Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 7/10/2006
Publication Date: 7/10/2006
Citation: Marek, T., Colaizzi, P.D., Howell, T.A., Dusek, D., Porter, D. 2006. Estimating seasonal crop ET using calendar and heat unit based crop coefficients in the Texas High Plains Evapotranspiration Network. In: Proceedings of the ASABE Annual International Meeting, July 9-12, 2006, Portland, Oregon. 2006 CDROM. Paper No. 062206. Interpretive Summary: Water use by irrigated crops is often expressed as the product of a crop coefficient times a weather-estimated reference crop water use value. The crop water use is often called simply evapotranspiration (ET). The 1998 FAO (United Nations, Food and Agricultural Organization) report FAO-56 outlines a procedure to estimate crop water use (ET) that has become widely used throughout the world. The Texas High Plains Evapotranspiration (TXHPET) network utilizes many procedures from the FAO-56 report to be standard. One principle difference is FAO-56 expresses the crop growth cycle (crop calendar) in terms of days after planting while the TXHPET network uses a growing degree day basis (heat units) for the crop calendar basis. This paper compared the two crop calendar approaches using field observations from Etter, TX. The growing degree crop calendar appeared to better represent the crop growth cycles for corn, sorghum, and soybeans; however, the FAO-56 procedures would be useful for crops that are newer to the region without a prior data base.
Technical Abstract: 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.