Impacts of warming winters on recharge in a seasonally frozen bedrock aquifer

Under conditions of a changing climate, winters are predicted to be warmer and wetter in the northern hemisphere. Yet, the impacts of increasing midwinter snowmelt and rain-on-snow (ROS) events on groundwater recharge are poorly understood, particularly in seasonally frozen shallow bedrock. To characterize the mechanisms and antecedent conditions inhibiting or enabling winter recharge in seasonally frozen bedrock, a field investigation was conducted in eastern Ontario, Canada over the winter of 2019-2020. Since rock outcrops are known areas of recharge in nonwinter months, a low-lying granitic outcrop and adjacent soils were heavily instrumented. Both hydrogeologic and cryospheric conditions of the surface, unsaturated and saturated zones were monitored at a high temporal resolution (10-15-minute intervals). Climate data collected during the study indicated that winter conditions were warmer and wetter than long-term (30 year) averages, which allows for parallels to be drawn between observations from this winter and what could be expected for future winter conditions under climate change. Hydraulic head and temperature measurements in two bedrock wells indicated rapid midwinter recharge occurred in response to most ROS and snowmelt events despite the presence of basal ice in the snowpack and shallow frozen ground. Volumetric water content and temperature measurements in two soil profiles revealed that where the soil-bedrock interface was unfrozen, this became the primary pathway permitting infiltration to bypass the frozen surface layer. A major midwinter ROS event generated ponding on surface which subsequently froze, ultimately reducing effective recharge from the event and inhibiting future snowmelt recharge. Estimates of net recharge per ROS and/or snowmelt event indicated that moderate events were more effective at recharging the bedrock aquifer than higher intensity events and events in winter were more effective than in fall. Implications of these findings suggest that rock outcrops provide a window for rapid localized recharge during midwinter warming or ROS, where basal ice or frozen soil would otherwise inhibit vertical infiltration. Study results provide field-based evidence for both enhanced and impeded winter recharge in seasonally frozen bedrock aquifers due to warming winters under climate change.