Hydrologic effects of frozen soils in the upper Mississippi River basin

The variable infiltration capacity (VIC) macroscale hydrologic model was modified to improve its performance in cold regions by adding frozen soil and energy balance snow accumulation and ablation algorithms. Frozen soil penetration was determined by solving thermal fluxes through the soil column. Infiltration and runoff response parameterizations were modified to reflect the simulated ice content of the soil. The revised model was tested using point data from the University of Minnesota Rosemount Agricultural Experiment Station and subsequently applied to two subcatchments in the upper Mississippi River basin. The point tests showed that the model was able to reproduce observations of the snowpack, soil liquid water content, and freezing and thawing front depths. Comparisons of simulated discharge from the two subbasins showed that the frozen soil algorithm reduced infiltration during winter and spring thaws and increased rapid runoff response. However, the magnitude of the increased runoff response is relatively modest, at the scale of the two upper Mississippi subbasins tested.