Quantifying the effects of changing snowpack dynamics on hydrologic partitioning at multiple scales
AuthorLongley, Patrick Cullen
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In the Western U.S. seasonal snowpacks have historically been important for recharging groundwater and generating surface water resources. Regional warming threatens snow- packs throughout the West by increasing the frequency of winter rain, reducing snowpack size, and leading to earlier snowmelt. These changes are likely to impact both small scale ecosystem function and large scale water availability by shifting streamflow earlier into the year and reducing summer low flows. Additionally, shifting from snow to rain spreads water inputs throughout more of the year which favors a decrease in runoff and an increase in evapotranspiration. We leverage regional scale model results and catchment scale experi- mental data to consider how changes in snowpack dynamics affect hydrologic partitioning at multiple scales. Using the Basin Characterization Model (BCM) we found that shifts from snow to rain without warming led to consistent increases in annual evapotranspiration in the Southwest, but had a diverging effect on recharge and runoff. Diverging changes in water input rate (monthly rain or snowmelt rate) led to increased runoff at the expense of recharge in the western Sierras, where atmospheric rivers can deliver precipitation at a rate that exceeds the snowmelt rate, and generally had the opposite on more interior regions. Additionally, changes in recharge and runoff were largest in areas where winter precipitation exceeded soil water storage.It is difficult to predict how individual catchments will be affected by changes in snow- packs. Effects will vary based upon climate and subsurface properties. We studied Sagehen Creek watershed, a 27 km2 watershed in Northern California, as an example of how chang- ing snowpack dynamics affect catchment-scale subsurface storage. Specifically, we used streamflow records and isotopic tracers to quantify how reduced snowpacks have affected storage and summer lowflows. Leveraging multiple techniques to study subsurface storage we found that storage decreased by 10 to 20 mm in the past 60 years in Sagehen, which is connected with a long term decline in lower elevation snowpacks of 4.5 cm/decade.