SIERRA NEVADA SNOWPACK VARIABILITY AND BASEFLOW GENERATION

Identifying sources of baseflow involves characterizing spatial snowmelt distribution as well as ground-based measurements in representative basins. Blending of satellite snow-covered area with spatial energy-balance estimates throughout the snow season provides a history of snowcover accumulation and depletion across the Sierra Nevada. Spatial maps of snow water equivalent from the 2004-2006 water years highlight elevation bands that contribute significantly to snowmelt. In the Upper Merced River basin, 40% of the elevation lies between 2,100-2,700 m, well above the rain/snow transition zone, which lies closer to 1,500 m. Elevations above the highest operational snow site (~2,700 m) contribute a disproportional share of the basin snowmelt relative to elevations from 2,700 m down to the snowline. The 2004 ablation season went from an above-normal mid-season snowcover of 120% to a below-average snowpack of 84% by April 1 due to an especially warm and dry air mass that rapidly depleted snowcover at all elevations. In 2005, the April 1 snowpack was 160% of normal, and SCA depletion at each elevation band lagged that of 2004 by about one month; 2006 was similar. South facing slopes accumulated at least 10% more snow than north-facing slopes, but were depleted at least twice as fast. Using both stable isotopes and specific conductance source waters for the Upper Merced River were determined for 2006. The primary sources of baseflow to the Merced River were groundwater, lateral subsurface flow (water that travels above the bedrock) and overland flow. Using a three-component mixing model the mean contribution of groundwater to baseflow was greater than 25% of total streamflow. Overland flow accounted for less than 30%, and came primarily from elevations above 2,800 m. Lateral subsurface flow and groundwater were mainly from elevations near the rain-snow transition. Lateral subsurface flow is more sensitive to changes in snowmelt timing at higher elevations than at lower elevations.