The model to estimate potential evapotranspiration (PET) was developed using periodic function. Average PET during 30 years for 67 areas was 2.36 mm day<sup>-1</sup>, and those were distributed with the range of 1.42∼ 3.45 mm day<sup>-1</sup>. The period of PET change was 16∼32 years according to area.