CONTROLS ON THE MAGNITUDE OF BEDROCK RECHARGE AT THE HILLSLOPE- AND WATERSHED-SCALE IN TWO PAIRED, SNOW-DOMINATED WATERSHEDS

Mountains in arid and semi-arid regions receive a disproportionately large amount of precipitation compared to their bounding valley aquifers due to orographic effects; however, little is known about how precipitation is partitioned and transported within the mountain block. Recent research has illuminated dynamic interactions between soil and bedrock reservoirs on research hillslopes, showing that bedrock permeability is potentially a major control on the volume of bedrock groundwater recharge and streamflow response dynamics. This study focuses on: i) estimating the magnitude of bedrock recharge at the hillslope scale and ii) evaluating the magnitude of recharge at the watershed scale in two paired catchments underlain by bedrock with different permeabilities. Bedrock well hydrographs are used to estimate bedrock recharge at the hillslope scale from snowmelt and storm events. A chloride mass balance is used to estimate watershed-scale recharge. Bedrock storage is compared using Mean Residence Time (MRT) estimates, core sample analysis, and outcrop fracture mapping. Mean effective porosity derived from outcrop fracture mapping is 0.1 for the watershed underlain by argillite and 0.01 for the granitic catchment. Mean residence time for the argillite watershed is approximately 4.74 years, while the fractured granite watershed has a MRT of 2.12 years, indicating the argillite has greater storage than the granite. Bedrock well hydrographs in the granitic catchment exhibit a flashy response to input and slow recession, while hydrographs in the argillite catchment show rapid response and rapid recession, demonstrating different recharge and drainage dynamics in each watershed. Additionally, each watershed’s stream hydrograph displays a different response and recession. The argillite catchment has a subdued response to events, while the granitic watershed displays a flashy response and significant increase in streamflow. The streamflow response and well dynamics imply a strong bedrock influence on watershed dynamics. This research expands current knowledge of the magnitude and controls on recharge to the bedrock reservoir in mountainous terrain, demonstrating the importance of understanding the role that bedrock plays in partitioning and transmitting flow through mountain blocks.