Illinois Drought: Update, Illinois State Water Survey

Illinois Drought Update

Illinois Drought Update, July 7, 2005
Illinois State Water Survey, Department of Natural Resources


1. According to the US Drought Monitor, most of Illinois is in a state of moderate or severe drought, with extreme drought in North Central and North Eastern Illinois (Figure 1).

Previous work at the Illinois State Water Survey defined two categories of drought based on the percent of normal precipitation. For a 3-month time period, a drought was considered “moderate” if precipitation was between 45 and 60 percent of normal and “severe” if precipitation was below 45 percent of normal. For a 6-month time period, a drought was considered “moderate” if precipitation was between 56 and 70 percent of normal and “severe” if it was less than 56 percent of normal.

The current situation is about 4 months old, starting on March 1. Interpolating between the 3-month and 6-month drought percentages, a 4-month drought is considered “moderate” if precipitation is between 49 and 63 percent of normal and “severe” for anything below 49 percent. Using this two-category classification, the Northwest, Northeast, and Central climate divisions in Illinois are in severe drought and the rest of the state is in moderate drought.

2. It has been the 3rd driest March-June period in Illinois since records began in 1895. Precipitation for this period is about 7.6ins below the statewide average of 16.1ns (Figure 2b). This is almost 50% below normal. Precipitation is above 50% below normal in the Northwest, Northeast, and Central Regions and about 37% below normal in the South.

3. Above normal temperature in June (Figure 2a), especially in Northern Illinois, has exacerbated the effects of below normal precipitation. 

4. Soil moisture, river flow, and shallow groundwater levels continue to decline (Figure 2c, d, e). 

5. The best chance of significant rainfall next week in southern Illinois is from hurricane Dennis, which currently is in the Caribbean. It is not expected to bring much rain to northern Illinois, which is expected to stay dry.

6. It is probable that the drought in central and especially North Central and North Eastern Illinois will continue to worsen.

7. The drought in Illinois is part of a band of drought extending south from Lake Michigan, across Illinois and through eastern Missouri, Arkansas, and eastern and southern Texas (Figure 1). 

8. The drought is caused by a weaker than normal flow of southerly moist air from the Gulf of Mexico combined with high pressure over the region, which restricts the penetration of storms from the west. The Great Plains states generally have received higher than normal precipitation.


9. Precipitation amounts and deviations from normal for the period March 1 through July 6 are shown in Figure 3.

10. Percent of normal precipitation for the Midwest in 2005 and 1988 are shown in Figures 4 and 5. The geographical extent of the 1988 drought was much larger than the current drought.

11. Figure 6 shows precipitation and temperature in July-August following the 8 most severe March-June droughts in Illinois (excluding 1992 when the eruption of Mt. Pinatubo influenced climate worldwide). These data show about a 60% statistical probability of a dry and warm summer following a dry spring.

12. Figure 7 shows 2005 tied with 1934 as the third driest March-June on record in Illinois. 1936 and 1988 were two driest years.

13. Figure 8 shows average streamflow percentiles for June 23-July 6 at selected USGS gaging stations. Streamflow in much of Illinois falls in the lowest was below the current streamflow in only 10% of the years on record. Streamflow levels continue to fall throughout the state, mostly unaffected by recent rains.

14. Lake Michigan is currently 1.4 feet below normal, and has been below normal for the past 6 years. Its level has remained unchanged since mid-May, whereas typically it would rise 0.4 feet during that period in an average year. The Ohio River has below normal flow for this time year, as impacted by generally dry conditions in its watershed from Illinois to Pennsylvania. In contrast, the Mississippi River is at normal flow levels. The Illinois River experienced one of its lowest total June flows on record, as a combined result of both the dry conditions in northern Illinois and the general reduction in the Lake Michigan diversion that has occurred since 1998.

