Quantifying seasonal snow and groundwater contributions to streamflow across the Upper Colorado River Basin, 1986-2020

In the southwestern United States, the Upper Colorado River Basin (UCRB) faces substantial water availability challenges. Snowmelt dominates hydrology in the basin, with much of the streamflow originating as meltwater. Snowmelt either becomes surface runoff or groundwater recharge, but this partitioning is not well constrained. Furthermore, groundwater recharge from snowmelt can discharge back into streams becoming an important component of streamflow. On average, over half of the streamflow in the UCRB is estimated to originate from groundwater discharge to streams, highlighting the importance of baseflow in sustaining surface water. Yet we have not quantified past spatio-temporal variability of baseflow and its contributions to streamflow, nor do we understand variations in streamflow and baseflow sources under shifting hydroclimates. Here we describe the development and application of linked models of baseflow and streamflow to characterize sources and transport pathways of both baseflow and streamflow in the UCRB at a seasonal timestep from 1986-2020, including the lagged delivery of groundwater to streams over longer timescales. Results suggest that baseflow yields are greatest in headwater catchments during spring, and that a majority of baseflow is derived from water that takes over one season to move through the subsurface to streams. We also find that although streamflow and its sources vary seasonally, on average across the basin, about half of streamflow is from baseflow, which is particularly important for sustaining streams outside the snowmelt season and at lower elevations.