A formula of watershed evapotranspiration by Penman or Thonthwaite or Lowry-Johnson was used to measure its quantity of evapotranspiration until now. These formula were derived for Foreign country and, it is rather difficult to apply the above formulas to the Korean watershed. These measuring methods are merely used to measure the monthly quantities of evapotranspiration. At the research CE of a coefficient of evapotranspiration for a watershed were newly presented, which was utilized for the calculation of Beken's formula in the DAWAST model.