Estimating Reference Evapotranspiration With an Atmometer

Measurement or estimation of crop water use is important in scheduling irrigations for efficient watermanagement. Atmometers have been used to measure evaporation, but recent models have not been tested againstevapotranspiration (ET) estimated from weather data. Measurements of ET by atmometers sold under the brand name ETgagewith three types of fabric covers and two evaporation surface geometries were compared to ET estimates calculated fromweather data. Measurements by atmometers with GoreTex or canvas 54 covers and flat or convex evaporating surfaces werecompared to alfalfabased modified Penman reference ET (ETr ) estimates for three growing seasons. Also measurements byan atmometer with a canvas 30 cover and a convex evaporating surface were compared to grassbased PenmanMonteithreference ET (ETo ) estimates for one growing season. Correlation between ET measurements by atmometers and the ETr orETo estimates from weather data were good. Evaporation from the atmometer with a flat evaporation surface and a GoreTexcover 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). Theatmometer with a convex evaporation surface and GoreTex 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 withETo (R 2 = 0.81, SE = 2.0 mm). The results indicated that with the proper regression equation, atmometers could be used toestimate ET for irrigation scheduling where evapotranspiration estimates are not available from a weather station or theweather station is located at a distance from the area of use greater than recommended.