Estimating reference crop evapotranspiration under limited climate data in West Texas

West Texas, USA Estimation of crop reference evapotranspiration (ETo) is essential for many aspects of water resources planning and management such as irrigation scheduling. Available widely used methods for calculating ETo include American Society of Civil Engineers' Standardized Reference Evapotranspiration and Food and Agriculture Organization's Penman-Monteith equations (FAO-ETo). These methods use complete climate datasets to estimate daily ETo, whereas simple evapotranspiration models based on radiation and temperature use limited climate data. In this study, daily ETo estimated using the temperature based Hargreaves-Samani (HS) equation were compared and evaluated with those estimated using the standard FAO-ETo at different stations of West Texas Mesonet. The results showed that the HS equation with original coefficients underestimated daily ETo values as compared to FAO-ETo data. New coefficients of the globally, monthly and regionally calibrated HS equation against FAO-ETo data were derived and proposed for more accurate daily ETo estimates in West Texas. Based on the results of global, monthly and regional calibration scenarios, ETo estimated by the calibrated and validated HS equation using fitted month-specific coefficients showed better agreement with FAO-ETo both within and outside the calibration region. No significant improvement in ETo estimation was observed for the HS equation using interpolated coefficients derived from station-specific calibrated coefficients as compared with commonly calibrated coefficients derived based on datasets of all selected meteorological stations in West Texas.