Understanding the hydrologic effects of frozen soil

Frozen soil plays an important role in the hydrologic cycle of cold regions by increasing the amount of moisture retained in the soil column through the winter and decreasing the infiltration rate of the soil during spring melt. This study investigates how the spatial distribution of snow and frozen soil influence the hydrologic response of a catchment. A field observation program was conducted over three winters (1997–1998, 1998–1999 and 1999–2000) in southeastern Minnesota. Observations of snow depth and soil temperatures were used to identify statistical distributions that can be used to represent their spatial distributions. The statistical distributions of snow and soil temperature were used to represent sub-grid variability of snow and frost within grid cells of the variable infiltration capacity (VIC) macroscale hydrologic model. The influence of the spatial distributions are secondary to the effects of the snow and soil frost algorithms, but they improve the representation of the observed processes. Partial snow cover increases the exchange of sensible heat with the atmosphere during the spring, and the advection of sensible heat from bare to snow covered patches increases the rate of spring melt. The spatial distribution of soil ice increases winter and spring infiltration and soil moisture drainage, while maintaining the increased peak flow response caused by the presence of significantly frozen soil. Discharge records from gauged catchments in the upper Mississippi River basin were used to identify the statistical significance of observed (e.g. vegetation type and precipitation) and simulated (e.g. soil moisture and snow cover) catchment characteristics on spring runoff response. Ice content was determined to be a significant predictor of both the runoff ratio and peak flow response, indicating that frozen soil plays an important role in spring runoff response in the upper Mississippi basin.