Hydrology

The hydrology of mountain permafrost can be very complex (Giardino et al., 1992; Krainer and Mostler, 2002; Schrott, 1996; Schrott, 1998; Woo et al., 2008) and affect both the thermal regime due to convective heat transfer, and slope stability due to changes in the effective stresses. Water may flow within the active layer, below the base of the permafrost, through taliks and even through open channel systems in the frozen soil. To determine the influence and relevance of any permafrost body in a study area as a hydrologic resource, it is necessary to establish a water balance for the watershed area that the permafrost features encompass. Some tasks to establish hydrological information include:

o Measure climate data as indicated above that determine the amount of precipitation, water storage and storage capacities including water retard.

o Measure water flow at several locations with groundwater wells (flows into and out of permafrost areas are expected to be predominantly subsurface) including upslope run-offs.

o Measure snow depth and snow water equivalent (SWE) along a series of transects through the watershed prior to and at least once during snow ablation. If snow depths are known, SWE can also be calculated with reasonable accuracy for regions with long measurement records.

The objective of the measurements is to isolate a base flow component that could be attributed to slow release of water contained within the body of frozen ground and to groundwater originating from different sources located above.

Hydraulic properties of the various soil types encountered can be measured by carrying out laboratory tests from collected soil samples, or from slug or pump tests carried out in the field. Detailed site investigations are needed to determine the number and location of groundwater wells required for a specific problem.

Several different probes are available to measure moisture contents and pore pressures. Rist and Phillips (2005), for example, carried out field tests using TDR probes, vapour traps and lysimeters to measure suction and spatial variability of water infiltration within the active layer during freeze-thaw cycles. Other suction probes, such as tensiometers, are available to measure in situ suction (e.g. Ng et al., 2008; Springman et al., 2003; Thielen and Springman, 2005) in which water is replaced as the saturating medium in winter by an alcohol (circa 2% by volume) / water mixture.

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