Date of Award

Spring 1-1-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

First Advisor

Mark W. Williams

Second Advisor

Eve-Lyn S. Hinckley

Third Advisor

John F. Knowles

Abstract

Snow-dominated mountainous watersheds of the western US that provide vital freshwater resources are facing increased precipitation variability due to climate change, including changes in the timing and amount of snowmelt. During years of low snow accumulation, groundwater can supply a large portion of streamflow that natural and human ecosystems rely on, yet groundwater dynamics in mountainous areas are poorly understood due to complex topography and geology and heterogeneity in flow processes. Isotopic and geochemical surface water and groundwater data are used to examine groundwater recharge dynamics and hydrologic connectivity in the mixed alpine-subalpine Como Creek headwater catchment of the Boulder Creek Watershed. 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 an opportunity to investigate the impacts of precipitation variability on mountainous hydrologic systems. Mean subalpine groundwater residence times are 2-6 years, with a mean catchment water residence time of 1.1 years. Net subalpine groundwater recharge accounts for an average of 36% of event (snowmelt, rain) precipitation. The fast turnover of the subalpine groundwater reservoir suggests it has limited storage capacity and is consistent with a high recharge, snowmelt-dominated system. Geochemical (Na+, Mg2+, Ca2+, Si, ANC) and isotopic (δ18O) tracers are analyzed via End-Member Mixing Analysis (EMMA) to determine the source waters contributing to streamflow over 2011-2014. Results show that mean annual streamflow is derived of 36% soil water, 35% snowmelt, and 29% groundwater. Stream-groundwater interactions are high in the subalpine and most snow or rain infiltrates the subsurface before entering the stream. Observed recharge from the 2013 rain event indicates that the subalpine aquifer-stream system has the possibility of transitioning to a hybrid (snow-rain) influenced system with greater recharge from seasonal rains. Observed changes in hydrologic connectivity over 2011-2014, including multiple major groundwater recharge events during a year, 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.

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