Abstract
The streambed is the critical interface between the aquatic and
terrestrial systems and hosts important biogeochemical hot spots within
river corridors. Although the streambed characteristics are
significantly different from those of its surrounding soil, the
streambed itself has not been explicitly represented in watershed
models. We developed an integrated hydrologic model that explicitly
incorporated a streambed layer to examine the hydrological effects of
streambed characteristics including hydraulic conductivity (K), layer
thickness, and width on the exchange fluxes across the streambed as well
as the streamflow at the watershed outlet. The numerical experiments
were performed in the American River Watershed, a headwater, mountainous
watershed within the Yakima River Basin in central Washington. Despite
having a negligible effect on the watershed streamflow, an explicit
representation of the streambed with distinctive properties dramatically
changed the magnitude and variability of the exchange flux. In general,
larger streambed K along with a thicker streambed layer induced larger
exchange fluxes. The exchange flux was most sensitive to the streambed
width or the mesh resolution of the streambed. A smaller streambed width
(or a finer streambed resolution) increases exchange fluxes per unit
area while reducing the overall exchange volumes across the entire
streambed. The amount of baseflow decreased by 6% as the streambed
width decreased from 250 m to 50 m. This finding is important because
these hydrological changes may in turn affect the exchange of nutrients
and contaminants between surface water and groundwater and the
associated biogeochemical processes. Our work demonstrated the
importance of representing streambed in fully distributed, process-based
watershed models in better capturing the exchange flow dynamics in river
corridors.