The model of evapotranspiration climatonomy is expanded by the incorporation of feedback to account for parameter dependencies on soil moisture. The concept is applied to climatic and hydrologic data for the 46-km2 Mabacan River watershed in Laguna, Philippines, in a humid tropical climate where average runoff (about 1.2 m/yr) exceeds evapotranspiration (about 0.7 m/yr) significantly. Of primary concern was the development of methods for parameter evaluation or watershed calibration. The numerical model requires input of mass and solar energy (monthly precipitation and global radiation) to predict monthly runoff, soil moisture storage, and evapotranspiration. For the investigated 12-yr period, averages of monthly runoff and the root-mean-square value of departures from the annual mean are 100 ± 22.3 mm from observations and 100 ± 20.5 mm from model simulation, with a linear correlation coefficient of 0.89. Computed monthly evapotranspiration was comparable to empirical data obtained at the International Rice Research Institute (about 10 km north of the watershed and about 65 km southeast of Manila). Weaknesses of the parameterization are discussed with the aid of model-simulated runoff for each month from January 1965 to December 1968, a sequence of a “dry”, a “wet”, and a “normal” year.



Now at the Department of Horticulture and Forestry, University of Nebraska, Lincoln, Nebr.