Impact of Tropical Storm Isaac on Illinois Soil Moisture

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.

Four-inch soil moisture, expressed as the fraction of water in the soil by volume. Click to enlarge.

Eight-inch soil moisture, expressed as the fraction of water in the soil by volume. Click to enlarge.

Twenty-inch soil moisture, expressed as the fraction of water in the soil by volume. Click to enlarge.

Rainfall Totals from Tropical Storm Isaac

Tropical Storm Isaac dropped a lot of rain on Illinois over the Labor Day weekend. As noted in an earlier post, tropical storms are rare in Illinois but they do happen. However, this one was especially interesting because the heavy rains fell over areas in drought across the state. In a normal year, such heavy rains would have produced widespread flooding in low areas, streams, and small rivers. But not this year. I have not heard any reports of significant flooding. This suggests that the dry soils absorbed most of the rainfall. And what little runoff was produced, ended up in streams that were much below-normal. More on this in another post.
The first map shows the 7-day rainfall totals from Tropical Storm (TS) Isaac. The areas in red had over 4 inches of rain. Areas north of Interstate 80 saw little of the rainfall.
The second map shows the 30-day rainfall totals. Parts of Illinois already had substantial rainfall before TS Isaac. With the addition of the rainfall from Isaac, much of central and southern Illinois had accumulated over 4 inches of rain. In fact, some areas had over 6 inches of rain (areas shaded in yellow or red).
The third map shows the 30-day rainfall departures from normal. Areas in green and blue are above normal. The areas in the two shades of blue are 2 to 4 inches above normal. The areas in purple and violet (?) are 4 to 8 inches above normal. Clearly, areas with much above normal rainfall will have a significant impact on the drought.

Here are the 7-day rainfall totals that includes all of the rainfall from Tropical Storm Isaac. This is based on the radar/raingauge product from the NWS. Click to enlarge.

Here are the 30-day rainfall totals showing how wet Illinois has been. Several areas in southern and central Illinois have received over 6 inches of rain. Click to enlarge.

Here are the 30-day rainfall departures from normal. Areas in the blues and purples are more than 2 inches above normal. Click to enlarge.