET is comprised only of evaporation of water
from the soil surface. As the crop emerges and begins to develop
leaf area, an increasingly larger portion of ET comes from transpira-
tion from the crop's leaves. When leaves completely shade the soil
surface, ET consists primarily of transpiration. Actually, during most
of the growing season of typical agronomic crops, transpiration is
responsible for the largest portion of the water lost from the field.
*Assistant Professor of Agronomy, Professor, and Associate Professor of
Agricultural Engineering, respectively, IFAS, Gainesville, Fl. 32611.
Even during early crop development when the soil surface is exposed
to direct sunlight, evaporation is small when the soil surface is dry.
Clearly, the largest seasonal requirement for water in most field crop
situations is that required to meet the transpirational needs of the
plant and not for evaporation from the soil surface.
Calculated seasonal ET values for several agronomic crops are
given in Table 1 and range from about 15 inches (38 cm) for tobacco
to approximately 25 inches (64 cm) for corn. For most agronomic
crops which produce leaf canopies that fully cover the soil surface,
variations in the amounts of water required for ET are primarily
dependent on the time of season during which the crop is grown,
the energy (radiation from sun, dryness of air, wind, etc.) imposed
on the crop, and the length of the growing season. Net irrigation
requirements (NIR) necessary to supplement rainfall and satisfy the
ET requirements in 80% of the years are given in Table 1. For ex-
Table 1. Seasonal Evapotranspiration and Net Irrigation Requirement for
Several Agronomic Crops1.
Crop ET (in.) NIR 80%
Corn 25 12
Grain Sorghum 20 6
Peanuts 22 7
Soybeans 23 7
Small Grains 20 9
Tobacco 15 7
'From Rogers and Harrison, Water Resource Council Report No. 5 as calculated
from U.S. Soil Conservation Service, Technical Release No. 21.
ample, in 8 of 10 years for corn in Florida, 12 inches (25 cm) of ir-
rigation water supplementing rainfall would provide adequate water
to meet the crop's ET demand of 25 inches (64 cm). Because ET is
quite responsive to many weather variables including radiation,
temperature, humidity, and windspeed and also to numerous crop
characteristics, values of both ET and NIR will vary from season to
season. Data reported in Table 1 must only be considered as repre-
senting average environmental and crop conditions.
Efficient irrigation management requires some knowledge of
rather short-term ET rates. Daily water use rates as influenced by
crop age for tobacco, soybean, sorghum, peanut, and corn are
presented in Figures 1 to 3. Data for tobacco were obtained from
' (from: Harrison and Whitty, Extension circular no. 270 A)
3 5 70 9 II 13
Weeks After Transplanting
S.04 (from: calculations using Penman method)
10 30 50 io70 90 110 130
Days After Emergence
Figure 1. Daily water use rates for tobacco (upper) and soybean (lower)
as affected by crop growth. Data are for a late March planting
date for tobacco and an early June planting date for soybean.
st p second crop
o (from' Doss and Ashley, 1965. USDA-ARS publ. no. 41-112)
15 15 15 15 15 15 15
Apr. May Jun. Jul. Aug. Sep. Oct.
. 04 (from' Stansell etal. 1976. Peanut Sci. 3'44-48).
20 40 60 80 100 120 140
Days After Emergence
Figure 2. Daily water use rates for a ratoon sorghum (upper) and peanut
(lower) crop as affected by growth stage.
.32- CORN full season hybrid
. 24 ,' short season hybrid
.04 -- (from: calculations using Penman method)
*-- (from: Wright, Rhoods, and Stanley, Extension circ. no. 486)
10 30 50 70 90 110 130
Days After Planting
Figure 3. Daily water use rates as affected by growth stage for early
March planted corn.
experiments conducted at Gainesville, Florida, and reported in IFAS
Extension Circular 270A. The curves for soybean and corn (dashed
line) were calculated by the Penman method using average weather
data for north central Florida. Water use by a short season corn
hybrid is also plotted in Figure 3 using data reported by Wright,
Rhoads, and Stanley in IFAS Extension Circular 486. The sorghum
and peanut water use rates were obtained from experiments con-
ducted at Thorsby, Alabama, and Tifton, Georgia, respectively.
Rates of ET for most agronomic crops tend to be low during the
early growth stages, increase significantly as leaves attain full
coverage of the soil surface, and generally decline slightly as leaves
senesce and maturity approaches. Although ET may be somewhat
variable from day to day depending on environmental conditions,
information in Figures 1 to 3 should serve as a general guideline and
may be of value for irrigation scheduling decision-making.
Sensitive Crop Growth Stages for Water Deficits
Severe water stress at any crop developmental stage will result
in some growth reduction. However, certain stages of growth are
sensitive to even slight water stresses. Knowledge of these particular-
ly sensitive growth stages and ET rates during these growth periods
can be helpful when deciding whether to irrigate or delay for a few
days in anticipation of rainfall.
Table 2 gives a summary of crop growth stages which are most sen-
sitive to water stresses, the approximate days after planting at which
the critical stages occur, and the expected maximum daily water use
during the indicated growth periods. Days after planting and daily
water use rates are only to be used as general guidelines since date
of planting, variety, plant population, and numerous environmental
factors will cause the values to vary. The ET estimates given,
however, are representative of a typical crop planted at a recom-
mended date and population on a relatively clear day during the in-
dicated crop growth stages. Generally, the ET requirements during
the most sensitive growth stages are similar for various crops and
range between 0.20 and 0.28 inches (0.51 to 0.71 cm) per day.
