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Water Use and Irrigation Management of Agronomic Crops 1
E. B. Whitty and C. G. Chambliss2To obtain maximum yields from agronomic crops, plants should remain relatively free of water stress. Although different crops may vary in their responses to water deficits, the amount of water used by a crop is closely associated with final vegetative and grain yield.
EVAPOTRANSPIRATION (ET)
Evapotranspiration (ET) is a term used to describe the water loss from land on which vegetation is growing. The evaporation component (evapo-) of ET is the process by which water in the soil is changed to the vapor state and moved into the atmosphere. This is the same evaporation process that results in water being lost from the surface of a lake or an ocean.The second component of ET (-transpiration) refers to the vaporization and loss of water from the leaves of a crop through the small pores or stomata in the leaf. Although this is also an "evaporation process," it is termed transpiration because the evaporated water has been taken up by the plant roots from the soil, moved up through the plant stem and evaporated from the plant leaves.
If the amount of water which is evaporated from a given soil surface area (the evapo- part) is added to the amount of water which is transpired from the leaves above that soil surface (the -transpiration part), the resulting amount is the total amount of 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).
At planting time, ET rates consist 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 results from transpiration from the crop's leaves. When leaves completely shade the soil surface, ET consists largely of transpiration. Actually, during most of the growing season of typical agronomic crops, transpiration is responsible for the largest portion of the water loss from the field. Even during early crop development when the soil surface is exposed to direct sunlight, evaporation is small if the soil surface is dry. Clearly, the largest seasonal requirement for water in most field crop situations is to supply transpirational needs and not for evaporation from the soil surface.
Seasonal ET
Calculated seasonal ET values for several agronomic crops range from about 15" (38 cm) for tobacco to approximately 49" (124 cm) for sugarcane (Tables 1 and 2 ). For most agronomic crops that produce leaf canopies which 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 water stress imposed on the crop, and the length of the growing season. Net irrigation requirements (NIR) (see equation below) necessary to satisfy the ET needs in 80% of the crop years are also given in Tables 1 and 2 . For example, in 8 of 10 years for corn, 12" (25 cm) of irrigation water supplementing rainfall would provide adequate water to meet the ET demand of 25" (64 cm). Since ET is quite responsive to many weather variables including radiation, temperature, humidity, and wind speed 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 values representing average environmental and crop conditions. Also the NIR values given in Tables 1 and 2 do not consider irrigation efficiency. See the section on "Efficient Irrigation Management" for further information.SENSITIVE CROP GROWTH STAGES FOR WATER DEFICITS
Severe water stress at any developmental stage of crops will usually result in some growth reduction. However, certain stages of growth are sensitive to even slight water stresses. Knowledge of these particularly 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 3 gives a summary of crop growth stages which are most sensitive to water stresses, the approximate days after planting at which the critical stages occur, and the expected maximum daily water use rates during the indicated periods. Days after planting and daily water use rates are only to be used as general guidelines since dates of planting, variety, plant population, and numerous environmental factors will cause the actual values to vary. The ET estimates given, however, are representative of a typical crop planted at a recommended date and population on a relatively clear day during the indicated crop growth stage. Generally, the ET requirements during the most sensitive growth stages are similar for the various crops and range between 0.20 and 0.28" (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 stresses are usually the growth stages during which either seed numbers or seed weights are being established (Table 1 ). Crop yield is generally less reduced by water stress if the stress occurs before the reproductive stages of growth.
Some crops, including corn, sorghum, and small grains, have relatively short periods of growth during which seed numbers are determined, and severe water stress at this 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 severely affected by short term stresses. For a crop such as tobacco, where leaf production is most important, water stresses during most growth stages can be detrimental to yield.
Water deficits may also affect crop management and production other than the direct effect on plant growth. The efficacy of many herbicides and other pesticides depends on soil moisture. Plants under moisture stress may not respond to foliar applied chemicals, or in some cases, may be damaged by chemical burns. Nutrient utilization and fertilization practices are influenced by the moisture status of the crop plants. Application of pesticides must be scheduled according to irrigation applications or to moisture stress in the crop.
