Controls on Snowmelt Partitioning to Surface and Groundwater Flow

Abstract:

High altitude mountainous regions are vital source areas of water and their snow-dominated hydrologic processes are particularly sensitive to climate change. Yet, basic questions remain about snowmelt’s partitioning between surface and subsurface flow and the role it plays in replenishing alpine groundwater. High geologic heterogeneity in mountain regions and inter-annual climate variation challenge our ability to address these questions and anticipate related water resource vulnerabilities. This study compares bedrock and colluvial aquifer system responses to snowmelt on Niwot Ridge in Colorado over five years (2008-2012) to evaluate the role of snowmelt in annual groundwater recharge over varying temporal and spatial scales. We monitor water tables and conservative tracers in source water samples to infer groundwater interaction with snow melt based on unique signatures of source waters. We find that snowmelt is the most important hydrologic alpine groundwater recharge event with water table rises of up to 8m and 4m in response to the freshet in the bedrock and colluvial aquifers, respectively. However, the nature of groundwater response not only depends on climate conditions and the geologic setting, but also on wind scour and topography that affect seasonally frozen soil extent and thus soil infiltration capacity during snowmelt. Areas with high scour and small overlying snow experience smaller annual water table rise and recharge comes from a mix of snowmelt and summer rain sources. This finding suggests that localized recharge processes where seasonally frozen soil is present may not be as vulnerable to declining snowpacks as aquifers more exclusively reliant on snowmelt for recharge. Because wind scour indices can be determined using remotely sensed information, it is possible to anticipate this groundwater response over large scales and in remote regions.