Performance of Atmometers in Estimating Reference Evapotranspiration in a Semi-Arid Environment.

Evapotranspiration (ET) based irrigation scheduling requires accurate measurements of reference evapotranspiration. Mathematical formulas with localized weather data can be used to accurately predict alfalfa or grass reference evapotranspiration rates. When local weather data are unavailable, a physical measurement of ET can be taken with an atmometer. The objective of this study was to determine if an ETgage Model E atmometer, equipped with a canvas #54 cover, could be used to effectively estimate alfalfa reference ET in semi-arid conditions of Southeast Colorado. The ASCE Standardized Alfalfa Reference ET Equation (ASCE ET_rs) was used as the standard for comparison of atmometer ET values to determine atmometer performance. Four years of alfalfa ET, as determined by an atmometer (ET_gage), were compared to ASCE ET_rs. Daily as well as 2, 3, 5, and 7 day sums of daily ET_gage and ASCE ET_rs were compared using simple least-squares linear regression. Coefficients of determination (R²) between daily ET_gage and ASCE ET_rs for all years were greater than or equal to 0.80. Throughout the study, the atmometer tended to underestimate ASCE ET_rs. Average seasonal underestimation of ASCE ET_rs measured by the atmometer ranged from 9.06% to 18.9%. Root Mean Square Error (RMSE) and Mean Bias Error (MBE) ranged from 1.14 to 1.82 mm d^-1 and   -0.66 to -1.51 mm d^-1, respectively. The atmometer underestimated daily ASCE ET_rs 88% of the time, with an average underestimation of 1.30 mm d^-1. Under estimation of ASCE ET_rs measured by the atmometer occurred most often on days when mean daily horizontal wind speeds were greater than 2 m s^-1 or when mean daily air temperatures were below 20 °C. The atmometer performed best when the alfalfa was at reference condition and localized calibration equations for reference and non-reference conditions were also developed to improve accuracy.