15. The Water Survey records monthly level readings on 35 public water supply (PWS)

 reservoirs in Illinois, or roughly one-third of all PWS reservoirs in Illinois. These reservoirs typically have full pools through the end of June in most years with water levels starting to fall in July. In contrast, of the 33 reservoirs reporting at the end of June 2005, 30 had water levels below their normal pool. The reservoir levels range from 0.0 to 2.3 feet below normal, with most around 0.5 foot below normal. Twelve of the 33 reservoirs are one foot or more below their normal pool. With continued hot and dry weather, the levels of these reservoirs can be expected to drop more quickly than normal because of a combination of low inflows, above-normal evaporation, and high water-use withdrawals.

16. A few reservoirs in the State are susceptible to shorter drought periods that last from 6 to 9 months and could experience water supply concerns later this fall, but most reservoirs are designed to store water over droughts that last multiple years and their communities may not have acute concerns in a continuing drought condition until the end of the calendar year.

17. Most of the surface water impacts early in a drought are commonly related to adverse aquatic habitat conditions, with the potential for fish kills in shallow rivers and streams. There are a few communities that withdrawal water directly from rivers or streams that could potentially experience shortages by late summer; but it is too early in this year’s drought for any PWS reservoirs to be experiencing substantial impacts to their water supply.

18. A number of communities across Illinois have implemented water use restrictions, including communities in the Chicago, East St. Louis, Rockford, and Peoria metropolitan areas. These restrictions do not appear to be associated with a lack of water from the communities’ supply sources, but instead are associated with the extremely high water use rates associated with outdoor water use and the inability of the distribution systems to maintain adequate pressure under these high uses. In the St. Louis Metro East area there were a number of water main breaks near the Illinois American water plant due to the extra pressure needed to distribute the high rates of water. Most communities that do not experience distribution problems will likely choose not to impose water use restrictions until later in a drought when reservoir levels are already low and the threat of water shortages is more imminent. However, conservation in the early months of the drought, during hot and dry weather when water use is at its highest, has the greatest potential to benefit future water levels.

19. Shallow groundwater levels in the Water Survey's observation well network around the state were below average levels for June by 1.7 feet. Two wells located at Fermi National Laboratory (DuPage County) and Bondville (Champaign County) reported their lowest levels for June since their records started in Nov-1988 and Mar-1982, respectively. While the Water Survey has no quantitative data on impacts to rural wells, anecdotal evidence based on telephone calls received in the Groundwater Information Office suggests that impacts are being felt the number of requests for information regarding groundwater availability has increased and several callers have noted their wells have gone dry. There have also been references to increases in water hauling to supplement low water levels in shallow, bored wells, common in southern and central Illinois. Impacts to the state's principal aquifers are not believed to be serious, although restrictions on water use are being implemented in many communities due to facility capacity. There have also been reports that in the northwestern portion of the state (Lee, Whiteside, Bureau Counties), irrigation systems are applying large quantities of groundwater to crops causing water level declines greater than previously observed. This has caused domestic well pumps and sand points to be lowered in response to the increased drawdown.

20. According to the National Weather Service, no large scale organized precipitation is expected in Illinois through the next five days. There continues to be a small chance of showers and thunderstorms at individual locations in the meantime, but no widespread rain events. Surface high pressure is expected to dominate the weather in the Midwest over the next two weeks, leading to above normal temperatures during most of the period. The active storm track will be well north of Illinois. The greatest likelihood of large scale precipitation in Illinois in the next two weeks is likely to involve an incursion of tropical moisture into the Gulf Coast with tropical storm Dennis. Some forecast models project precipitation from Dennis to reach at least the southern portion of Illinois, but the movements of tropical systems are generally not well known more than three days in advance. The Climate Prediction Center monthly and seasonal forecasts for Illinois call for normal chances for any given temperature and precipitation anomaly. However, as noted above, climate statistics for years with the driest March through June periods in Illinois history tend, as noted in Figure 6, to favor continuing drier than normal and warmer than normal conditions through the summer.

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