Air permeability and capillary rise as measures of the pore structure of snow: an experimental and theoretical study

Air permeability and capillary pressure are macroscopic snow properties tha t are influenced by the pore structure of the snow cover. Formulas for pred icting fluid transport, species elution, and acoustive wave propagation req uire parameterization of one or both of these properties. We report paired measurements of permeability and capillary rise from snow samples at field sites in Hanover. New Hampshire, and Sleepers River Research Watershed, Dan ville, Vermont. We augment these data with laboratory tests on sieved snow and glass beads. Our measurements demonstrate a linear relationship between permeability and the ratio of porosity and the square of capillary rise, w hich we corroborate theoretically using a simple conduit model of the pore space. We propose that scatter in the data results, in part, from the effec t of crystal shape on air flow and imbibition contact angle. Since the early measurements and classification schemes of Bader in 1939, m any investigators have expanded the database of permeability observations f or a wide range of snow types. We summarize these data and report our own r ecent observations from the New England sites and from an additional site i n Manitoba, Canada. Our measurements are in the high range of reported valu es. However, after normalizing our data by the square of grain diameter, th ey follow the empirical function of Shimizu fairly closely. This agreement supports our measurements, and demonstrates the usefulness of Shimizu's fun ction for snow types other than the relatively dense, fine-grained snow use d in his analysis. Our normalized permeability data for low density snow, as well as the Shimi zu function, are below theoretical predictions for suspensions of spheres a nd infinite cylinders. Hy extending the model for spheres to oblate spheroi ds and discs, we estimate permeability that is in closer agreement with our data. We suggest that a decrease in surface-to-volume ratio as snow ages m ay account for a relative increase in normalized permeability. Copyright (C ) 1999 John Wiley & Sons, Ltd.