EFFICIENT IRRIGATION MANAGEMENT
The most sensitive growth stages for most crops coincide with time intervals during which the crop is also utilizing the most water (Table 3 ). The most important 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 period of a few days. This emphasizes the importance of designing the irrigation system so that it will be able to apply water in amounts and at rates sufficient to supply maximum ET demands. Furthermore, all of the water which is pumped does not become available for ET. Some water is unavoidably lost during the delivery to the crop. In Florida, most 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 a 75% efficiency, if 1" (2.5 cm) of water is pumped, only 0.75" (1.9 cm) reaches the soil surface and is available for ET. To apply 1" (2.5 cm) to the crop, 1.33" (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. Irrigation efficiency can be improved by use of low pressure systems and by irrigating at night.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 added attention should be given to irrigation management decisions during those stages.
Some general guidelines for irrigation management of several agronomic crops are given in the following paragraphs. On the coarse-textured soils dominating much of Florida, more frequent irrigations with smaller amounts of water (1" or less) allow for more efficient storage of rainfall which may occur shortly after 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 used for crop transpiration. Efficient irrigation requires careful management and is attainable if an understanding of water use and stress responses of the crop is applied.
Corn, Tobacco, and Peanuts
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" (15 cm) deep in the crop root zone. When the soil tension at that depth approaches 20 to 25 centibars, irrigation water should be applied. It is well documented that corn is extremely sensitive to water stresses during silking and tasseling, but research also indicates that 2 weeks of midday wilting during early vegetative growth can reduce yields by as much as 10 to 15%. Irrigation of peanuts when tensiometers at 12" (30 cm) depths indicate soil tensions of 30 centibars has also proven effective. Tensiometers allow the manager to apply irrigation water before crop stress symptoms become visible. If tensiometers are not utilized, an accounting method can be quite effective.Accounting methods are more practical for determining when to irrigate with overhead sprinklers than are tensiometers and other moisture-sensing devices placed in the soil. Accounting methods are much like keeping a bank account ledger, in that records of rainfall and irrigation are maintained and water use by the crop is estimated. Water use estimates can be based on the experience of the irrigation manager, but may become difficult when several systems at various locations must be monitored. Use of weather instruments can improve the reliability of water use estimates and can be coupled with computer programs that are available to quickly provide reliable irrigation recommendations. In general, when using the accounting method, irrigation should begin when 50 percent of the available water in the root zone is depleted. Estimates of the water-holding capacity of the soil and changes in the rooting depth as the crop grows must be estimated, if not actually measured.
Estimates are based on soil type and are available from soil surveys and other references. In general, sandy soils contain less than an inch of available water per foot of depth, while soils that contain significant levels of clay or organic matter can hold well over an inch of available water per foot of depth. The rooting depth of the crop varies with species, age of plant, and soil properties. More detailed information on irrigation scheduling can be found in Extension Bulletin 249 Basic Irrigation Scheduling in Florida (http://edis.ifas.ufl.edu/AE111).
Soybeans
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 2 weeks of midday wilting during vegetative growth resulted in only small (2 to 5%) yield reductions. Thus, it appears that some water stress can be tolerated by soybeans during vegetative growth without significantly reducing yields, but more liberal applications of irrigations water are necessary from early pod-set to maturity.Sugarcane
There are two factors to consider in reviewing the water requirements of sugarcane; one is the actual amount of water required to produce the sugarcane, the other is the management of the water table in the cane field.Approximately 80% of all sugarcane grown for commercial sugar production in Florida is grown on organic soils in which the water table should be maintained at as high a level as possible, without reducing yields, in order to reduce soil subsidence. A water table depth of 24" (61 cm) or greater is usually maintained on well-decomposed organic soils. According to a seven-year study conducted at the Everglades REC, a water table of 30" (76 cm) resulted in the best sugar tonnage per acre, but a water table of 15" (38 cm) reduced production only 5%. The rate of subsidence with a 20" (51 cm) water table for example, is less than half of the rate for soil with a 36" (91 cm) water table.
