Modeling Soil Temperatures and the Mesic-Frigid
Boundary in the Great Lakes Region, 1951-2000
J. Schaetzl, Bruce D. Knapp and Scott A. Isard
Understanding the spatial and temporal variation in soil temperatures is important to classification, land use and management. To that end, mean annual soil temperature (MAST) data for Wisconsin and Michigan were modeled in order to (1) determine the effects of the Great Lakes and their snowbelts on soil temperatures, and (2) better estimate the location of the boundary between the mesic and frigid soil temperature regimes in this region. The location of the mesic-frigid (M-F) line is particularly difficult to determine where east-west gradients in air temperature cross north-south trends in snowfall due to Lake Michigan. Additionally, the soil temperature regime of several Great Lakes’ peninsulas near the M-F line is in question. In order to determine the accuracy of our soil temperature model, soil temperature data output from it were compared to data derived from thermocouples implanted in soils at 39 sites in northern Michigan that had been collecting data several times daily for more than six years. Error statistics for the model show that it has essentially no mean bias when examined on an annual basis or for winter, and only a bias of 0.1o C for the warm season. The M-F line in Wisconsin and Michigan is slightly north of most previously estimated locations, and is strongly influenced by the snowbelt in southern Michigan. Soils in deep snow areas stay warmer in winter than do soils inland, increasing their MAST and forcing the M-F line north of where air temperatures alone might have placed it. Lake-effect areas also stay cold longer into the spring season, and cool down more slowly in fall. Soil temperatures in these areas are, therefore, more moderated on an annual basis, as indicated by coefficients of variation.