CHANGES IN SNOW WATER STORAGE AND HYDROLOGIC PARTITIONING IN WESTERN NORTH AMERICA (Invited Presentation)

Montane snowpacks in the western United States (US) act as a natural reservoir, storing water during cold months and releasing water throughout warm months when downstream human and ecological demands are highest. In alpine catchments, snowmelt-derived runoff represents up to 80% of total annual discharge, which is governed by varied interactions between snowfall, wind, topography, and vegetation. Climate warming has caused fundamental hydrologic changes in alpine catchments, impacting precipitation phase (rain versus snow) and reducing the spatial heterogeneity of effective precipitation, with unknown effects on current and future water availability sensitivities. In an alpine watershed in the Front Range of Colorado, we evaluate changes in the magnitude and duration of snow water storage and release by comparing effective precipitation seasonality to that of surface water inputs using a Snow Storage Index (SSI). We employ the Distributed Hydrology Soil Vegetation Model and Weather Research and Forecasting Model-based projections of future climatic conditions, to spatially represent and compare the SSI in 2-m historical and end-of-century warming simulations. We find that historically colder conditions and wind redistribution of snowfall resulted in snow drifted subregions with large SSI values (e.g., > 0.75). But proportionally more rainfall, reduced snowpack size and extent, and earlier snowmelt with warming caused SSI decreases in snow drifted areas of 25-100%. Yet, in contrast to the historically wind scoured regions of the catchment, the snow drifted areas experienced a subsequent increase in hydrologic partitioning of annual precipitation to runoff (+10%) with warming, signifying a potential buffer for total alpine catchment and annual discharge loss in a future climate. Fine-scale analyses of hydrologic sensitivities to warming are important because climate conditions are expected to further reduce alpine snow water storage and alter hydrologic partitioning and runoff generation for use downstream.