Regional Scale Estimates of Baseflow and Factors Influencing Baseflow in the Upper Colorado River Basin

Groundwater and surface water are interconnected and can be considered a single water resource, and thus it is important to understand groundwater contributions to streamflow, or baseflow, within a region. Previously, quantification of baseflow using chemical mass balance at large numbers of sites was not possible because of data limitations. However, a new method using regression-derived daily specific conductance values with conductivity mass balance hydrograph separation allows for baseflow estimation at sites across large regions. This method was applied to estimate baseflow discharge at 229 sites across the Upper Colorado River Basin (UCRB). Subsequently, relationships between baseflow discharge and climate, soil, topography, and land cover characteristics were evaluated using principal component analysis (PCA). Results indicate that approximately half of the streamflow in the UCRB is baseflow derived from groundwater discharge to streams. Higher baseflow yields typically occurred in upper elevation areas of the UCRB, and PCA identified precipitation, snow, sand content of soils, land surface slope, percent grasslands, and percent natural barren lands as being positively correlated with baseflow yield. Temperature, potential evapotranspiration, silt and clay content of soils, percent agriculture, and percent shrublands were negatively correlated with baseflow yield. These findings indicate that baseflow contributes a significant fraction of total streamflow in the UCRB and provide insights into watershed characteristics that may affect baseflow volume. Results demonstrate the interconnected nature of groundwater and surface water and highlight the importance of managing them as a single water resource.