Landscape-river corridor storage affects watershed hydrologic dynamics: inferences from a large-scale model

Natural water storage units (e.g., surface depressions, wetlands) on the landscape and along river corridors are hotspots of hydrologic and biogeochemical activity. As a result, they play a significant role in mediating flow generation, the transport of water, and the fate of its constituents, such as nutrient loads, across space and time. Most of the current generation hydrologic and land surface models lack spatial specificity for characterizing these areas in the uplands and river corridors and their storage-discharge functions. These existing models further lack the capacity to differentiate surface water interactions among the upland landscape, river corridor, and channel interfaces, which is key to accurately simulating water quantity and quality. Thus, hydrological processes both distal and proximal to a river network are treated similarly, and their effects are frequently spatially aggregated (i.e., lumped). We initiated a large-scale modeling study, using the Soil and Water Assessment Tool (SWAT), to enable spatially explicit attribution of landscape-river corridor storage potential across a major river basin in the Midwestern United States. We assimilated a spatially explicit river corridor boundary in SWAT, and further modified the model to incorporate variable floodplain hydraulic geometry. We estimated the extent and water-storage capacity of surface depressions for each subbasin and partitioned their storage-discharge functions at two spatial levels, namely the upland and the river corridor. A 5-year hydrologic simulation using this novel modeling approach suggests that simulated hydrologic processes across the watershed can be significantly different compared to the same model without these modifications. These results confirm the importance of considering landscape-river corridor storage for efficient prediction of floods and droughts - in addition to variations in water quality.