A first-principle, physics-based watershed model: WASH123D

Hsin-Chi Lin, Earl Edris, and David Richards The approaches to watershed-scale modeling can be classified into three broad groups: parametric methods, stochastic approaches, and physics-based mathematical models. In the past 30 years, the watershed modeling communities have employed parametric-based models, of which the most famous is the Hydrological Simulation Program-Fortran (HSPF) (Bicknell et al., 1993); all other parametric models are similar to HSPF, e.g., Storm Water Management Model (SWMM) (Huber and Dickinson, 1988), Chemical, Runoff, and Erosion from Agricultural Management Systems (CREAMS) (Knisel, 1980), STORM (Hydrologic Engineering Center, 1977), Areal Nonpoint Source Watershed Environment Response Simulation (ANSWERS) (Beasley and Higgins, 1981), and Simulator for Water Resources in Rural Basins – Water Quality (SWRRBWQ) (Arnold et al., 1991) for watershed management and assessment including ecological exposure assessments and total maximum daily load (TMDL) calculations. Evolved from the pioneer model STANFORD WATERSHED IV (Crawford and Linsley, 1966), HSPF has dominated watershed simulations for more than 20 years. Physics-based, process-level chemical transport and hydrological models have been practically nonexistent until recently. It is easy to understand that only the physics-based, process-level fluid flow and thermal, salinity, sediment, and biogeochemical transport models have the potential to further the understanding of the fundamental biological, chemical, and physical factors that take place in nature. It is precisely for this reason that the U.S. Environmental Protection Agency (EPA) ecological research strategies (EPA, 1997) had clearly stated that the first-principle, physics-based models should be used in ecological system assessment on a watershed scale.