Controls of streamflow generation in small catchments across the snow-rain transition in the Southern Sierra Nevada, California

Processes controlling streamflow generation were determined using geochemical tracers for water years 2004-2007 at eight headwater catchments at the Kings River Experimental Watersheds in southern Sierra Nevada. Four catchments are snow-dominated, and four receive a mix of rain and snow. Results of diagnostic tools of mixing models indicate that Ca2+, Mg2+, K+ and Cl- behaved conservatively in the streamflow at all catchments, reflecting mixing of three endmembers. Using endmember mixing analysis, the endmembers were determined to be snowmelt runoff (including rain on snow), subsurface flow and fall storm runoff. In seven of the eight catchments, streamflow was dominated by subsurface flow, with an average relative contribution (% of streamflow discharge) greater than 60%. Snowmelt runoff contributed less than 40%, and fall storm runoff less than 7% on average. Streamflow peaked 2-4weeks earlier at mixed rain-snow than snow-dominated catchments, but relative endmember contributions were not significantly different between the two groups of catchments. Both soil water in the unsaturated zone and regional groundwater were not significant contributors to streamflow. The contributions of snowmelt runoff and subsurface flow, when expressed as discharge, were linearly correlated with streamflow discharge (R-2 of 0.85-0.99). These results suggest that subsurface flow is generated from the soil-bedrock interface through preferential pathways and is not very sensitive to snow-rain proportions. Thus, a declining of the snow-rain ratio under a warming climate should not systematically affect the processes controlling the streamflow generation at these catchments. Copyright (c) 2012 John Wiley & Sons, Ltd.