Alumni & Clinton McClure Professor Auburn University
Watershed models have been increasingly applied to investigate the impacts of environmental changes on water quantity and quality. Thus, accurately representing physical processes in watershed modeling is vital to achieving reliable results. Chanell morphology, such as channel width and depth play an important role in streamflow dynamics, flow velocity, erosion, sediment transport, and stream ecology. However, the representation of channel geometry is often oversimplified in watershed models. One example is the widely used Soil and Water Assessment Tool (SWAT), which relies on empirical equations related to the watershed’s drainage area to represent channel width and depth. This study compares SWAT’s default channel width and depth representation against values derived from aerial measurements as well as estimates from an empirical model developed for the Coastal Plain region of the southeastern U.S. We test our methodology in the Mobile River Basin, a large river basin draining to the Mobile Bay estuary along the Gulf of Mexico coast. We derived channel width for over 100 stream segments using aerial imagery and elevation profiles generated from USGS Streamstats. Our findings indicate that, overall, SWAT overestimates channel width by a factor of 3. Regarding channel depth, the values assigned by SWAT showed a good match with estimates from the empirical model. For streams wider than 200 meters, SWAT substantially underestimated channel width. In testing the effects of channel geometry on model simulations we found that daily peak flows were greatly increased by replacing SWAT’s default channel width with the values derived from aerial imagery. Simulated water quality was also affected, with sediment loading increasing 290% using channel width values measured via Streamstats. Our findings demonstrate that channel width is misrepresented in SWAT with underlying impacts on simulated water quantity, quality, and ecological flows such as maximum flows, which can be important regulators of aquatic biodiversity.
