Permafrost Controls on Hydrological Processes in a Changing Climate

Abstract

The primary control on local hydrological processes in northern regions is dictated by the presence or absence of permafrost, but is also influenced by the thickness of the active layer and the total thickness of the underlying permafrost. As permafrost becomes thinner or decreases in areal extent, the interaction of surface and subpermafrost ground water processes becomes more important (Figure 1). The inability of soil moisture to infiltrate to deeper groundwater zones due to ice rich permafrost maintains very wet soils in arctic regions. However, in the slightly warmer regions of the subarctic, the permafrost is thinner or discontinuous. In permafrost-free areas, surface soils can be quite dry as infiltration is not restricted, impacting ecosystem dynamics, fire frequency and latent and sensible heat fluxes. Other hydrologic processes impacted by degrading permafrost include increased winter stream flows, decreased summer peak flows, changes in stream water chemistry, and other fluvial geomorphological processes. Hydrologic changes witnessed include drying of thermokarst ponds, increasing importance of groundwater in the local water balance and differences in the surface energy balance. Arctic hydrology differs substantially from the hydrology of temperate zones, largely due to the interactions of extremes in climate and the land surface characteristics. That is, the presence of permafrost strongly affects hydrologic processes and the fact that its distribution is currently undergoing warming and in some places degradation. Even though the principles governing groundwater movement in arctic regions are the same as those in more temperate regions, groundwater in the far North is affected by the presence of permafrost that acts as an aquitard. Above the permafrost is the active layer, a zone that freezes in winter and thaws in summer. This seasonally thawed zone supplies the summer moisture to plants and for evaporative flux. Suprapermafrost (above permafrost) aquifers are a source of freshwater for some villages near the Arctic Ocean; but for most villages, water is pumped from freshwater lakes in summer and stored in heated tanks for winter use. Subpermafrost aquifers are those consisting of permeable material below the permafrost. Permafrost affects groundwater recharge, movement, and discharge. Ice-rich permafrost prevents infiltration of rainfall or snowmelt water, often maintaining a moist to saturated active layer where the permafrost table is shallow. Permafrost also blocks the lateral movement of groundwater, and acts as a confining unit for water in subor intrapermafrost aquifers. Discharge of water confined beneath the permafrost is possible only through unfrozen zones (taliks), or higher permeable zones such as faults or springs that may perforate the permafrost layer. Although a huge quantity of water is stored in or below the permafrost, this water has historically not been an important component of the hydrologic cycle. However, as permafrost degrades, profound changes in interactions between groundwater and surface water may occur that affect the volume of runoff reaching the Arctic Ocean. A detailed understanding of the processes responsible for these storage phenomena is lacking. With projected increases in surface temperature and decreases in surface moisture levels, the active layer thickness will probably increase, permafrost areal extent will decrease and permafrost will become thinner, leading to subtle but predictable ecosystem responses such as vegetation changes. Permafrost in arctic regions exerts a significant influence upon hydrologic and ecosystem dynamics through controls on vegetation and drainage. In relatively flat areas where the frozen layer is near the surface, the soil moisture contents are usually quite high. These areas have relatively high evapotranspiration and sensible heat transfer, and a low conductive heat transfer due to the insulative properties of thick organic soils. The climax vegetative species and soil forming processes are dominantly controlled by the closely coupled permafrost and hydrologic conditions. As permafrost degrades, the soil moisture holding capacity increases, soil drainage improves and moisture is no longer held near the surface but percolates to deeper reservoirs. As permafrost becomes thinner or absent, groundwater contributions from springs become more important. Figure 1. Discontinuous permafrost imparts strong local controls on hydrologic processes. As permafrost degrades, the surficial hydrological processes will changes markedly, initiating a cascade of impacts to local ecology and surface energy balance.