How Ready Is The Hydrologic Sciences For the Loss of Seasonal Snowpacks (And What Can Be Done)?

Upland ecosystems and downstream communities have relied on seasonal snowpacks to act as 'mountain water towers' that store winter precipitation and release it when ecological and societal demands are high. Historical patterns of snow accumulation and melt have led to predictable streamflow timing and amount. As a consequence of climate change, snowmelt amounts, rates, and timing have changed unevenly in the western U.S. Observations of streamflow and catchment water budgets provide substantial insight into the effects of declining mid-latitude snowpacks. However, hydrological understanding (i.e. the equations in physically-based models) will be severely tested by the predicted loss of seasonal snowpacks by the end of the 21^stcentury: 1. How are declines in snow storage likely to impact upland evapotranspiration and streamflow exports? 2. How will changes in snow storage impact water supply and flood forecasts and what can be done to mitigate likely declines in forecast accuracy?

Empirical evidence supports snowmelt as a more efficient streamflow generator than rainfall, implying that streamflow volumes may decline if snow shifts to rain. However, a shift from snow to rain during winter when energy availability is low and vegetation is largely inactive may increase runoff efficiency in some locations. The extent that snowpack loss will impact upland evapotranspiration will depend largely on whether that water can be stored in the subsurface, a property which varies among catchments. A better understanding of upland storage could be combined with vegetation management to reduce evapotranspiration losses as snowpacks decline. The loss of snow storage also increases challenges for water managers, who are tasked with mitigating more winter peak flows from rainfall, while having less snowpack storage to supplement reservoirs. In addition, forecasts that use a few point snow measurements to predict streamflow volumes for reservoir operations will degrade as snowpacks decline. Combating declining forecast skill and less snow storage will require more-informed water management of built storage using physically-based weather/climate models with better medium-term precipitation forecasts and more parsimonious hydrology models that better capture key processes driving future streamflow changes.