Streamflow, Baseflow, and Hydrologic Partitioning in Response to Changing Snowmelt in the Upper Rio Grande Basin, 1980 - 2015

Understanding how changing climatic conditions affect streamflow volume and timing is critical for effective water management. In the Rio Grande Basin of the southwest U.S., decreasing snowpack, increasing minimum temperatures, and decreasing streamflow have been observed in recent decades, but the effects of hydroclimatic changes on baseflow, or groundwater discharge to streams, have not been investigated. In this study, we determine how trends in precipitation, snowpack accumulation, and snowmelt rate relate to streamflow, baseflow, and the hydrologic partitioning of baseflow and runoff at 12 sites in the Upper Rio Grande Basin (URGB) from 1980 to 2015. Streamflow was partitioned into baseflow and runoff components at a daily time step using conductivity mass balance hydrograph separation. Annual trends in streamflow, baseflow, baseflow index, precipitation, snowmelt rate, and peak snow water equivalent (SWE) were evaluated from 1980 to 2015 using the non-parametric Mann-Kendall trend test. Results indicate that baseflow forms a large component of total streamflow, contributing an average of 49% of total discharge in the URGB. Trends in streamflow were almost always associated with trends in baseflow, with statistically significant decreases in streamflow and baseflow at 8 and 7 sites, respectively. Statistically significant increasing trends in baseflow index occurred at 3 sites, while no trends were observed at 9 sites, demonstrating that the hydrologic partitioning of runoff and baseflow did not change for most streams. Decreases in snowmelt rate were more frequently associated with decreases in baseflow and streamflow than were decreases in precipitation and SWE, highlighting the importance of snowmelt rate as a process controlling streamflow. Findings from this study help to advance the understanding of the hydrologic system in the URGB, as well as the impacts of climate fluctuations on water supply and water budgets.