Spatially-Distributed Estimates of Groundwater Discharge to Streams in the Upper Colorado River Basin

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 groundwater discharge to streams is critical for sustaining flow in streams and rivers, there is a need to develop methods to better quantify groundwater discharge to streams across large regions. We adapted and applied the spatially referenced regression on watershed attributes (SPARROW) water quality model to assess the spatial distribution of groundwater discharge to streams, the fraction of stream flow supported by groundwater discharge, and estimates and drivers of the amount of stream flow that originated as groundwater that is lost during in-stream transport in the Upper Colorado River Basin (UCRB). The model estimates an average of 1.8×10¹⁰ m³/yr of groundwater discharge to streams in the UCRB, greater than 80% of which is lost during in-stream transport to the Lower Colorado River Basin via processes including evapotranspiration and water diversion for irrigation. On average, 56% of the stream flow in the UCRB originated as groundwater. Observed relationships between groundwater discharge to streams and spring-time snow cover suggest that future changes in climate may result in decreased groundwater discharge to streams, but a greater reliance on groundwater for sustaining stream flow. Our results indicate that surface waters in the Colorado River Basin are dependent on groundwater discharge to streams, and that management approaches that consider groundwater and surface water as a joint resource will be needed to effectively manage current and future water resources in the Basin.