Application of the precipitation-runoff modeling system model to simulate dry season runoff for three watersheds in south-central Guam

The Precipitation-Runoff Modeling System model was calibrated and verified using existing hydrologic and climatic data for the Maulap and Imong River watersheds for simulation of dry season runoff from three gaged areas that contribute to the Fena Reservoir water supply. The model was applied to the Almagosa River watershed by transferring calibrated parameters and coefficients because data were not available for daily diversions of as much as 3.9 cubic feet per second of runoff at Almagosa Springs. Application of the model in the watershed of Fena Reservoir can provide a physically based method for estimating reservoir recharge during the dry season, January through May. Estimated recharge can be examined in relation to the effect of varying intensities of monthly reservoir-water production in order to identify a basis for the rational release of water. Differences between simulated and observed monthly mean runoff for dry season months in the verification period (November 1980 through December 1981) ranged from-0.04 cubic feet per second (-3.51 percent) to 0.74 cubic feet per second (30.34 percent) at Maulap River and from 0.03 cubic feet per second (1.3 percent) to 1.19 cubic feet per second (27.95 percent) at Imong River. On the basis of runoff simulations for the four complete dry seasons included in the total calibration and verification periods (1981 and 1984-86), the total volume of runoff during the 5month dry season can be predicted to within 20 percent of actual runoff at Maulap River, and to within 27 percent at Imong River.