Water Towers of the West: Where are they and how have they changed?

Abstract:

Snow-dominated mountainous regions rely on accumulated snow and snowmelt for water supply, and the storage of water in the snowpack delays and extends the downstream delivery of meltwater through the spring and summer months, when water demand is highest. Climate warming, however, has reduced snowfall across the western United States, reducing the annual snowpack and shifting snowmelt toward earlier in the year. These hydrologic changes alter the timing of surface water inputs (SWI) associated with rainfall and snowmelt and ultimately the catchment and regional hydrologic signatures. In this research, we derive an index of seasonal snowpack water storage, termed the Water Tower Index, which is based on differences in seasonality between precipitation and SWI. We define mountainous water towers as regions where there is significant temporal misalignment between precipitation and SWI, indicating a delay in meltwater production due to water storage in the snowpack. We used output from the Variable Infiltration Capacity (VIC) model from 1950-2014 to evaluate trends in WTI associated with regional warming. Evaluating the western U.S. by snow fraction, we determined that, on average, areas with an annual average snow fraction of >0.25 have experienced a significant decrease in annual WTI. This suggests that the delay in the timing between falling precipitation and SWI generation is becoming smaller, likely due to changing precipitation phase (snow to rain) and earlier snowmelt, both a product of climate warming. Future changes in WTI have broad implications for water availability and regional annual hydrologic behavior.