Frozen Ground Controls on Hydrological Processes

Abstract:

Frozen ground establishes a unique discipline of hydrologic science where the hydrologic regime is intimately coupled with the thermal regime to the extent that one may not be completely understood without correct characterization of the other. In permafrost regions, material properties may change drastically on a scale of centimeters to meters, particularly in the vertical dimension due to distinct changes in soil and thermal characteristics. Properties may vary just as dramatically in the horizontal dimension across the boundary of discontinuous permafrost. Although the spatial extent of permafrost changes on relatively slow time scales in response to disturbance or a changing climate, this too introduces an added level of complexity. Permafrost may nearly eliminate the interactions between near-surface and sub-permafrost aquifers, which in essence defines the hydrologic response of every watershed that is directly influenced by permafrost. Even though the principles governing water movement in permafrost areas are the same as those in more temperate regions, interactions of extremes in climate and the land surface characteristics render permafrost hydrology different from the hydrology of temperate latitudes. Ice-rich permafrost prevents deep percolation of rainfall or snowmelt water, often maintaining a moist to saturated active layer above the permafrost table. Most hydrologic activities are confined above-ground or in the thin active layer, which supplies summer moisture for baseflow and/or plant transpiration. Limited storage capacity of the thawed active layer does not support extended baseflow in a stream, though the proportion of baseflow increases as the percentage of permafrost extent decreases. In areas where permafrost is discontinuous or where it has thawed substantially near the surface, local hydrology may display a markedly different character as there are stronger exchanges between the surface water and the ground water system, or water may drain laterally resulting in drier surface conditions. Understanding the interdependence of frozen ground, hydrology and ecosystems is critically important to enable accurate projections of future conditions in the high latitudes.