Distributed Hydrology-Soil-Vegetation Model (DHSVM)
Geographic Information System-based Modeling System for Watershed Analysis (GISWA)
Water is integral to the biological, chemical, and physical processes that occur in a watershed. Topography, soil characteristics, vegetation, and climate interact in a complex manner to determine the types, intensities, and locations of runoff production and the associated transport of sediments, chemicals, and organic debris in a landscape. Natural or human-induced changes to one or more of these forcing variables will alter the hydrologic response of a drainage basin. Prediction of the resulting changes in water quality and runoff production requires an accurate, explicit representation of the relationships between hydrology, vegetation, and climate. The GIS-based modeling system for watershed analysis (GISWA) provides a physically-based approach that accounts explicitly for the spatial distribution of land-surface processes. The level of model detail is sufficient to represent important processes and feedbacks within the hydrology-vegetation system in a physically realistic manner, consistent with the types of data typically used in watershed analysis. The modeling system has been applied over a range of scales, from plot to large watershed at subdaily to daily time scales.
GISWA is an enhanced version of the Distributed Hydrology-Soil-Vegetation Model (DHSVM) of Wigmosta et al. (1994). GISWA provides an integrated representation of watershed processes at the spatial scale described by digital elevation model (DEM) data. The model includes:
- a multi-layer canopy model for evapotranspiration
- energy balance models for canopy snow interception and ground snowpack processes
- a multi-layer rooting zone model
- three-dimensional saturated subsurface and surface flow models
- one-dimensional road drainage and channel network flow models
Figure 2. Model representation of hydrologic features showing flow directions for DEM grid cells, road locations and drainage directions, culvert locations, and stream channels. Water moving downslope coincident with the grid cell flow directions may be intercepted and diverted by the road network. Click for a larger version.
The modeled landscape is divided into computational grid cells centered on DEM elevation nodes (Figure 1). Each grid cell is assigned a surface cover type and soil properties that may vary throughout the basin. Digital elevation data are used to model topographic controls on absorbed shortwave radiation, precipitation, air temperature, and down-slope water movement. At each time step, the model provides a simultaneous solution to the energy and water balance equations for every grid cell in the watershed. Each cell may exchange water with one or more of its neighbors resulting in a three-dimensional redistribution of surface and subsurface water across the landscape. As water moves downslope toward a stream channel it may be intercepted by a road network (Figure 2). This intercepted water moves through the road drainage network until it reaches a culvert or stream channel. If the road intersects a stream channel, the water is input to the appropriate channel reach and routed through the channel system. The active road drainage/channel network may expand and contract as grid cell water tables rise and fall below their channel beds. Discharge from culverts without a defined channel is subject to reinfiltration as it moves downslope below the culvert. A more complete description of the model system and its components is given in papers by Wigmosta et al., 1994; Wigmosta and Lettenmaier, 1994; Wigmosta and Perkins, 1997; Storck et al. 1995; and Wigmosta and Lettenmaier 1999.
Two studies are summarized to demonstrate the range of GISWA applications for evaluating the hydrologic responses to environmental change.