Streamflow and groundwater response to precipitation variability in a snow-dominated, subalpine headwater catchment, Colorado Rocky Mountains, USA
MS: University of Colorado Boulder, 2016.
Snow-dominated mountainous watersheds of the Western US that provide vital freshwater storage and resources are facing changes in the timing and amount of snowmelt due to climate change. Increased precipitation variability will impact the hydrologic connectivity of mountainous catchments and resulting water availability. During years of low snow accumulation, groundwater can supply a substantial portion of annual streamflow that natural and human ecosystems rely on, yet groundwater influences on streamflow generation in mountainous areas are poorly understood due to complex topography and geology and a diversity of flow processes. Isotopic and geochemical surface water and groundwater data are used to examine groundwater recharge dynamics and hydrologic connectivity in the forested, subalpine Como Creek headwater catchment of the Boulder Creek Watershed in the Colorado Front Range. Interannual variations in surface water-groundwater interactions are investigated across years that exhibited near-record snowfall in Colorado (2011), the worst snow drought in decades (2012), and a rare, heavy rain event (2013), providing a unique opportunity to investigate the impacts of precipitation variability on mountainous hydrologic systems. Mean subalpine groundwater residence times are 3-7 years, with an estimated mean catchment water residence time of 1.1 years. Net subalpine groundwater recharge is estimated to account for an average of 19% of annual precipitation, with slower recharge rates and less recharge observed with below average snowfall. Results indicate that the subalpine aquifer-stream system is recharge-driven and snowmelt-dominated, but has the possibility of transitioning to a hybrid (snow-rain) influenced system with greater recharge from seasonal rains. Geochemical (Na+, Mg2+, SO42-, Si, ANC) and isotopic tracers (δ18O) are analyzed via hydrologic mixing models and End-Member Mixing Analysis (EMMA) to determine the appropriate source waters contributing to Como Creek streamflow over 2011-2014. Mean relative contributions to streamflow were 36% soil water, 35% snowmelt, and 29% groundwater. Observed changes in hydrologic connectivity over recent years, including multiple major groundwater recharge events for one year, a drying out of the subsurface following snow drought, and decreased catchment water residence time due to large rain inputs, provide insight into the response of mountainous headwater catchments to precipitation variability under a changing climate.