Estimating Evapotranspiration with Water-Production Functions or the Blaney-Criddle Method

Abstract

ABSTRACT SEASONAL evapotranspiration (ET) for cotton (Gos-sypium hirsutum L.), corn (Zea mays LJ, alfalfa (Medicago sativa LJ, and barley (Hordeum vulgare LJ can be estimated with either the Blaney-Criddle method or county yield data with a water-production function. As used here, a water-production function is an em-pirical relationship between yield and evapotranspira-tion. Blaney-Criddle crop coefficients vary depending upon crop season ET and yield. The original Blaney-Criddle coefficients (K) resulted in ET estimates that were very close, except for corn, to the ET determined from maximum county yields and linear water-produc-tion functions. The modified Blaney-Criddle method described by Doorenbos and Pruitt (1977), overestimated seasonal Er compared to Er computed using maximum county yields and linear water-production functions. Blaney-Criddle crop coefficients determined from plot studies in NM, AZ, and CA are larger than the original coefficients and result in estimates of yearly ET that are greater than those determined using county yields and the linear water-production functions. Blaney-Criddle crop coefficients representing non-irrigated ET and yield levels are less than 35 percent of the K value for small ir-rigated plot studies.