Sensitivity of Hydrologic Partitioning to Snowpack Dynamics, Como Creek, CO

Friday, 19 December 2014
Theodore B Barnhart1, Noah P Molotch1,2, Adrian Adam Harpold3,4, John F Knowles1 and Suzanne P Anderson5, (1)University of Colorado at Boulder, Geography / INSTAAR, Boulder, CO, United States, (2)Jet Propulsion Laboratory, Pasadena, CA, United States, (3)University of Colorado at Boulder, Institute of Arctic and Alpine Research, Boulder, CO, United States, (4)University of Nevada Reno, Natural Resources and Environmental Science, Reno, NV, United States, (5)University of Colorado at Boulder, INSTAAR and Geography, Boulder, CO, United States
Snowmelt is the primary source of surface water in the western United States and many other regions on Earth. 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 (R) at Como Creek, a snowmelt dominated catchment on the Colorado Front Range. Como Creek is a 4.5 km2 headwater catchment spanning 2900-3560 m and is part of the Niwot Ridge Long Term Ecological Research Station and the Boulder Creek Critical Zone Observatory. We use observations of snow water equivalent (SWE), ET, and precipitation (P) from Niwot Ridge, CO, and discharge from Como Creek to explore relationships between snowpack dynamics and snowmelt partitioning. Measurements of ET are collected adjacent to Como Creek at the Niwot Ridge Ameriflux site and are assumed representative of the hydrologic fluxes in Como Creek. Analyses from point data show that years with higher peak SWE/P ratios partition proportionally more snowmelt to ET (pValue: 0.045). For example, water year (WY) 2005 has a peak SWE/P ratio of 0.49 and a growing season ET normalized by WY precipitation (ET/P) ratio of 0.48 while WY 2008 has a peak SWE/P ratio of 0.83 and an ET/P ratio of 0.82. Observations also show that years that experience later peak SWE (DOY=142) partition proportionally less snowmelt into ET (ET/P=0.42) compared to years that experience earlier peak SWE (DOY=86) and partition proportionally more snowmelt to ET (ET/P=0.56). Further point analyses also suggest that more rapid snowmelt results in proportionally less snowmelt partitioned to ET and more partitioned to runoff. To explore the underlying processes responsible for these relationships at the catchment scale we use the Regional Hydro-Ecologic Simulation System (RHESSys) to model how snowmelt is partitioned between ET and R under observed conditions and under a variety of climate change induced snowmelt timing, magnitude, and rate scenarios.