Water and Atmospheric Resources Monitoring Program - , Illinois State Water Survey

Prairie Research Institute State Scientific Surveys

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A Guide to Illinois Climate Network (ICN) Soil Moisture Data

ICN soil moisture data are presented in units of water fraction by volume (wfv). To better understand what the values mean, the data can be related soil field capacity and wilting point.

Field capacity is roughly defined as the amount of water a soil can hold before draining. At field capacity, the pores of the soil contain both air and water. It is not the same as saturation in which the soil's pores are completely filled with water.

The wilting point refers to when a soil dries to a point that it is difficult for plants to extract water and so the plants wilt. The soil still contains moisture in its smallest pores, but plants are unable to access it.

The difference between the soil's field capacity and wilting point is considered the plant water available.  The closer the soil moisture value is to field capacity, the more water is available for plant use. The closer the value is to the wilting point, the less water is available.

Field capacity and wilting point are dependent upon the soil's texture. For most ICN stations, the soil texture is silt loam.  The field capacities and wilting points of the soils at the stations have not been measured; however, we are able to obtain typical values for these parameters from the scientific literature. These are presented in the table below. Comparing the ICN data to the values can give you a better understanding of the soil's moisture levels.

*    The numbers provided are typical values for the soil texture and should only be used as guidelines.  Source:  Rowell, David L.  Soil Science:  Methods and Applications.  Longman Scientific & Technical.  1995.