How wide is the problem? Leveraging alternative data sources to enhance channel width representation in watershed modeling

Watershed models have been increasingly applied to investigate the impacts of environmental changes on water quantity and quality and to support decision-making. Thus, accurately capturing physical processes and the characteristics of the watershed system is vital to achieving reliable results. Channel geometry features such as cross-sectional shape, bankfull channel width, and depth affect hydraulic parameters like hydraulic radius, wetted perimeter, and cross-sectional area. These, in turn, play important roles in streamflow dynamics, flow velocity, erosion, nutrient transport, and stream ecology. However, the representation of bankfull channel width is often oversimplified in watershed models. Power-law regression equations relating geometric parameters and upstream drainage area are commonly used to calculate bankfull channel width in watershed models. This approach has limitations and may resonate in the misrepresentation of channel geometry with implications for water quantity and quality estimations. This study evaluates how bankfull channel width is represented in a popular watershed model and presents alternative data sources derived from aerial measurements, empirical models, LiDAR, and a global database to mitigate potential misrepresentations. To assess the impacts of bankfull channel width on water quantity and quality, we designed a series of modeling experiments through the Soil and Water Assessment Tool (SWAT) model. We test our methodology in the Alabama-Coosa-Tallapoosa river basin, a large watershed system draining to the Gulf of Mexico coast. Our findings indicate that, overall, the regression equation used in the ArcSWAT program (SWAT's GIS interface) can overestimate bankfull channel width by as much as three times in our study domain. In testing the effects of bankfull width on model simulations we found negligeable implication for water yield predictions. On the other hand, 1-day maximum flows were greatly increased (20%) by using channel width values from the alternative data sources. Simulated water quality was also affected, with stream water temperature showing better agreement with observations under the proposed scenarios. Sediment loadings increased by as much as 118% under the alternative data sources of channel width. Nitrate and phosphate loadings had opposite responses and showed decreases as high as 8.3 and 18.8%, respectively, relative to the default model. Our findings demonstrate that bankfull channel width is misrepresented in watershed models with underlying impacts on simulated water quantity, quality, and ecological flows. Our analysis found bankfull channel width as an important parameter affecting high flows and water quality, and the freely available data sources tested in this study showed good matches with field measurements, making our findings readily available and broadly useful to the modeling community.