Impacts of Historic Climate Variability on Seasonal Soil Frost in the Midwestern United States

The present study examines the effects of historic climate variability on cold-season processes, including soil temperature, frost depth, and the number of frost days and freeze-thaw cycles. Considering the importance of spatial and temporal variability in cold-season processes, the study was conducted in the midwestern United States using both observations and model simulations. Model simulations used the Variable Infiltration Capacity (VIC) land surface model (LSM) to reconstruct and to analyze changes in the long-term (i.e., 1917-2006) means of soil frost variables. The VIC model was calibrated using observed streamflow records and near-surface soil temperatures and then evaluated for streamflow, soil temperature, frost depth, and soil moisture before its application at the regional scale. Soil frost indicators-such as the number of frost days and freeze-thaw cycles-were determined from observed records and were tested for the presence of significant trends. Overall trends in extreme and mean seasonal soil temperature from 1967 onward indicated a warming of soil temperatures at a depth of 10 cm-specifically in northwest Indiana, north-central Illinois, and southeast Minnesota-leading to a reduction in the number of soil frost days. Model simulations indicated that by the late-century period (1977-2006), soil frost duration decreased by as much as 36 days compared to the midcentury period (1947-76). Spatial averages for the study area in warm years indicated shallower frost penetration by 15 cm and greater soil temperatures by about 3 degrees C at 10-cm soil depth than in the cold years.