An Integrated Media, Integrated Processes Watershed Model – WASH123D: Part 1 – Model Descriptions and Features

Parametric-based, lumped watershed models have been widely employed for integrated surface and groundwater modeling to calculate surface runoff and pollution loads on various temporal and spatial scales of hydrologic regimes. Physics-based, process- level, distributed models that have the design capability to cover multimedia and multi-processes and are applicable to various scales have been practically nonexistent until recently. It has long been recognized that only such models have the potential to further the understanding of the fundamental biological, chemical, and physical factors that take place in nature hydrologic regimes; to give mechanistic predictions; and most importantly to be able to couple and interact with weather/climate models. However, there are severe limitations with these models that inhibit their use. These are, among other things, the ad hoc approaches of coupling between various media, the simplistic approaches of modeling water quality, and the excessive demand of computational time. This paper presents the development of an integrated media (river/stream networks, overland regime, and subsurface media), integrated processes (fluid flows and thermal, salinity, sediment, and water quality transport) watershed model to address these issues. Rigorous coupling strategies are described for interactions among overland regime, rivers/streams/canals networks, and subsurface media. Generalized paradigms of reaction-based water quality modeling are presented. The cultivation of innovative, numerical algorithms and the implementation of high performance computing to increase the computational speed are discussed. Various application-dependent numerical-options to simulate scalar transport are provided. The necessities to include various options in modeling surface runoff and river hydraulics are emphasized. Several examples are used to demonstrate the flexibility and efficiency of the model as applied to regional-level large scale and project-level small scale problems.