QUANTIFYING BASEFLOW USING GROUNDWATER LEVELS IN THE UPPER COLORADO RIVER BASIN

Water from the Colorado River and its tributaries supplies 40 million people with municipal water and irrigates nearly 5.5 million acres of land. The vitality of the Colorado River faces significant uncertainty in light of frequent and prolonged droughts due to climate change. Progressing knowledge concerning groundwater and surface water interactions of this litigious resource is critical in informing water resource managers and ultimately easing tension among Colorado River water users. Baseflow is considered a proxy for groundwater discharge to streams. Groundwater is vital in sustaining streamflow via the mechanism of baseflow, particularly during periods of low precipitation and overland flow. Previous baseflow studies within the Upper Colorado River Basin indicate disparate values for baseflow contribution. This study aims to quantify baseflow to the Roaring Fork River, a major tributary within the Upper Colorado River Basin. The Roaring Fork River flows along the western margins of Colorado’s Southern Rocky Mountains. We employed an approach based on groundwater level data retrieved from the Colorado Division of Water Resources (CDWR). We compiled groundwater level data observed between 2000 and 2022 from over 150 wells in the Roaring Fork subbasin. We then interpolated static water level elevations and created contour maps for groundwater levels, which leads to the inference of hydraulic gradients in the vicinity of the Roaring Fork River. We also analyzed existing estimates for hydraulic conductivity. On the basis of hydraulic gradient and hydraulic conductivity, a groundwater discharge of 1.39 m³/s to the Roaring Fork River is estimated. In parallel, baseflow separation via a graphical method is conducted, which leads to a smaller estimated baseflow than that from this groundwater level approach. This study demonstrates the potential of utilizing existing groundwater level data to complement baseflow study. Enriching the arsenal of baseflow analysis will help contribute to sustainable water resource management.