Correlation Lengths of Vertical Flowpaths
in Melting Snowpacks, Colorado Front Range, USA
Mark W. Williams1, Richard Sommerfeld2, Sam Massman1, and Mark Rikkers1
The melting of snow is known to be an inhomogeneous process. The spatial distribution of melt-
water 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 snowmelt runoff models that could better characterize
snowmelt hydrographs. Near-infrared aerial photos of melting snow have been analyzed using a
moving window analysis that can characterize correlation lengths in the reflectance of the snow
surface. Near-infrared is sensitive to snow grain size, which indicates the concentration of melt-
water; the grains grow faster if the liquid water content is higher. The probability of finding such
correlation lengths was about 0.22 in May 1997, when the melt had just started, and rose to 0.68
by June when the melt was well established. Correlation lengths for all sampling dates ranged
from 5 to 7 m. Liquid water content at the snow surface was sampled with a dielectric sensor at
0.5-m intervals on two 100-m2 grids. Semi-variograms showed a sill at 5 to 6 meters. The liquid
water measurements at the snow surface suggest that the correlation lengths derived from the in-
frared aerial photos represent surface expressions of vertical flowpaths through the melting snow-
pack. A circular array of 16 snowmelt 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 corre-
lated over a distance of 5 to 7 m. These three independent methods all suggest a correlation
length of 57 m for vertical flowpaths draining ripe snowpacks in the Rocky Mountains.
1
Institute of Arctic and Alpine Research and Department of Geography, University of Colorado, Boulder,
Colorado 80302, USA
2 Rocky Mountain Forest and Range Experimental Station, U.S. Forest Service, Department of Agriculture,
Fort Collins, Colorado 80525, USA
3
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