In this paper, a three dimensional numerical evapotranspiration model was applied to the basin of Lake Biwa.
To compare the results by simulation with other studies, the numerical model was considered to be valid. Moreover, the basic knowledge to estimate evapotranspiration from a large field was obtained, such that, for water surface, daily evapotranspiration was closely related to the wind speed averaged over the day. As for land surface, except for forest, daily evapotranspiration was also related to the average wind speed influenced by the radiation.