4.6: Crop Coefficients

Evapotranspiration from crops depends on the type of crop, stage of growth, water content of the soil, and the amount of energy available to evaporate water. The reference crop evapotranspiration rate (ET_o) is used to represent the amount of energy available. The ET for crops is computed relative to the ET_o using the crop coefficient (K_c):

ET = K_c ET_o

The crop coefficient consists of the basal crop coefficient (K_co) which represents crops with adequate soil water to maintain transpiration tempered by a stress factor (K_s) to account for water stress and a factor (K_w) to adjust for increased evaporation from a wet soil surface. The crop coefficient is calculated by:

K_c = K_co K_s + K_w

where: K_co = basal crop coefficient for unstressed crops with a dry soil surface, K_s = stress factor to account for effects of water stress on ET, and K_w = soil wetness factor to account for increased evaporation from wet soils. The crop coefficient depends on the growth and development of the crop canopy. A measure of crop canopy development is the leaf area index (LAI). The LAI is the ratio of the amount of leaf area relative to the underlying land area. For example, if the total surface area of one side of the leaves is 2,600 in2 for a 3-ft square area of a field (i.e., 1,296 in² ), then the LAI is about 2. The maximum LAI for many irrigated crops often exceeds 5 but depends on the crop variety, plant density and geographical location of the field. An example of the LAI for an annual crop during the year is illustrated in Figure 4.13. The basal crop coefficient (K_co) resembles the LAI curve during the season (Figure 4.13). Early in the growing season, the basal crop coefficient is small for an annual crop. As the crop sprouts and seedlings start to grow, transpiration contributes a larger portion of daily water use, thus, the crop coefficient increases with canopy development. At some point the canopy develops sufficiently so that the crop coefficient reaches a maximum value. This time is referred to as the effective cover date. After effective cover, the crop coefficient is essentially constant for a period even though the plant canopy continues to expand. The crop coefficient decreases as the crop matures and leaves senesce. For crops that are harvested before senescence, the crop coefficient may remain at or near the peak value until harvest. Some perennial crops, such as citrus, maintain a near constant canopy from one season to the next year. Conversely, some perennial crops, such as fruit trees and grasses, emerge from dormancy and develop vegetation during the initial periods of growth. The initial crop coefficient for a crop breaking dormancy is often higher than for annual plants. When the plant canopy is small the soil surface is not completely shaded and evaporation from wet soil contributes significantly to ET. When the soil surface is dry the rate of evaporation is small. Following a rain or irrigation the evaporation rate increases. Therefore, the crop coefficient increases immediately following a rain or irrigation (Figure 4.13). As the soil dries the crop coefficient decreases back to the rate for dry soil surfaces. As the canopy expands, the crop shades larger portions of the soil surface and absorbs energy that earlier would have been used to evaporate water from the soil. The effect of evaporation from wet soil, therefore, decreases as the canopy develops. The crop ET rate decreases when plants are stressed by lack of water. Processes involved in reducing ET are complex but relate to the increased difficulty for the plant to extract soil water. For computing irrigation water requirements, the effect of water stress on ET can be estimated by decreasing the crop coefficient as in Figure 4.13.

Figure 4.13. General shape of crop coefficient curve for an annual crop and the relationship to leaf area index.