(Editor’s note: Producers often like to know what kind of yield they might expect from a given level of expected available water during the crop season for various crops. This is true for both dryland and irrigation production. Daily and seasonal variations from the basic long-term trends will affect irrigation scheduling in any given year. This can be accounted for by using a computer program such as K-State’s KanSched. The following is an excerpt from a new K-State publication that discusses this, and other water-related topics, Agricultural Crop Water Use, L934: http://www.ksre.ksu.edu/bookstore/pubs/L934.pdf/ The figure and table numbers shown in this article are those used in the actual publication. – Steve Watson)
Yield and crop water use are closely linked and linearly related; meaning the more crop evapotranspiration (ET), the more yield until the production limit is reached. This is illustrated in Figure 8.
Basic crop water use curve
Threshold ET is the amount of crop water use needed to grow the crop until the seed-producing segment of the yield. In forage crops, when the entire above-ground portion of the crop is harvested, the threshold ET would be zero, and the y axis would be the weight of dry matter production. Often the crop water production function is referred to as the crop water use curve, which is the curvilinear line of Figure 8.
This line includes the ET amount plus additional water applied to a field either by rainfall or irrigation but was lost to runoff, drainage, or evaporation. Drainage water also is called deep percolation and is water that moves past the crop root zone and therefore cannot be accessed by the plant. The water use curve represents the average long-term yield response of a crop for a particular location. The crop’s root depth and the soil water holding capacity determine the amount of water that can be held in the soil for the crop to use.
When rainfall and/or irrigation water is added to the field in amounts that keep this water availability in the upper range for optimum growth, there is less room for water storage in the soil; also, wetter soils have slower infiltration rates. Both of these factors increase the potential loss of water due to drainage or runoff when it rains.
Irrigation water applications should be scheduled so no water is applied unless there is sufficient root zone soil water storage available for the application; however, every irrigation system has an associated efficiency, which means some applied water may not be used by the crop. The general objective of irrigation is to keep the soil water in the optimum range, so less storage of rainfall after an irrigation event may occur, since perfect weather forecasts are not possible.
Crop yield and water use relationships for important Kansas crops are shown in Figure 9, with the threshold ET values and yield slope shown in Table 4. Corn, soybeans, grain sorghum, and sunflowers are all spring-planted, summer-grown crops, while winter wheat is fall-planted, grows until winter dormancy, resumes growth in spring, and matures in early to mid-summer. Corn and grain sorghum are generally used as feed grains, although they are also stock for ethanol production. Corn tends to be grown in areas with irrigation or higher rainfall instead of grain sorghum due to higher yield potential. Grain sorghum initiates grain yield at a lower ET threshold, which can give it a production advantage over corn in lower rainfall areas under dryland or limited irrigation conditions.
Seasonal variations and implications for irrigation scheduling
The crop water production functions are useful planning tools but represent the long-term response of crops to growing conditions. Crop water use varies based on the seasonal weather conditions. This is illustrated by a long-term water use study on corn at Garden City. The study had six levels of irrigation treatment, as shown in Table 5.
Figure 10 shows the yields for each of the six irrigation treatments for each of the seven years of the study. The precipitation ranged from above normal to extreme drought at the site during the study period. Yield for the higher water treatments were generally good, although in some years, yield was suppressed due to hail.
In general, notice the variation of yield decreases with increasing irrigation. This is more easily seen in Figure 11, which shows the relative yield of the study. Relative yield is the yield of an individual treatment divided by the maximum yield of the year multiplied by 100 to make it a percentage. This removes the year-to-year yield variation effect. The irrigation application depths for the highest yield level ranged from about 8 inches to about 19 inches (the seven 100 percent yield data points of Figure 11), which dramatically illustrates the need to schedule irrigation using current-year conditions versus long-term averages.
The individual year relative yields are shown in Figure 12; note the yield response curve of 2011, the drought year. This was the only year with yield failure at the dryland treatment level. More than 7 inches of irrigation was needed to achieve 20 percent relative yield level, just slightly less than the full irrigation treatment application in 2009.
The range of full irrigation treatment application depth demonstrates the need to use some form of irrigation scheduling. The day-to-day variation in water use, when combined with seasonal rainfall variations, can result in wide fluctuation of the annual irrigation requirement. KanSched, an ET-based irrigation scheduling program, is available to assist producers in scheduling irrigation:
Danny Rogers, Biological and Agricultural Engineering
Jonathan Aguilar, Southwest Research and Extension Center Water Resources Engineer
Isaya Kisekka, Southwest Research and Extension Center Irrigation Research Engineer
Philip Barnes, Biological and Agricultural Engineering
Freddie Lamm, Northwest Research and Extension Center Irrigation Research Engineer
Loyd Stone, Soil and Water Agronomist, professor emeritus (pending)