About the Horizon Point
Crop Water Use Application

Water is one of the biggest factors 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. It will also help growers qualify for financial incentives to adopt weather-based irrigation scheduling.

Subscribers to the Crop Water Use (CWU) application are given a secure account and website link that can be saved to a desktop computer and mobile phone. You can share your unique link with others such as a crop scout or farm employee to monitor soil water deficit in your fields and update rainfall events and irrigation applications from their mobile devices.

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 soil dryness indexes to help farmers determine when to irrigate.

Crop Water Use. The University of MO Extension automatically downloads weather data each day from a network of 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 crop heat units for corn, rice, and cotton. Calendar days are used for soybeans. This information is used to estimate the daily crop water use (ETc).

Soil Water Balance. Daily soil water deficit is reported using a system similar to a checkbook 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 as ETc, the balance becomes negative.

As the crop grows and periods of low rainfall occur, the values in the H2O deficit will become more negative. The app shows you when to trigger the next irrigate. A Dryness Index warns you when the soil water deficit is coming close to the Management Allowed Depletion (MAD) level of the field. The symbols (*,**,***, and XXX) represent 70%, 80%, 90%, and 100% of MAD, respectively.

For more information about the Crop Water Use Application,
contact Gene Stevens at 573-379-5431 or
StevensW@missouri.edu