Illinois Drought: Update, Illinois State Water Survey

Illinois Drought Update

Illinois Drought Update, August 26, 2005
Illinois State Water Survey, Department of Natural Resources

SUMMARY. August precipitation across most of Illinois has been about normal or above normal, and near-surface soil moisture and streamflows have increased after rain events. However, precipitation since March remains much below normal over most of west central and northern Illinois, deeper soil layers remain dry, groundwater levels are below normal, baseflows in streams and rivers remain low, and reservoir and lake levels are below normal.

1. DROUGHT STATUS. According to the U.S. Drought Monitor, most of northern and western Illinois remains in a severe or extreme drought (categories 2 and 3 in their 4-category drought classification). Much of eastern and southern Illinois has seen improvements in recent weeks, according to the Drought Monitor. Most of those areas now are considered only “abnormally dry” (Figure 1).

2. PRECIPITATION. Statewide precipitation for August 1-25 was 3.21 inches, which is 10 percent above normal. Furthermore, precipitation was more evenly spread across the state than in previous months. As a result, there have been short-term gains on the drought. However, in the long-term the statewide rainfall deficit since March 1 is 7.24 inches, or 67 percent of normal (Figure 2). Precipitation on August 25 was somewhat disappointing, with the heaviest swath receiving 1 to 2 inches across the southern part of the state from St. Louis to Evansville. Only limited relief was received in west-central portions of the extreme drought area, with just a few places reporting more than 0.50 inches of rain.

3. LAST 100 YEARS. As of August 25, statewide March-August precipitation is the 6th driest on record (Figure 3). Normal precipitation for the rest of August would result in March-August precipitation in 2005 being the 7th lowest on record. As of August 25, March-August precipitation in northwest, northeast, and central Illinois is the 3rd lowest on record (Figure 4).

4. SOIL MOISTURE. Soil moisture in the 0 - 72 inch layer indicates that soils remain very dry across central and north central Illinois (Figure 5), especially below 20 inches.

5. GROUNDWATER. Shallow groundwater levels in Illinois remain below average. Water levels at ISWS shallow observation wells at Fermi National Laboratory (DuPage County) and Bondville (Champaign County) are at their lowest levels for August since records started in November1988 and March 1982, respectively. The water level at Fermi Lab is 10.05 feet below land surface (1.97 feet below normal) and at Bondville 7.30 feet below land surface (1.89 feet below normal). Water levels in both wells fell about 0.9 feet since late July. The Groundwater Information Office continues to receive reports from well drillers lowering pumps or redrilling wells to greater depths as a result of lowered water levels in aquifers in northern and northwestern Illinois, including Kane (Campton Township), LaSalle, and Whiteside Counties (highlighted in Figure 5).

            Irrigation demand appears to have decreased during the past two weeks along the border of Bureau and Whiteside Counties. Groundwater levels were observed by the ISWS on August 3, 2005 to have risen by 1 to 4 feet in the buried Sankoty aquifer. By contrast, groundwater levels in observation wells completed in the surficial Tampico aquifer (overlying the Sankoty) declined about 3 to 4 inches in Whiteside, Lee, Bureau, and northern Henry counties. Historical records suggest that this trend will continue, possibly until December, despite the lessening of irrigation demand.

6. ILLINOIS STREAMFLOWS. Streamflow levels in most portions of Illinois rose considerably in the past three weeks as a result of heavy rainfall, particularly following a series of storms from August 16th to 18th (Figure 6). The most significant increases occurred for the Kishwaukee River in northern Illinois and the Little Wabash River in southeastern Illinois, both of which had been experiencing 10-year low flow conditions in early August. August 2005 average flows for most of Illinois will fall in the below normal range, and over a third of the State is expected to be in the well-below normal range (lowest 10th percentile) for the month. The Green River in northwestern Illinois continues to experience record low flows for this time of year, and earlier this month it experienced its second lowest flow conditions on record (dating to 1940).

