Projected Large-Scale Regional Baseflow Decreases in Response to Changing Climate in the Upper Colorado River Basin

Recent studies indicate that, on average, greater than half of the annual streamflow in the Upper Colorado River Basin (UCRB) originates as baseflow - a proxy for groundwater discharge to streams. While a multitude of studies have predicted how future changes in climate and anthropogenic activities will influence surface runoff in the UCRB, less is known about baseflow response to changing climatic conditions. Given the quantified importance of groundwater in sustaining streamflow, effective water resource management in the basin requires estimates of how baseflow discharge to streams will respond to projected warming and shifts in precipitation and evapotranspiration fluxes. We applied a combined modeling approach to estimate future baseflow conditions in the UCRB. Specifically, baseflow estimates from site-specific conductivity mass balance models were used to calibrate hybrid statistical-deterministic-geospatial SPARROW (SPAtially Referenced Regressions On Watershed attributes) models. The SPARROW model then was driven by statistically downscaled projections of future climate and runoff to estimate future baseflow. At the local stream-reach scale, baseflow is predicted to change by +/- 10%, depending on the future climate scenario and time-horizon, but at the regional-scale, baseflow delivered to the outlet of the UCRB is predicted to decrease by 30-40% by the year 2080 regardless of the climate scenario. These regional-scale decreases in baseflow are driven largely by increases in evapotranspiration associated with increased temperatures, offsetting any local-scale increases in baseflow due to increased precipitation. These findings further highlight the importance of considering groundwater and surface water as an interconnected resource when planning for future water availability.