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The Colorado River has been identified as the most overallocated river in the world. Considering predicted future imbalances between water supply and demand and the growing recognition that base flow (a proxy for groundwater discharge to streams) is critical for sustaining flow in streams and rivers, there is a need to develop methods to better quantify present-day base flow across large regions. To accommodate the snowmelt-dominated hydrology of the region, we applied a conductivity mass balance hydrograph separation approach, using streamflow and specific conductance measurements, to estimate base flow at 229 streamgages across the Upper Colorado River Basin (UCRB). We adapted and applied the spatially referenced regression on watershed attributes (SPARROW) water quality model to assess the spatial distribution of base flow, the fraction of streamflow supported by base flow, and estimates of and potential processes contributing to the amount of base flow that is lost during in-stream transport in the UCRB. 

Percentage of base flow delivered to the Lower Colorado River from incremental reaches of the Upper Colorado River

Percentage of base flow delivered to the Lower Colorado River from incremental reaches of the Upper Colorado River

(Public domain.)

Baseflow (groundwater flowing to streams) is estimated to contribute over 50% of the total streamflow in the Upper Colorado River Basin and is thus crucial for sustaining ecological and human water needs in this highly managed area. Baseflow may be sensitive to changing climate, but the sensitivity is not well constrained. To estimate baseflow response to climate change, we tested how warm/wet, median, and hot/dry future climate scenarios affect baseflow in the basin using a hydrologic model. Results show that the largest declines in baseflow may occur in the headwater streams, and total baseflow delivered to the Lower Colorado River Basin may decline by up to 33%, although delivery may increase in the near future by 6% under a warm/wet climate. We hypothesize that basinwide baseflow declines because of greater increases in evapotranspiration relative to precipitation in the future. Baseflow loss during in-stream transport is projected to increase by 1 - 5%. The changes in baseflow may affect both human and ecological water users in an area where water supply doesn’t always meet demand.

Read more about declining baseflow.

Learn more about Integrated Water Availability Assessments: Upper Colorado River Basin.