Snow cover, frost depth, and soil water across a prairie pothole landscape

The prairie pothole is an important hydrologic component of the Prairie Pot hole Region of North America. Snow cover and frozen soil affect runoff and soil water recharge, but little is known about their temporal and spatial c haracteristics across a prairie pothole. A 2-ha prairie pothole near Morris , MN was instrumented to assess snow cover, soil water, and frost depth at 11 locations across two transects that traversed the major and minor axes. Measurements were made over three winters beginning in 1990, Snow cover was deepest on the leeward side of a ridge located on the NW end of the major axis and shallowest and least variable in the bottom of the depression. Max imum rate of descent of the freezing front in the soil approached 25 mm d(- 1) whereas maximum depth of soil freezing neared 1.2 m across the experimen tal site during this study. Rate of soil freezing during the fall was more rapid at positions of higher elevation and with thinner snow cover. Elevati on and snow cover accounted for as much as 56% of the variability in rate o f soil freezing across the site. Snow cover accounted for as much as 46% of the variation in maximum frost penetration; penetration was deepest at pos itions with the least snow cover. Thawing of the soil profile was most rapi d in the bottom of the depression and appeared to be accentuated by infiltr ation of surface water through localized unfrozen soil pores or cracks, Max imum surface thaw rate approached 40 mm d(-1). Soil water recharge occurred during spring snowmelt, but snow cover accounted for <20% of the variation in recharge. This study suggests that topographic features across a prairi e pothole can influence snow cover and, therefore, frost depth. Thickness o f snow cover, however, has little influence on positional soil water rechar ge within a pothole during snowmelt because frozen soil can impede infiltra tion and enhance runoff.