A simple model to estimate monthly forest evapotranspiration in Japan from monthly temperature

The estimation of evapotranspiration ( E ) in forested areas is required for various practical purposes (e.g. evaluation of drought risks) in Japan. This study developed a model that estimates monthly forest E in Japan with the input of monthly temperature ( T ). The model is based on the assumptions that E equals the equilibrium evaporation rate ( E eq ) and that E eq is approximated by a function of T . The model formulates E as E (mm month −1 ) = 3·48 T ( °C) + 32·3. The accuracy of the model was examined using monthly E data derived using short‐term water balance (WB) and micrometeorological (M) methods for 15 forest sites in Japan. The model estimated monthly E more accurately than did the Thornthwaite and Hamon equations according to regression analysis of the estimated E and E derived using the WB and M methods. Although the model tended to overestimate monthly E , the overestimation could be reduced by considering the effect of precipitation on E . As T data are commonly available all over Japan, the model would be a useful tool to estimate forest E in Japan.