Abstract
The Hargreaves equation has two parameters: one is an empirical coefficient for estimating solar radiation, and the other is a fitting parameter. This study aims to develop the calibration equation for the fitting parameter, which is useful to analyze monthly water balance throughout Japan. The followings are clarified: 1) the fitting parameter is estimated by a multiple linear regression model with the square root of the annual mean range between mean monthly maximum and minimum temperatures, the proximity to a coast, and the annual mean temperature; 2) the modified Hargreaves equation with the above-mentioned model gives a good estimation accuracy with a root mean square error of 0.34 mm ± 0.10 mm (Ave.±S. D.).
