GROUNDWATER DISCHARGE TO SURFACE IN PERMAFROST REGIONS UNDER A WARMING CLIMATE

The role of permafrost plays in regulating shallow groundwater flow and discharge to surface waters in cold regions is not well understood. Thinning or thawing of permafrost alters the dynamics of water cycle near the Earth’s surface. As subsurface temperatures change, the permeability of frozen ground increases as a result of thawing or decreases as a result of freezing. Consequently, groundwater flow and discharge to surface waters is promoted or impeded. This study examines the impact of air temperature change on permafrost temperature, thickness of the active layer above permafrost, and groundwater discharge to surface over seasonal and decadal time scales. A coupled groundwater flow and heat transport model was developed to simulate groundwater flow and heat transport in a 2D hillslope setting. Modeled processes include saturated groundwater flow, conductive and advective heat transport, latent heat release or absorption during freezing and thawing, permeability varying with ice-water saturation, as well as seasonal and long-term temperature variations exerted on the land surface. Modeling results show that in a yearly cycle, groundwater flow occurs in the active layer for five months from late spring to early fall. Maximum groundwater discharge to the surface lags the maximum subsurface temperature by approximately two months. Under an increasing air temperature scenario of 3 ºC per 100 years, the active layer thickness can increase by threefold over a 40-year period. Groundwater discharge to the surface can experience a similar threefold increase over the same period. These modeling results are consistent with reported increasing trends in recent decades in winter baseflow for many rivers in permafrost regions. The results of this study imply that with increased warming there will be more groundwater flow in the active layer and increased groundwater discharge to rivers if sufficient upgradient water is available to replenish the increased discharge. There could be an overall lowering of the water table in the recharge area if insufficient resource of water is available. More studies quantifying hydrologic changes in permafrost regions are needed to evaluate the impact of permafrost hydrodynamics on water cycle in different geologic settings as well as consequent effects on biogeochemical cycling.