While high porosity and low thermal conductivity make snow a protective blanket against extreme cold, this same openness makes it quite permeable to flows of air and water. Snow acts as both a source and a transmitter of water, quickly routing it through to the ground, which then channels much of the water to streams or rivers. Knowledge of the snow melting rate and snow hydrological processes (Marsh, 1990; Bales and Harrington, 1995) allows a more accurate prediction of flooding and of the distribution of water for agriculture and forest growth. Possible increased rainfall and snowmelt as a result of global warming increase the urgency for accurate runoff predictions (Jones, 1996). As well as its hydrological significance, snow acts as a buffer to chemical species transported by wind, rain, and meltwater (Bales etal, 1989; Davis, 1991; Harrington and Bales, 1998).
Water flow through snow is similar to that through other granular materials but is complicated by freeze-thaw effects, metamorphism of the ice matrix, and textural layering. Because snow is highly permeable, water moves through it rapidly with typical speeds of 1-20 cm h-1. Within this coarse material, gravitational forces dominate capillary forces. Low viscosity wetting fluids (such as water) develop instabilties which can concentrate flow in preferential channels, or flow fingers, ahead of the background wetting front. Stratigraphic inhomogenieties in permeability and capillary tension can impede and laterally divert the flow, as well as trigger the formation of flow fingers.
Unlike soils, snow is sufficiently open to allow the movement of air in the interstitial pore space. The interstitial air flow, most often caused by wind-induced pressure variations across the rough surface of snow, is known as ventilation. Only in very high permeability snow, such as hoar, does buoyancy-induced natural convection occur. Both natural and forced convection accelerate transport of water vapor and chemical species through snow and firn. An understanding of the mechanics of chemical transport in snow and firn is essential both to understanding air-snow transfer processes of chemical exchange (Albert et al., 2002) and to interpreting the species concentrations in polar cores, which are vital records of paleoclimate history (e.g. Waddington et al., 1996).
This section describes the theory of saturated (one-phase) and unsaturated (two-phase) flow through snow and discusses the macroscopic properties of permeability and capillarity that affect the flow. Just as thermal conductivity relates to the structure of the ice matrix, flow properties relate to the pore structure. The section concludes with a discussion of the basic features of uniform water flow and of unstable flow through flow fingers.
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