Snowmelt runoff efficiency mediated through multi-year climatic controls on groundwater recharge in the Wasatch Mountains, Utah, USA.

Rapid population growth, pronounced warming, and changes in the amount, timing and form of precipitation present significant challenges to managing water resources in the coming century. Like many cities in the western U.S., Salt Lake City relies on mountain snowmelt for both surface and groundwater supplies. Unlike other major cities, mountain water sources are immediately adjacent to the city and opportunities to develop surface water storage are minimal. Consequently, this rapidly growing urban region is extremely sensitive to both the amount and the timing of annual snowmelt. Any reduction in surface water supply must be matched by increased groundwater withdrawal from aquifers recharged by the same mountain snowpacks. This tight coupling between surface and groundwater supplies provides a unique challenge to water managers tasked with providing sustainable water resources in a changing climate. Working in partnership with local water utilities, this study addresses that challenge using over a century of climate and discharge data for the seven catchments that are the primary water supply for the urban region.

The seven catchments in the study are located in the Wasatch Front Mountains of Utah and range in elevation from 1300m to 3500m. Mean annual precipitation and temperatures (PRISM) vary between catchments from 791mm to 1290mm and from 3.3C to 6.9C. Mean annual streamflow (SLC Public Utilities, USGS) ranges from 150 mm to 820 mm (normalized by watershed area). Although all catchments have warmed approximately 1.5C over the last five decades, surprisingly neither annual nor seasonal temperatures were related to water yield. In contrast, a multiple linear regression model using antecedent groundwater storage and snowmelt dynamics was able to accurately predict annual water yield (R2 >= 0.9 for all catchments). This model suggests that climate change will have the greatest impacts on surface water supply during two time periods: one during annual snowmelt and the second during the time scales of groundwater recharge. Initial analyses suggest that the timescale for groundwater recharge is controlled by precipitation, melt dynamics, and temperature during the preceding three to five years. Combining multiple years of climate to predict streamflow may be more effective than simply using an annual time step.