For grain crops, yield is determined by both the total number of
seeds produced and by the weight of each seed. Thus, any stress
which causes a reduction in either the number of seeds produced
or the weight per seed will result in yield reductions. Growth stages
which are most sensitive to water stress are usually the growth stages
during which either seed numbers or seed weights are being
established (Table 2).
Growth periods during which seed numbers are determined are
relatively short for corn, sorghum, and small grains and severe water
stress at this crop growth stage may be quite detrimental to grain
yield. Conversely, crops such as soybeans and peanuts determine
seed numbers over relatively longer periods, and are not as severe-
ly affected by short term water stresses. For a crop such as tobacco,
where leaf production is most important, water stresses during most
growth stages can be detrimental to yield.
Efficient Irrigation Management
Generally, the most sensitive growth stages coincide with the
period during which the crop is also utilizing the most water (Table
2). Thus, proper irrigation management decisions must be made
when the crop is using large amounts of water and when the crop
may progress from being well-watered to severely stressed in a few
days. This emphasizes the importance of designing the irrigation
system to provide water in sufficient quantities to meet maximum
ET demands. Furthermore, all of the water which is pumped does
not become available for ET. Some water is unavoidably lost during
delivery to the crop. In Florida, many agronomic crops are irrigated
by sprinkler irrigation systems. Sprinkler systems deliver water with
approximate efficiencies of 70 to 75%, depending on the system and
environmental conditions. Therefore, for an irrigation system with
Table 2. Sensitive growth stages, dates of occurrence, and maximum daily water use requirements of several
Approximate Expected Maximum Water
Days After Use Requirements During
Crop Sensitive Growth Stage1 Planting Critical Growth Stage3
Tobacco Two to three week 50-60 0.22-0.25
period near flowering2
Corn Tasseling and silking 65-75 0.22-0.28
Sorghum Early boot through bloom 45-70 0.20-0.25
Peanuts Flowering through cor- 45-90 0.22
pletion of pod set
Soybeans Early to late bean fill 50-100 0.20-0.25
'Growth stage at which yield is most sensitive to water stress.
2Represents maximum water use period. Data are limited for growth stage sensitivities.
3Values should only be used as estimates for maximum rates since many environmental factors affect water use. The range in values given
for a particular crop represents values obtained from different experiments or changes associated with crop development during the critical
a 75% efficiency, if 1 inch (2.5 cm) of water is pumped only 0.75
inch (1.9 cm) reaches the soil surface and is available for ET. To apply
1 inch (2.5 cm) to the crop, 1.33 inches (1.0 0.75) must be pumped.
Amounts of water actually pumped must be increased above the ET
requirements (presented above) to allow for the delivery losses.
Although the preceding paragraphs have referred to critical crop
growth stages, this does not suggest that stress at other periods will
not reduce yields. The critical growth periods only imply that add-
ed attention should be given to irrigation management decisions dur-
ing those stages.
Some general guidelines for irrigation management of several
agronomic crops are given in the following paragraphs. Research has
indicated that corn and tobacco, two of the more sensitive crops to
water stress can be effectively irrigated on sandy soils with the aid
of tensiometers placed 6 inches (15 cm) deep in the crop root zone.
When the soil tension at that depth approaches 20 to 25 centibars,
water should be applied. It is well documented that corn is extremely
sensitive to water stresses during silking and tasseling, but recent
research also indicates that two weeks of midday wilting during early
vegetative growth can reduce yields by as much as 10 to 15%. Ir-
rigation of peanuts when tensiometers at 12 inches (30 cm) indicate
soil tensions of 30 centibars has also proven effective. The use of
tensiometers has been thoroughly described in several extension
reports (IFAS Circulars 486, 487, and 532). Tensiometers allow the
manager to apply irrigation water before crop stress symptoms
become visible. If tensiometers are not utilized, the accounting
method described in IFAS Extension Circular 431 is also quite
The appearance of midday wilt appears to be a reasonable indicator
for applying water for soybeans during reproductive growth.
However, research results have indicated that two weeks of mid-
day wilting during vegetative growth resulted in only small (2 to 5%)
yield reductions (IFAS Agronomy Facts No. 129). Thus, it appears
that some water stress can be tolerated by soybeans during
vegetative growth without significantly reducing yields, but more
liberal applications of irrigation water are necessary from early pod-
set to maturity.
On the coarse-textured soils dominating much of Florida, more fre-
quent irrigations with small amounts of water (1 inch or less) allow
for more efficient storage of rainfall which may occur shortly after
an irrigation. The goal of efficient irrigation management should be
to minimize the loss of water to runoff, deep percolation, and
evaporation and to maximize the portion of irrigation water which
is transpired by the crop. Efficient irrigation requires careful
management, and is attainable when an understanding of water use
and stress responses of the crop are known.
This publication was promulgated at a cost of $668.73, or 13 cents per
copy, to provide information on concepts of evapotranspiration and
water requirements of agronomic crops and to promote efficient ir-
rigation practices. 6-5M-84
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLOR-
IDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES,
K. R. Tefertlller, director, In cooperation 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 or national origin. Single copies of Extension publications (excluding 4-H and Youth
publications) are available free to Florida residents from County Extension Offices.
Information on 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.