A scaling hierarchy for hydrologic response to snowmelt in mountain basins (Invited)

Snowmelt is a dominant source of runoff in mountain regions throughout the world, but these regions are characteristically difficult to monitor. Because of heterogeneity in topography, climate, geologic substrate, and vegetation, local-scale hydrologic measurements are often of limited utility in describing basin-scale hydrologic response. The objective of this research is to identify controls on hydrologic response to snowmelt at basin, hillslope, and plot scale in the Rocky Mountain Front Range, Colorado, USA. At basin scale, we analyze snow depletion patterns using MODIS images and develop simple conceptual models to estimate spatial and temporal patterns of snowmelt runoff generation within a 2,700 km^2 basin. These analyses suggest that the magnitude and timing of snowmelt runoff production varies primarily by elevation within the basin, and the majority of runoff is generated at elevations above around 3,000 m. Within this runoff-producing zone we monitor snow depth, snow cover, and soil moisture on hillslopes and link these measurements with a physically-based distributed model of hydrologic response. Results show that study hillslopes produce runoff through both rapid shallow subsurface flow and saturation excess overland flow. A hillslope with an east-facing aspect produces snowmelt runoff primarily in late May-early June, approximately the same time as peak basin-scale runoff production, whereas a hillslope with a north-facing aspect produces runoff in late June to early July, when the basin-scale runoff is receding. Hillslope analyses indicate that slope aspect and terrain shading are primary controls on the timing of runoff production from hillslopes as a whole. Local hydrologic responses within study hillslopes exhibit further heterogeneity in the timing of snow depletion and soil moisture response, which relates to tree distribution and microtopography. Analyses at these nested scales suggest a scale-based hierarchy of controls on snowmelt runoff generation, where at basin scale, spatial and temporal variability in runoff generation varies primarily by elevation; at hillslope scale, variability in runoff relates to topographic features such as slope aspect, and at plot scale, tree distribution and microtopographic features affect snow depletion and soil moisture behavior. A scale-based analysis of hydrologic response therefore provides a context for selecting appropriate field measurement sites and for understanding how complex local behaviors relate to basin-wide processes.