Evaluation and Calibration of Empirical Methods to Estimate Reference Evapotranspiration in Northwest Texas.

The influence of the various climatic factors on water requirement of crops is estimated by the reference crop evapotranspiration (ETo). Estimated daily ETo is used to determine the water requirement of crops using the crop factor method. ASCE’s Standardized Reference Evapotranspiration Equation (ASCE-ET) and FAO Penman-Monteith equation (FAO-ETo) use complete weather data whereas many empirical methods based on radiation and temperature use limited climate data. The main objectives of this study were to (i) evaluate different empirical methods and (ii) calibrate parameters of empirical methods to estimate ETo in Northwest Texas using climate data from West Texas Mesonet (www.mesonet.ttu.edu). Daily ETo calculated between 2006 and 2016 by FAO Penman-Monteith (FAO-ETo) equation using observed weather data was compared to ETo estimated by different empirical methods (Hargreaves and Samani, Monthly Hargreaves and Samani, Valiantzas, Priestley-Taylor, Makkink and Stephens-Stewart). The required data to estimate FAO-ETo were collected for 48 weather stations from West Texas Mesonet. All selected methods underestimated ETo when original parameters were used. Calibrated parameters improved ETo estimation; however, all methods underestimated ETo when FAO-ETo is in the range of 8 - 16 mm. ETo estimated using Monthly Hargreaves and Samani equation with original and calibrated parameters were in good agreement with those estimated from FAO-ETo. This method can be used to estimate reference evapotranspiration using measured and forecasted daily maximum and minimum temperatures in Northwest Texas.