Estimating seasonal crop ET using calendar and heat unit based crop coefficients in the Texas High Plains Evapotranspiration Network

The Texas High Plains Evapotranspiration (TXHPET) network utilizes a heat unit-based approach (growing degree day concept) in the timing of various crop growth stages along with crop coefficients for computation of crop water use with the newly standardized ASCE/EWRI reference evapotranspiration (ET) equation. Mean crop coefficients of the TXHPET network are empirically derived from measurements using the large, monolithic lysimeters located at the USDA-ARS at Bushland, TX. Average regional crop growth stages are adjusted using multi-year, in-season, recorded data from irrigated producers through the Texas Cooperative Extension’s Agri-Partner program. A comparison of the seasonal and crop stages of the TXHPET network as compared with the FAO-56 method, which utilizes date based timing for the crop growth stages, for the major crops of corn, grain sorghum and soybeans is presented. This analysis was conducted to determine if additional, producer requested crops not evaluated with weighing lysimeters would have adequate accuracy based on previously measured crops using the FAO-56 method for irrigation scheduling purposes through the TXHPET network. While there were substantial ET differences in several growth stages of the crops evaluated, the total seasonal crop ET difference as evaluated indicated that use of the calendar based FAO-56 method could provide some degree of guidance to producers irrigating crops not yet evaluated with regional weighing lysimeters.