Estimating reference evapotranspiration with an atmometer

Measurement or estimation of crop water use is important in scheduling irrigations for efficient water management. Atmometers have been used to measure evaporation, but recent models have not been tested against evapotranspiration (ET) estimated from weather data. Measurements of ET by atmometers sold under the brand name ETgage with three types of fabric covers and two evaporation surface geometries were compared to ET estimates calculated from weather data. Measurements by atmometers with Gore-Tex or canvas 54 covers and flat or convex evaporating surfaces were compared to alfalfa-based modified Penman reference ET (ETr) estimates for three growing seasons. Also measurements by an atmometer with a canvas 30 cover and a convex evaporating surface were compared to grass-based Penman-Monteith reference ET (ETo) estimates for one growing season. Correlation between ET measurements by atmometers and the ETr or ETo estimates from weather data were good. Evaporation from the atmometer with a flat evaporation surface and a Gore-Tex cover showed the highest correlation to ETr and the lowest standard error of the estimate (R-2 = 0.92, SE = 1.8 mm). Evaporation from the atmometer with canvas 54 covers had the next highest correlation (R-2 = 0.87, SE = 2.2 mm). The atmometer with a convex evaporation surface and Gore-Tex cover showed lower correlation and higher standard error (R-2 = 0.65, SE 3.9 mm). Measurement of ET by an atmometer with a convex surface and a canvas 30 cover correlated well with ETo (R-2 0.81, SE = 2.0 mm). The results indicated that with the proper regression equation, atmometers could be used to estimate ET for irrigation scheduling where evapotranspiration estimates are not available from a weather station or the weather station is located at a distance from the area of use greater than recommended.