A higher water table maintained to reduce subsidence will require more water for irrigation during the dry season and more pumping for drainage during the wet season.
The actual amount of water required to produce 2.2 lb (1 Kg) of cane ranged from 196 lb (89 Kg) for plant cane (freshly planted sugarcane) to 260 lb (118 Kg) for ratoon cane (sugarcane which was harvested and allowed to grow back). The water use efficiency for the ratoon cane is less than that for the plant cane. The water required to produce 2.2 lb (1 Kg) of sugar ranged from 1948 lb (884 Kg) in plant cane to 2485 lb (1115 Kg) in the ratoon cane.
REFERENCES
Harrison, Dalton S., Memorandum on "Water use (ET) Values for Crops and NIR (Net Irrigation Requirement) Values at 50%, 80%, and 90% Rainfall Probabilities." February 1, 1983.Florida Agriculture in the 80's Commodity Committee: Sugarcane Committee Report.
Snyder, George H. et al, Water Table Management: For Organic Soil Conservation and Crop Production in the Florida Everglades, Bulletin 801, Agricultural Experiment Station, IFAS, University of Florida, pp. 14-15.
Shih, S. F. and Gascho, G. J., Water Requirement for Sugarcane Production. Transactions of the ASAE, American Society of Agricultural Engineers, St. Joseph, Michigan, Vol. 23, No. 4, 1980, pp. 937.
Tables
Table 1. Seasonal evapotranspiration and net irrigation requirements for several agronomic crops.1
Crop
Seasonal ET (in.)
NIR-80
Corn
25
12
Grain Sorghum
20
6
Peanuts
22
7
Soybeans
23
7
Small Grains
20
9
Tobacco
15
7
1 From Rogers and Harrison, Water Resources council Report No. 5 as calculated from U. S. Soil Conservation Service, Technical Release No. 21.
Table 2. Consumptive use, or ET (Evapotranspiration) of sugarcane for Everglades area of Florida.
Month
ET
NIR-80
January
1.4
0.5
February
1.1
0.5
March
2.5
0.9
April
3.4
1.8
May
4.8
1.7
June
6.0
1.2
July
6.5
1.6
August
6.7
1.7
September
5.1
0.7
October
5.2
2.2
November
3.2
1.7
December
2.6
1.5
Total
49.5
17.9
NIR-80 = Net irrigation at 80% rainfall probability. Divide by irrigation efficiency for gross irrigation requirements. For sugarcane, seepage irrigation is used and the efficiency is 30 to 50%. Table 3. Sensitive growth stages, dates of occurrence, and maximum daily water use required of several agronomic crops.
Crop
Sensitive Growth Stage1
Approx. Days
After Planting
Expected Maximum Water Use Requirements During Critical Growth Stage2 (in/day)
Tobacco
2 to 3 week period near flowering3
50 - 65
0.22 - 0.25
Corn
Tasseling and silking 65 - 75
0.22 - 0.28
Sorghum
Early boot through bloom
45 - 70
0.20 - 0.25
Peanuts
Flowering through completion of pod set
45 - 90
0.22
Soybeans
Early to late bean fill
50 - 100
0.20 - 0.25
1 Growth stage at which yield is most sensitive to water stress. 2 Value should only be used as estimates for maximum rates since many environmental factors effect 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 period.
3 Represents maximum water use period. Data are limited for growth stage sensitivities.
Footnotes
1. This document is SS-AGR-155, one of a series of the Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First printed December 1992. Revised April 2002. Reviewed March 2005. Please visit the EDIS Web site at http://edis.ifas.ufl.edu.2. E. B. Whitty, professor and C. G. Chambliss, associate professor, Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.
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