The model of evapotranspiration climatonomy which was developed by H. Lettau was applied to the 46-square kilometer Mabacan River Watershed at Calauan, Laguna, Philippines. Essentially, the study concerned the development of appropriate methods to refine and evaluate the parameterization procedures. The numerical model required input of mass (precipitation) and solar energy (global radiation) to estimate runoff, evapotranspiration and soil moisture storage.