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Senior Thesis in Environmental Earth Sciences: Winter/Spring 2021

Spatiotemporal Variation in Snowmelt Runoff Efficiency: Unmixing the Effect of Melt-Season Moisture Depth on Vegetation Water Consumption and Bedrock Evaporation Rates Using High-Resolution SWE Maps

Dartmouth College

With thanks to my advisors:
Carl Renshaw (professor, Department of Earth Sciences)
Evan Dethier (graduate researcher, Department of Earth Sciences)

Wetterhahn Symposium 2021

Accurate modeling of the melt-season runoff from alpine catchments is critical for forecasting yearly water availability in snow-dominated regions. Realtime runoff models often rely on an estimate of runoff efficiency, the ratio of streamflow to precipitation. In large mountain catchments, it is difficult to isolate the physical processes that cause annual variation in runoff efficiency because the drivers of runoff efficiency in such basins are spatially heterogeneous and vary over time. We leverage the high spatiotemporal resolution of seven years of distributed snow water equivalent (SWE) maps acquired by the NASA Airborne Snow Observatory (ASO) in the headwaters of the Tuolumne River (California, USA) to model melt-season runoff efficiency based on the spatial distribution of SWE into vegetation and bedrock eco-zones and the impact of total seasonal moisture on vegetation water consumption. We calibrate our seasonal runoff model using unimpaired reservoir inflow data measured at the catchment’s outlet point, achieving a mean seasonal error of 3%. We also use the remainder term in our runoff efficiency calculations to estimate evapotranspiration (ET) for both eco-zones. In the bedrock zone, results indicate a constant runoff ratio of 0.95, suggesting that almost all precipitation falling on bedrock is converted to streamflow. In the vegetation zone, results show that vegetation runoff efficiency varies from 0.42 to 0.68, with a positive correlation between runoff efficiency and total melt-season moisture even though vegetation water consumption increases during wetter years. We conclude that catchment-wide runoff efficiency is lower in relatively dry years despite the suppression of vegetation ET during droughts.


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