Estimating potential evapotranspiration from climatological data in an arid environment

Abstract

Existing empirical evapotranspiration formulas were evaluated using climatic data for two consecutive years (1980 and 1981) from the Lower Colorado River Valley (Blythe, California). It was demonstrated that existing simple empirical models are inapplicable in arid zones where climatic ranges are wide. Extensive measurements of evapotranspiration rates using the Bowen ratio energy budget approach and selected climatological variables were made over well irrigated alfalfa sites in Avra Valley near Tucson, Arizona, for the season beginning in early May 1982 and ending in mid-October 1982. Forty-three days of 12-minute data gathered during the study were used as a basis for developing new models and for calibrating the Penman combination model. Several empirical models estimating evapotranspiration from various numbers of climatic variables were derived using a multiple linear regression technique. Their performance over the wide range of climatic conditions during the study prove the utility of certain empirical models for estimating evapotranspiration. The most promising is a simple solar radiation and wind model PET = a + bK + CU which uses only two climatic variables, incoming solar radiation (I%) and wind (U) which are widely available from routine weather measurements. The Penman combination model, with improvements suggested in literature, underestimated evapotranspiration as measured over irrigated alfalfa. A new arid zone wind function was developed using 12-minute data throughout the entire measurement period. The Penman model with the arid zone wind function performed very well throughout the season.