GROUNDWATER DISCHARGE AS A SOURCE OF STREAMFLOW IN SMALL, HIGH-ELEVATION, HEADWATER BASINS

Groundwater (GW) recharge in many snow-dominated, high-elevation headwater basins mainly occurs during seasonal snowmelt. Recharge occurs as infiltration of snowmelt through fractures and other pathways to the underlying mountain aquifer. GW will then flow down-gradient and may become a source of water for wetlands, springs, and streams. In snow-dominated watersheds, the annual stream hydrograph is dominated by snowmelt during spring and summer. GW discharge occurs year-round but its relative contribution to streamflow becomes greater as snow melts. Late season drainage from groundwater-fed wetlands and springs, along with direct GW discharge, can become the only sources of streamflow. Post melt, recharge to the aquifer nearly ceases and GW discharge begins to drain the aquifer as it maintains flow in wetlands, springs, and streams. Generally, mountain aquifers are replenished annually during snowmelt. However, snowpack can vary by year, sometimes much lower than average, and melt out much earlier. This can lead to compounding affects: The aquifer may not be completely refilled, and GW discharge becomes the dominant source of streamflow much earlier, continuing without replenishment and resulting in a greater loss from the aquifer. Therefore, variability in amount, timing, and phase of precipitation may result in significant impacts to groundwater recharge and discharge, affecting the hydrologic regimes of wetlands and springs, and late-season streamflow. The changes can ultimately result in harm to ecological functions and downstream water availability.

Our study site is the Senator Beck Basin (SBB) in the San Juan Mts of Colorado, USA. SBB is a small high-elevation research basin with a stream and wetland and instrumented with two meteorologic stations and a stream gauge; a SNOTEL station is nearby. Using 16 years of hydrologic and meteorologic data, we are investigating the how variations in inputs (e.g., SWE, discharge, etc.) and their timing influence GW contributions to maintain hydrologic functions. Our results show a significant variation in GW discharge between wet, dry, and average years. GW discharge is significantly influenced following consecutive dry years, suggesting that climate variability may have a detrimental effect on water availability in headwater basins.