Crop water use, also known as evapotranspiration (ET), represents soil evaporation and the water used by a crop for growth and cooling purposes. This water is extracted from the soil root zone by the root system, which represents transpiration and is no longer available as stored water in the soil. Consequently, ET is used interchangeably with crop water use.
The evapotranspiration process is composed of soil evaporation (E) and transpiration (T). Transpiration is the water transpired or lost to the atmosphere from small openings on the leaf surfaces. Evaporation is the water evaporated or lost from the wet soil and plant surfaces. Significant evaporation can take place only when the soil's top layer (1-2 inches) or plant canopy is wet. Once the soil surface is dry, evaporation decreases sharply. Thus, significant evaporation occurs after rain or irrigation. Furthermore, as the growing season progresses and canopy cover increases, evaporation from the wet soil surface gradually decreases. When the crop reaches full cover, approximately 95 percent of ET is due to transpiration and evaporation from the crop canopy where most of the solar radiation is intercepted.
Prevailing weather conditions, available water in the soil, crop species, and growth stage influence crop water use. At full cover, a crop is at the maximum ET rate (reference ET or potential ET [PET]) if soil water is not limited, namely, if the soil root zone is at field capacity. Different crops reach full cover at different growth stages and times after planting. To standardize ET measurements and calculations, a PET or reference crop ET (ETr) is used to estimate actual ET (ETa) for other crops. In humid and semihumid areas where water usually is not a limiting factor, grass is used as a reference ET crop. In arid or semiarid areas, alfalfa is more suitable as a reference ET crop because it has a deeper root system, which reduces its susceptibility to water stress resulting from dry weather. Also, the pan evaporation is used to estimate PET. The standard is a large pan of water in or near a field. Figures 1 and 2 show corn and soybean ET factors, respectively, over a growing season. The ETa is the water use of a particular crop at a given time. ETa of an annual crop reaches its maximum at full canopy and can be higher or lower than PET, depending on the crop. Actual ET can be calculated by multiplying PET by crop coefficient (KC). A crop coefficient is the ratio between ETa of a particular crop at a certain growth stage and PET. If the crop coefficient is less than one, the crop uses less water than PET.
Fig. 1. Variation of corn ET factor over the growing season. For example, on June 7 ET from a cornfield would be 40 percent of measured pan evaporation. On August 1 it would be 82 percent.
From Shaw, R.H. 1982. Estimation of soil moisture under corn. Research Bulletin 520, Department of Agronomy , Iowa State University.
Fig. 2. Variation of soybean ET factor over the growing season.
The actual soil water content also influences crop water use. As soil dries, it becomes more difficult for the plant to extract water from the soil. At field capacity (maximum water content), plants use water at the maximum rate. When the soil water content drops below field capacity, plants use less water. Crops differ in their response to water stress at a given growth stage. Different crops have different water requirements and respond differently to water stress. The range of water use for crops varies from one area to another. However, we can use some guidelines to determine crop water use. If we assume that corn uses approximately 0.3 (Table 1) of the pan evaporation at emergence, crop water use will increase as the crop reaches pollination, then water use will drop gradually. For soybeans, the factor would be 0.6 at 40 days after planting. When the canopy closes on the soybean field, a maximum value of 1.1 (110 percent of the water from the pan) is used 70 days after planting until leaves turn yellow. Then water use drops off. The total amount of water used by a corn and a soybean crop would normally be the same (22.3, 21.5, and 22.1 inches in north central, southwestern, and southeastern Iowa, respectively). However, the soybeans tend to use water a little later in the season than does corn. The critical growth stages of corn for water use will be at tassel until grain is fully formed. The highest water use by corn will be during July and August. For soybeans, critical growth stages for water availability are during bloom and fruit set. Shortage in moisture supply during these growth stages will cause yield reduction.
Table 1. Crop stage and relative water use as a portion of pan evaporation.
|Crop||Factor||Date or Stage|
|0.75||July 15 to silk|
|Soybeans||0.6||40 days after planting|
|0.9||60 days after planting
(when canopy closes)
|1.1||70 days through leaf turning|
This article originally appeared on pages 85-86 of the IC-484(11) -- May 29, 2000 issue.