Water is essential for plant growth, and too little or too much water can limit crop yield. It is important, therefore, to understand water use by crops. Crop water use occurs via evapotranspiration, which is the transfer of water from soil, plant, and water surfaces to the atmosphere as water vapor. Evapotranspiration, or ET, is the combination of evaporation (E) from wet soil and plant surfaces or standing water and transpiration (T), which is the movement of water from the root zone into the roots, through the plant, and to atmosphere through the plant leaves. In practice, it is difficult to separate out the evaporation and transpiration components from a crop field, so they are usually considered together as ET. Most of the water received as precipitation will return to the atmosphere as ET. The difference between ET and precipitation that is stored in the root zone determines irrigation requirements, so good estimates of ET throughout the growing season are crucial in proper irrigation scheduling.
Above: Graphic adaptation of photo by Lynn Betts, USDA Natural Resources Conservation Service
There are a number of plant, soil, and management factors that will affect ET. However, ET is primarily driven by weather conditions, including solar radiation, temperature, humidity, and wind speed. Data from weather stations that measure all of these parameters can be used to estimate ET. In South Dakota, the South Dakota State Climate Office maintains an automated weather data network of stations that can be used for estimating ET. Within the High Plains region, the High Plains Regional Climate Center provides weather data and ET estimates for a six-state area.
Since different crops transpire at different rates, a common practice is to use weather station data to calculate the ET of a reference crop (reference ET or ETref) and then use a crop coefficient (Kc) to adjust the reference ET to estimate the actual ET of the crop of interest (crop ET or ETc). There are two reference crops that are typically used for calculating reference ET: alfalfa (commonly used in the western US) and cool season grass (generally used in the eastern US and worldwide). Reference ET represents the atmospheric demand for water. The crop coefficient integrates crop height, growth stage, and other factors into a single term that is multiplied by the reference ET to estimate the actual crop ET:
ETc = ETref × Kc
There are crop coefficients available for most crops. However, crop coefficients are developed for use with either the alfalfa or grass reference crop, so it is important to match the crop coefficient with the reference crop it was developed for. A good source of crop coefficients for use with alfalfa reference ET is from the High Plains Regional Climate Center.
Additional information on using reference ET and crop coefficients can be found in the UNL Extension publication Estimating Crop Evapotranspiration from Reference Evapotranspiration and Crop Coefficients.
Many agricultural weather networks will report both reference ET and crop ET for common crops, which saves the user from having to make those calculations. This is true for South Dakota, where the State Climate Office reports alfalfa reference ET and crop ET for corn and soybeans on its "Daily Crop Water Use" http://climate.sdstate.edu/awdn/et/et.asp page. The South Dakota State Climate Office site is currently undergoing an upgrade, which will include improvements and additional features for the crop water use tools. Additionally, a new service from the National Weather Service provides forecasting of ET, and this is described in a separate article on ET forecasting: http://igrow.org/agronomy/corn/forecasting-crop-water-use/