Nearly ninety percent of Colorado River water originates from the snow-dominated watersheds in Colorado, Utah, and Wyoming. The river supports 40 million people and $1.4 trillion in economic activity. Dwindling stream water supply aggravates increased demand downstream in basins like the Colorado River. The Colorado River is currently grappling with a multi-decadal drought with legal and political tensions rising due to voluntary cuts in water use because of diminishing supply. 

Declining streamflow and reservoir storage, however, cannot be explained solely by lower precipitation in its upstream basins. In 2021, the Upper Colorado River Basin received 80% normal snowpack but delivered only 30% of average streamflow to its receiving reservoir. These trends show a need to better understand mechanisms of mountain discharge for better adaptive management. 

Hydrologic studies show increased evapotranspiration as a primary mechanism of streamflow reduction in a warmer climate. This reduction is largely because less snow cover decreases light reflection, resulting in warmer soils and longer growing seasons. But, scientists have previously done limited analyses at the resolution necessary to understand how groundwater recharge processes are linked to land surface conditions associated with snow, evapotranspiration, and soil moisture in complex terrains like the Colorado River Basin. 

So, in this new study, scientists used a high-resolution, integrated hydrological computer model. It included the deeper subsurface and determined that dwindling groundwater reserves may deplete mountain streamflow. Accelerated warming in mountain watersheds and associated impacts on snowmelt, evapotranspiration, and groundwater, may explain reductions in streamflow.

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