Estimation of Mean Monthly Pan Evaporation Based on Hargreaves Equation

Abstract

The Hargreaves equation can calculate monthly mean evaporation/evapotranspiration using only air temperature with two coefficients. First, it is confirmed that the Hargreaves equation with general coefficients gives better pan evaporation estimates than estimated results for pan evaporation from 143 meteorological stations in Japan and results generated using the Thornthwaite and the Hamon equations. In a further attempt to improve accuracy, two equations are determined to estimate the values of the two coefficients that can be applied throughout Japan with the exception of the Nansei Islands in the subtropical southwest of Japan. It is shown that the Hargreaves equation plus the two new equations gives an improved estimation accuracy with a root mean square error of 0.65mm ± 0.26mm (Ave.±S.D.) .