Given the importance of groundwater in sustaining Upper Colorado River Basin (UCRB) streamflow (over 50% of streamflow originates as baseflow), effective management of water resources in the basin requires estimates of how baseflow may change under projected climatic changes.听 We applied projections of future climate to a calibrated hybrid statistical-deterministic model (SPAtially Referenced Regressions On Watershed attributes, SPARROW) to estimate future baseflow changes under warm/wet, hot/dry, and central tendency climatic conditions for thirty-year periods centered around 2030, 2050, and 2080 compared to the historical period (1984-2012).听 Projected widespread baseflow declines, regardless of changes in precipitation, suggest increased evapotranspiration reduced baseflow.听 The largest relative declines in baseflow are projected to occur in high elevation source areas, and baseflow delivered to the UCRB outlet declining by between 11% and 40%.听 Results have implications for human and ecological water availability in one of the most heavily managed watersheds in the world.