Correlation lengths of meltwater flow through ripe snowpacks, Colorado Front Range, USA

Abstract

The flowpaths of meltwater through snow is known to be an inhomogeneous process. The spatial distribution of meltwater flowing from the bottom of melting snowpacks is the result of horizontal and vertical flow paths within the snowpack. The ability to characterize the spatial distribution of these meltwater flowpaths would be useful in developing models of snow melt runoff which could better characterize snow melt hydrographs. We analyzed near infrared aerial photos of melting snow using a moving window analysis which can characterize correlation lengths in the reflectance of the snow surface. Near infrared is sensitive to snow grain size, which in turn may indicate higher amounts of meltwater; the grains grow faster if the liquid water content is higher. Correlation lengths were five to seven metres and the probability of finding such correlation lengths increased from 0.22 in May 1997 when the melt had just started to 0.68 by late June when melt was well established. Liquid water content at the snow surface was sampled with a dielectric sensor at 0.5 m intervals on 100 m2 and 75 m2 grids. Semi-variograms showed a sill at five to six metres. The liquid water measurements at the snow surface suggest that the correlation lengths derived from the infrared aerial photos represent surface expressions of higher flow density through the melting snowpack. A circular array of 16 snow melt lysimeters each with areas of 0.2 m2 was operated for two years at Niwot Ridge in the Colorado Front Range. Variograms indicated that flows were correlated over a distance of five to seven metres. These three independent methods all suggest a spatial organization of five to seven metres for flowpaths draining ripe snowpacks in the Rocky Mountains.