Incorporating landscape heterogeneity to understand patterns of stream discharge across spatial and temporal scales in forested mountain watersheds

Abstract:

Numerous studies have indicated that catchment characteristics (e.g. geology, vegetation, and topography) modulate runoff generation processes that connect hillslopes to streams. However, there has been little direct quantification of the range of spatial scales and climatic forcing under which catchment characteristics and their topology influence patterns and thresholds in stream flow. We measured changes in discharge across 52 reaches (~200 m long) distributed across 5 nested watersheds ranging from 3.2 to 23 km² in the Tenderfoot Creek Experimental Forest, Montana. We performed dilution gauging from early snowmelt through late summer baseflow in the 2013 and 2014 water years to develop stage-discharge relationships for calculation of real-time stream discharge for each of the 52 reaches. We also computed indices of topography, geology, vegetation, and valley characteristics. Preliminary results suggest that the dominant controls, and their degree of influence on observed changes in discharge shifted across wetness states. Total contributing area to each reach was a significant predictor of discharge at high flow with the slope of the relationship decreasing across the annual recession. We examined incremental changes in discharge for each reach and determined that spatiotemporal variability in discharge was related to the underlying lithology. Contributing areas underlain by granite gneiss yielded more water per unit area than those with sandstone. However, yield was less predictable in areas overlying the granite gneiss formation. Our preliminary analysis has identified specific watershed characteristics and time periods at which they influence discharge across watersheds and spatial scales. We suggest that these results can support improved understanding of space-time variability of stream flow and the representation of watershed characteristics in simulation model structures.