Water is an important factor affecting crop yields. The University of Missouri Extension Service developed an on-line program (app) to help farmers produce higher crop yields by improving irrigation management. There is no charge to farmers. This bulletin gives information about the Crop Water Use (CWU) application and explains how to set up fields to track soil moisture.
How Does The Application Work? The application estimates crop water use from weather data. An equation for calculating evaporation from soil and plants (called evapotranspiration, ET) is used. The application also calculates daily soil water deficit balances for each field. Reports include indexes to help farmers determine when to irrigate.
Crop Water Use. The University of MO Extension automatically downloads data each day from a mesonet of agricultural weather stations located across Missouri. ET is calculated from solar radiation, temperature, humidity, and wind. CWU uses the Standardized short crop Penman-Monteith Evapotranspiration (ETo) equation which was developed by a task committee of the American Society of Civil Engineers. ETo is the amount of combined water lost from a reference crop (grass) and soil evaporation.
ETo is multiplied by a crop coefficient, which is specific for the crop and growth stage. The crop coefficients, except for rice, come from UN Food and Ag Organization publication FAO-56. Beginning at planting, growth stages are predicted from heat units for corn, rice, and cotton. Calendar days are used for soybean. This information is used to estimate daily crop water use (ETc).
Soil Water Balance. Daily soil water deficit is reported similar to a checkbook registry which tallies rainfall and irrigation as deposits and ETc as withdrawals. The soil water deficit becomes more negative in periods of low rainfall. When the soil profile is full (field capacity), the deficit balance is 0. As water is extracted, the balance becomes negative.
Acknowledgements. Funds for field research to validate this app were provided by Cotton Incorporated, Howard G. Buffett Foundation, and United States Department of Agriculture- Agricultural Research Service.
The app utilizes weather data from the Missouri Agricultural Weather Station network. For this reason, currently, it will not work outside of the state of Missouri.