Recent heavy rains have caused variable amounts of runoff in Illinois streams and increased shallow soil moisture, but in general have not replenished deeper layers of soil or percolated to the shallow groundwater to the extent needed to increase baseflows to streams. Thus, over the course of the next week and without much additional rainfall, many streams and rivers could be expected to return to the same low flow conditions that existed prior to last week’s heavy rainfall events.

7. WATER LEVELS AT PUBLIC WATER SUPPLY (PWS) RESERVOIRS. The Water Survey records month-end water levels for 35 public water supply (PWS) reservoirs in Illinois, with records generally extending back to the mid-1980s to early-1990s. Although the levels of some reservoirs have risen slightly during August in response to rainfall, most reservoirs (for which we have mid-month values) continue to have falling water levels following their normal seasonal trend. Many reservoirs are below their normal August levels – typically at levels that would occur, on average, once every five years. A few reservoirs are at levels that have not been experienced in August since the 1988-1989 drought. More information will be available following the end of month reports. The recession of lake levels during drought is typically a slow progressive process, with most reservoir levels falling no more than 1 foot per month – particularly during cooler weather. Thus, for most water supply systems we would not expect to see concerns for low water levels for several months to come; and, without additional rain, the recent reprieve in dry conditions may do little to reduce the potential for long-term water supply impacts.

8. FEDERAL RESERVOIRS. Carlyle Lake and Lake Shelbyville are roughly 1.1 and 1.0 foot below full pool, respectively, having dropped several inches since the beginning of August. The water level for Rend Lake is at an elevation of 405.3 feet, which is low for this time of year, but no longer one of the lowest levels on record for this time of year, as it was in July. Low water levels on the federal reservoirs do not affect water supplies.

9. MISSISSIPPI AND OHIO RIVERS. There were reported delays earlier this month in barge traffic in the lower Ohio River near Cairo and the Mississippi River near Keithsburg due to low water and sand bars. The rivers were reportedly closed for a short period at each location while dredging cleared the navigation channel. The water level in the Mississippi River at St. Louis has risen several feet over the last week and is now about five feet above the level where navigation problems might be expected to occur.

10. ILLINOIS RIVER. The water level in the Illinois River has risen significantly over the past week in response to rainfall throughout its watershed, and for now is in the range of normal flow levels for late August.

11. LAKE MICHIGAN. The level of Lake Michigan is at 578.0 feet, having dropped 0.1 foot since the beginning of August. The level in the lake can be expected to continue dropping until mid-winter as part of its normal seasonal cycle. Lake Michigan is now 1.4 feet below normal, and is 1.3 feet higher than its record low level for August, set in 1964.

12. OUTLOOK. A cold front with a substantial potential for rain is approaching Illinois currently, and the National Weather Service predicts that this will provide a varied amount of precipitation across the state on August 26-27, reaching 0.50 to 1.25 inches in most places. Unfortunately, the following period through the first week of September is forecast to be drier than normal across Illinois. The recent updates to the monthly and seasonal forecasts for the U.S. show no tendency away from the normal distribution of chances for September or Autumn precipitation in the Midwest.

            Fall is usually drier than spring and summer. Statewide precipitation averages 3.24 inches in September and 2.87 inches in October, compared with about 4 inches per month in summer. However, less water is lost to the atmosphere in fall as temperature decreases and crop growth slows and then ceases. Soil moisture generally starts to recharge in the fall, but with large soil moisture deficits, much above normal precipitation is needed to recharge soil moisture fully and to bring baseflows in rivers and streams up to normal. Shallow groundwater levels can be expected to continue to decline until deep soil moisture is replenished. Streamflows and lake and reservoir levels normally start to decline at this time of year, but below normal precipitation would result in further lowering of streamflows and lake and reservoir levels.

For more drought information please go to

Derek Winstanley: tel 217-244-5459; e-mail

Illinois Drought

Illinois State Water Survey

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Champaign, IL 61820-7463
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