Roles and Controls of Snowmelt in Alpine Groundwater Recharge

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High altitude mountainous regions are vital source areas of water. Hydrologic processes in these regions are particularly sensitive to climate change because of the presence of glaciers, snow, and permafrost. Yet, basic questions remain about the rate alpine groundwater is replenished, its source waters, and the size, flow paths and residence time of groundwater reservoirs. Inter-annual climate variation as well as the high geologic heterogeneity of remote mountain regions poses a central challenge to understanding the impact that changes to mountain water resources will have on future water supplies. This research characterizes alpine groundwater’s response to evaluate the role of snowmelt in annual groundwater recharge over varying temporal and spatial scales and includes a comparison of bedrock and colluvial aquifer system responses to snowmelt on Niwot Ridge near Boulder, Colorado over five years (2008-2012). We monitor water tables and analyze isotopic and chemical tracers in source water samples to infer groundwater interaction with snow melt based on unique signatures of source waters. Principal component analysis and end member mixing analysis was used to solve for stream water composition. We find that snowmelt is the single most important hydrologic event relating to alpine groundwater recharge with water table rises of up to 8m and 4m in response to the freshet in the bedrock and colluvial aquifer, respectively. The nature of the groundwater response depends on recent and current climate conditions as well as the geologic setting, and it varies even at small (100m) spatial scales. Groundwater is also an important component to streamflow in these alpine catchments with the timing of groundwater contributions to surface water also affected by the underlying geology. The annual flushing of groundwater prior to snowmelt’s presence in stream flow suggests these alpine groundwater aquifers are relatively small and they do not naturally buffer inter-annual water shortages. These findings have important societal implications to downstream water vulnerability, especially in light of forecasted declines in winter snowpacks, a key reservoir in itself.  This may reduce groundwater recharge volume leading to a confounding effect on natural water storage volume. Reductions to both of these reservoirs may lead to lower surface flows given the role that snowmelt and groundwater play in stream flow generation.