Abstract

Evapotranspiration (ET) data from atmometers were compared against evapotranspiration estimated by the FAO-56 Penman-Monteith equation, recommended method, in order to evaluate the accuracy of atmometers. Measurements by 3 atmometers with grass cover and 3 atmometers with alfalfa cover were compared, for one growing season, to Penman Monteith based grass and alfalfa equation–(ET0__PM and ETr__PM,, respectively). Comparison between cumulative Evapotranspiration measured by atmometers and ET0__PM or ETr__PM showed that Atmometers, for both grass and alfalfa, underestimate evapotranspiration by 12.5-21 and 15% respectively. The three Atmometers with alfalfa cover  give the same cumulative value (636 mm) compared to the atmometers with grass cover  which exhibit different results (atmometers 1 and 3 (467 mm) and atmometers 2 gives 419 mm).  Correlation between ET from atmometers and ETr__PM or ET0__PM estimates were generally good. Evaporation from atmometers with alfalfa cover showed the highest correlation to ETr__PM (R² varying from 0.68 to 0.72) whereas evaporation from atmometers with grass cover present the lowest correlation (R²ranges from 0.49 to 0.68). The results indicated that with the proper regression equation and a good calibration, atmometers could be used to estimate ET for crop water requirement where evapotranspiration estimates are not available from weather stations.

Key words: Atmometer, evaporation, evapotranspiration, irrigation, Penman-Monteith equation.