After a cool beginning of the month, soil temperatures are rising throughout the state in mid-May, according to Jennie Atkins, the Water and Atmospheric Resources Monitoring program manager at the Illinois State Water Survey, part of the Prairie Research Institute at Illinois.
Soil temperatures at depths of 4 inches under sod averaged 67.5 F in Illinois on May 15, 5 degrees higher than the long-term average. Daily highs were in the 60s and 70s for most of the state.
Temperatures began the month 3 to 5 degrees lower than normal as they fell back into the 40s in northern Illinois. However, temperatures have been steadily rising since the first week. Southern and west-central Illinois had the highest regional temperatures at 69 F on May 15. The coolest were in the north with an average of 63 F.
Under bare soil, temperatures were slightly higher, averaging 73 F on May 15 as temperatures reached into the 90s in central and southern Illinois.
Rainfall has been higher than normal so far for May as the state received 0.73 inches above the long-term average. Despite the wet conditions, soil moisture has been declining from the highs seen at the end of April. Soil moisture levels at 2 inches averaged 0.32 water fraction by volume on May 15, a decrease of 26 percent from May 1. Soils were wettest in the south with an average of 0.35 wfv. West-central Illinois was the driest at 0.28 wfv.
Similar, but smaller, declines have occurred at 4 and 8 inches. Soil moisture remains high at depths of 20 inches and deeper with no significant changes over the month.
The Illinois State Water Survey’s WARM Program collects hourly and daily weather and soil information at 19 stations across the state. Daily and monthly summaries can be found at the WARM website and in the Illinois Water and Climate Summary.