A Cotton Crop Coefficient-Based Model for Scheduling Irrigation in Georgia

A crop-coefficient model for scheduling irrigation on cotton in Georgia was developed and tested during the 2012 growing season.  The model uses cotton crop coefficients developed by various research groups in the Southeast.  Instead of using growing degree days as a trigger for changing the crop coefficient during the growing season, the model uses phenology (first bloom, first open boll, etc.)  The model uses a check-book approach to estimate when available soil moisture has been depleted by adding precipitation and irrigation to available soil moisture and subtracting FAO-56 ET adjusted by the crop coefficient from it.  The model was used to schedule irrigation in four experimental blocks at the University of Georgia’s Stripling Irrigation Research Park (SIRP).  Soil water tension in the experimental blocks was recorded continuously with a Watermark-based soil moisture sensing system.  The model performed well for the majority of the growing season with the model typically calling for irrigation when soil water tension exceeded 60 kPa at 16 or 24 inches.  However after first open boll, the model predicted higher crop water use than that indicated by the soil moisture sensors.  The problem was corrected by adjusting the crop coefficient used between first open boll and maturity.  The model will developed into a smartphone app and beta-tested during the 2013 growing season.