Wintertime Temperatures in the Fine-Textured Soils

of the Saginaw Valley, Michigan

Randall J. Schaetzl and Daniel M. Tomczak

Soil temperatures at four sites in Saginaw County, Michigan were monitored at 5 and 20 cm depth, on poorly-drained, loamy and clayey soils over the 1996-97 winter season. To determine the effects of land use on soil temperatures, two pairs of sites (forested and cultivated) were established. Our goal was to extend previous work that suggested that soils in Michigan freeze most frequently and for the longest duration in the Saginaw Valley area., and to present observational and quantitative data on soil freezing and fall-winter-spring soil temperatures for this region. Despite the snowy, warmer than normal winter, soils froze to depths >20 cm on cultivated sites, which were windswept and barren of snow for most of the winter, facilitating heat loss. Sites insulated by forest cover and leaf litter, as well as thin but persistent snowpacks, froze to depths of only 2 to 3 cm; temperatures at depth hovered near 1-2C for most of the winter. Nearer the surface, soils were generally colder and had higher daily temperature and variability than did soils at depth (20 cm). Forested soils were more moderated with respect to diurnal and weekly temperature change then were cultivated soils on open sites. Although both sites were located on poorly-drained soils, the wetter site developed more and larger ice lenses, and was colder throughout the fall and winter. We introduce the term pedothermic period for times in which soil temperatures have consistent temporal trends and characteristics, and similar within-period variability and ranges. We identified three pedothermic periods: ;ate fall (ends in mid December), winter (mid December to early-mid March), and spring (begins in early-mid March). Soil temperatures cool rapidly in fall, remain fairly constant under mid-winter snowpacks, but fluctuate greatly in spring after the snowpack is melted.