WATERSHED-SCALE HYDROLOGIC AND NONPOINT-SOURCE POLLUTION MODELS: REVIEW OF MATHEMATICAL BASES

Commonly used watershed-scale hydrological and nonpoint source pollution models were examined for strengths and limitations, and applicability to different kinds of watersheds and problems. Eleven models were reviewed: Agricultural NonPoint Source (AGNPS) pollution model, Annualized AGNPS, Areal Nonpoint Source Watershed Environment Response Simulation (ANSWERS), ANSWERS-Continuous, Dynamic Watershed Simulation Model (DSM), Hydrological Simulation Program-Fortran (HSPF), KINematic runoff and EROSion model (KINEROS), European Hydrological System model (MIKE SHE), Precipitation-Runoff Modeling System (PRMS), and Soil and Water Assessment Tool (SWAT). The components and capabilities, temporal scale, and watershed representation are compared, as is treatment of rainfall excess on overland/water balance, runoff on overland, subsurface flow, and runoff in channel. Some are continuous simulation models, useful for analyzing long-term effects of hydrological changes and watershed management practices (especially agricultural). Others are single rainfall event models applicable to severe actual or design single-event storms and evaluation watershed management practices (especially structural). Flow-governing equations and their solution methods are discussed so that the problems, situations, and conditions of appropriate use can be identified.