Observed soil moisture data are collected at 19 sites in Illinois by the Illinois State Water Survey’s Water and Atmospheric Monitoring program. The soil moisture is measured at 5 cm (2 inches), 10 cm (4 inches), 20 cm (8 inches), and 50 cm (20 inches). All amounts are reported as the fraction of water by volume of the soil. For example, a value of 0.35 means that 35% of the soil is occupied by water.
Silty clay loam soils, common in Illinois, are considered saturated at values of 0.45 to 0.50. Field capacity is in the neighborhood of 0.35. Values below about 0.20 would be at the permanent wilting point. As the name implies, that is the level where plants wilt and do not recover. Plant roots are unable to recover any water below this level because it is tightly bound to the soil particles.