Not Just Snowpack: Vegetation-Groundwater Controls on Summertime Streamflow in Colorado River Headwaters

Snowmelt-driven watersheds provide water for billions of people, yet warming temperatures threaten to reduce streamflow across these regions. One pathway for greater water loss is through increased evapotranspiration (ET), particularly during the warm summer growing months. However, the magnitude of summer transpiration and the water sources accessed by vegetation remain poorly understood. While snowmelt is the primary driver for peak runoff and supplies soil moisture for early summer transpiration, vegetation water use and its influence on summer baseflow are less well understood.

To study this pathway, we instrumented an 81 ha headwaters micro-catchment in the Upper Colorado River Basin (UCRB), where ET represents the largest annual water flux. This site includes eddy-flux towers, stream gages, shallow groundwater wells, sap-flux sensors, and a dense soil-moisture network. High-resolution eddy-flux observations show how ET is sustained even during extended summer droughts. Over three growing seasons, daily fluctuations in soil moisture, groundwater, and streamflow indicate roots intercept shallow groundwater to support the continued transpiration during these dry periods.

We extended this analysis basin wide across 18 headwaters catchments and observed that summer growing season conditions independently regulate streamflow, with effects rivaling those of snowpack. Warm summers suppress streamflow, causing high-snowpack years to be near-average, while cool summers elevate flow.

Together these results demonstrate upland vegetation suppresses summer streamflow in mountain headwaters by sustaining transpiration through shallow groundwater access during hot, dry periods. As warming continues, this vegetation-groundwater pathway will intensify summer streamflow declines across mountain regions, with significant implications for future water availability and management.