The approximate method to estimate potential evapotranspiration using monthly mean temperature Haruki NUMAJIRI Abstract The purpose of this study is to develop an easier method than Thornthwaite's to estimate the potential evapotranspiration. The data used are monthly climate (temperature) grid data of 1km resolution (Japan Meteorogical Agency). We calculated potential evapotranspiration by the Thornthwaite's method at first. We got an approximate expression as follows, verifying by

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... 's value; PE = ae bAT where PE is potential evapotranspiration in our method, AT is a yearly accumulated temperature, a and b are coefficients defined as follows; a = 37.816Tm 0.5056 b = -0.0069ln(Tm)+0.0245 where Tm is each monthly mean temperature. This method is much more simplified than Thornthwaite's method, and has practical advantages with enough accuracy.