In an earlier post, I discussed the sizable rainfall totals across central and southern Illinois from Tropical Storm Isaac. Its impact on soil moisture has been dramatic. We measure soil moisture at 19 sites across Illinois as part of the Illinois State Water Survey WARM program. Here are the 4, 8, and 20 inch soil moisture maps based on readings at midnight on September 3.
We measure the soil moisture as the fraction of water in the soil by volume. So a value of 0.4 means that 4/10 of the soil is water by volume. Or you could view it as 40 percent of the soil is water by volume.
- Values in the 0.4 to 0.5 range means the soil is close to or at saturation;
- Values in the range of 0.3 to 0.4 are typical of a moist soil;
- Values less than 0.3 are getting dry;
- values between 0.1 and 0.15 are pretty much at the lower limit of what plants can use.
Below are the maps for 4, 8, and 20 inches. For most of Illinois, the fraction of water by volume is in the range of 0.3 to 0.4 – that’s good, especially for this time of year. There are a few places over 0.4, which means they are getting close to saturation at that level of the soil. A month ago a lot of stations were in the 0.10 to 0.15 range, which is pretty much the lower limit of what plants can use.
These numbers suggest that soil moisture conditions in much of Illinois have improved dramatically after Tropical Storm Isaac. However, they still have room for improvement in northern Illinois. And the rains came too late for the corn crop and much of the soybean crop. Even so, it is reassuring that we have gone a long ways towards recharging the soil moisture as we move into fall.
By the way, you’ll notice the 0.06 values in west-central IL. Those are in an extremely sandy soil near Havana in the Illinois River floodplain. They drop quickly after a rain event and stay very low most of the year.