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Barnhart, Theodore B. ¹ ; Molotch, Noah P. ² ; Harpold, Adrian A. ³ ; Livneh, Ben  ⁴ ;  Knowles, John F. ⁵ ; Schneider, Dominik  ⁶ ; Anderson, Suzanne P. ⁷

¹ Department of Geography and INSTAAR, University of Colorado, Boulder, CO
² Department of Geography and INSTAAR, University of Colorado, Boulder, CO and Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
³ Department of Natural Resources and Environmental Science, University of Nevada, Reno, NV
⁴ National Oceanic and Atmospheric Administration, Boulder, CO
⁵ Department of Geography and INSTAAR, University of Colorado, Boulder, CO
⁶ Department of Geography and INSTAAR, University of Colorado, Boulder, CO
⁷ Department of Geography and INSTAAR, University of Colorado, Boulder, CO

Snowmelt is the primary source of surface water in the western United. Climate warming is forecast to impact the amount of precipitation that falls as snow and forms the mountain snowpack. Climate change induced alterations to snowpack translate to changes in snowpack magnitude, the timing of snowmelt, and changes in snowmelt rate. We ask how these perturbations may impact how snowmelt is partitioned between evapotranspiration (ET) and runoff (Q) at Como Creek and across the western United States. Como Creek is a snowmelt-dominated catchment on the Colorado Front Range. We use observations of snow water equivalent (SWE), ET, precipitation (P), and soil moisture to explore relationships between snowpack dynamics and snowmelt partitioning at Como Creek and meteorological forcing data and model output from the Variable Infiltration Curve model (VIC) at 1/16 of a degree to look at snowmelt partitioning relationships across the western United States. Analyses from point data show that years with higher snowmelt rates partition proportionally more snowmelt to Q whereas years with lower snowmelt rates partition proportionally more snowmelt to ET (r2=0.82, p=0.005). For example, water year (WY) 2011 had a snowmelt rate of 0.61 mm/day and a growing season ET normalized by WY P ratio of 0.34 while WY 2012 had a snowmelt rate of 0.40 mm/day and an ET/P ratio of 0.50. Soil moisture data shows that WY 2011 had higher peak soil moisture (60.2 %) compared to WY 2012 (21.7 %). This suggests a potential mechanism where rapid snowmelt in WY 2011 often overwhelmed atmospheric demand for water, bringing the soil to field capacity more frequently and for a greater duration, causing greater recharge to shallow groundwater. This work has implications for water resources management as a greater process understanding of snowmelt partitioning could be used to better-forecast runoff from mountainous regions.