Impacts of Changing Frozen Ground Regimes on Groundwater Recharge

Abstract

The spring snowmelt accounts for a large percentage of annual groundwater recharge in northern latitudes. Many factors control the proportion of snowmelt that becomes groundwater or runoff, including the volume of snow, the timing and rate of snowmelt, soil moisture content, and the extent of frozen ground. However, few studies have examined the impacts of frozen ground on groundwater recharge. Critically, although studies have found that midwinter snowmelt events are expected to increase with climate change, the effects of midwinter snowmelt on hydrologic partitioning and groundwater recharge have previously remained unclear. In this thesis, I use two complementary approaches to elucidate the complex relationships between snow cover, midwinter melt events, frozen ground, and groundwater recharge. First, I present a retrospective statistical analysis of groundwater, meteorological, and soil observations to determine the principal factors contributing to variation in winter and spring groundwater recharge. Secondly, I employ a physical model of heat and water transfer in the vadose zone to quantify the effects of warming air temperatures and resulting changing frozen ground regimes on groundwater recharge. Through these two studies, this thesis demonstrates that midwinter snowmelt and subsequent freezeback events are key drivers of hydrological partitioning, but the effect is sensitive to the temporal sequence of events. Midwinter snowmelts expose bare ground and play a critical role in increasing soil ice content and frost depth if followed by cold periods. The presence of frozen ground during snowmelt tends to increase runoff and decrease groundwater recharge. Therefore, midwinter snowmelt and subsequent freezeback events are key predictors of reduced groundwater recharge and can result in anomalously low recharge years when this sequence of events occurs. Model simulations reveal that, in regions with seasonally frozen ground, climatic warming can increase the number and magnitude of midwinter melt and subsequent freezeback events, reducing groundwater recharge. Furthermore, climatic warming reduces the amount and cover of snow and increases evapotranspiration, decreasing groundwater recharge regardless of frozen ground conditions. Overall, these findings present important implications for climate change impacts on groundwater recharge and groundwater supply in regions with seasonal snow cover and frozen ground. This thesis reveals the largely unrecognized role of changing frozen ground regimes in driving recharge variability, particularly anomalously low recharge years. I demonstrate how increased number and magnitude of midwinter melt and subsequent freezeback events may reduce groundwater recharge in warming climates, providing a scientific basis for predicting how inputs to aquifers may change with future warming. As climates continue to warm and groundwater extraction for human consumption continues to strain aquifers, understanding the process by which aquifers are replenished – specifically, groundwater recharge from the spring snowmelt – and how this process may change in the future, is imperative for successful assessment and management of this essential resource.