Lawrence Berkeley National Laboratory

The North American Monsoon occurs July–September in the central Rocky Mountains bringing significant rainfall to Colorado River headwater basins. This rain may buffer streamflow deficiencies caused by reductions in snow accumulation. Using a data-modeling framework, we explore the importance of monsoon rain in streamflow generation over historical conditions in an alpine basin. Annually, monsoon rain contributes 18 ± 7% water inputs and generates 10 ± 6% streamflow. The bulk of rain supports evapotranspiration in lower subalpine forests. However, rains have the potential to produce appreciable streamflow at higher elevations where soil moisture storage, forest cover, and aridity are low and rebound late season streamflow 64 ± 13% from simulated reductions in spring snowpack as a function of monsoon strength. Interannual variability in monsoon efficiency to generate streamflow declines with low snowpack and high aridity, implying the ability of monsoons to replenish streamflow in a warmer future with less snow accumulation will diminish.