Development of a Distributed Watershed Model Coupling River Flow, Surface Runoff, and Subsurface Flow

Rui-Tang Sung1 and Ming-Hsu Li2

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Abstract: Lumped watershed models are often suitable for long-term simulations but unable to provide adequate spatial and temporal resolutions. A distributed watershed model was developed in this study to couple river flow, surface runoff, and subsurface flow for resolving hydrological responses in different spatial and temporal scales. River flow, surface runoff, and subsurface flow are explicitly and interactively solved. The river flow is simulated by the 1-D diffusive wave approach for each river segment with the conservation of mass and the continuity of stage for river junctions. River cross-sectional characteristics, including cross-sectional area, wetted perimeter, top width, are parameterized as functions of water depths. The surface runoff is simulated by the 2-D diffusive wave approach and eight flow directions are allowed for runoff in/out of each surface grid. The interactions between river flow and surface runoff is determined by the continuity of stage and the conservation of mass when water of two regimes are connected and stage differences exist. When surface grid is ponding and the corresponding river stage is lower than its river bank, the surface discharge is estimated by the weir formula. Daily evapotranspiration is estimated by multiplying the potential evapotranspiration, either prescribed or computed, by the crop coefficient determined from land use. The subsurface flow is a quasi 3-D approach, including soil moisture movements in the vertical direction and groundwater fluxes in the horizontal direction, that water flows are described by the Darcy’s law. Vertical soil moisture distributions are updated after groundwater table is determined. The interactions between subsurface water and surface water (river and surface runoff) is described by the direct connection approach that flux and head continuities are conserved. Each subsurface grid is designed as a variable-saturated soil column right below its corresponding surface grid. River grids are designed not to overlap surface grids for conserving surface area. Time step size will be automatically reduced by examining the Courant number to increase numerical stability. The newly developed model is then applied to simulate daily flow of the Shiehman Reservoir watershed from 1977 to 2005. Results demonstrate our approach of integrated watershed modeling is suitable for applications in continuous and long-term watershed study, such as climate change impact and land use change analyses.

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Keyword: Distributed Hydrologic Model, Watershed, Geographic Information Systems