A STUDY OF SOIL TEMPERATURE CLIMATOLOGY: COTTONWOOD, SOUTH DAKOTA SOIL TEMPERATURES FROM 1982 TO 2004

Soil temperatures have been collected at some sites in South Dakota for over 20 years. The information is important for agriculture, hydrology, climatology, and other interests. The Cottonwood, SD, site has over 20 years of soil tempera ture data at the 5, 10, and 20 cm depths and several years at the 50 and 100 cm depths. This study used morning observed soil temperatures at the 5, 10 and 20 cm depths to develop a procedure for predicting the beginning and end of the annual freeze/thaw cycle at other similar sites across the state. Maximum, mean and minimum morning soil temperatures, sorted by Day of Year (DOY) and depth, were plotted to show the annual soil temperature oscillations and variability at each depth. The 5 cm and 10 cm temperatures were each divided into spring and fall sets. The first DOY observed temperature above 32oF (0oC) and the last DOY observed temperature at or below 32oF (0oC) were determined for each spring. The first DOY temperature at or below 32oF (0oC) and the last DOY temperature above 32oF (0oC) were determined for each fall. From these, percentiles were calculated to indicate the expected beginning and end of the spring and fall freeze/thaw cycle. The first DOY soil temperatures at or below 40oF (5oC) in the fall were also determined to compare time between soil temperatures reaching 40oF (5oC) and 32oF (0oC). A forecast model could be useful to the agricultural community and others for the potential period before soil freeze-up. The procedure developed for the Cottonwood data will be used at other sites across the state to develop maps of when frozen soils are expected in South Dakota.