Illinois State Water Survey - Drought Assessment
June 5, 2000
"DROUGHT CONTINUES IN CENTRAL AND WESTERN ILLINOIS.
CONCERNS REMAIN OVER WATER SUPPLY."
1. Heavy rainfall occurred across much of Illinois in May and early June and caused some flooding in northern areas. Rainfall was lower in central and western Illinois and drought conditions were not alleviated greatly (Figure 1). Low rainfall in these regions since July 1999 is the cause of the water shortages.
2. Some reservoirs that were previously below normal have been replenished, but the reservoirs of greatest concern have not seen substantial relief (Figure 2).
3. Soil below about 2 feet in depth continues to be dry across central and southwestern Illinois (Figure 3).
4. Shallow ground-water levels in western and much of southern Illinois remain low.
5. The climate outlook continues to show an above-average (about 40%, compared with 33%) risk of below normal rainfall and an above-average (about 45%) risk of above normal temperatures in June through August.
6. Water conservation measures have been enacted in some communities in central and western Illinois, and should be encouraged in other communities in this region.
May rainfall across Illinois averaged 4.52 inches, 13 percent above the long-term average. Early June rainfall has been heavy in northern Illinois, about average in central Illinois, and below average in southern Illinois.
Rainfall in May was quite variable across Illinois, ranging from 35 percent above average in the southwest to 19 percent below average in the west. Rainfall totals in Illinois from July through November, 1999, averaged 45 % below average (i.e., 55 % of normal), with the driest conditions in southern Illinois. December 1999 through February 2000 saw some recovery with precipitation averaging 111 percent of average; district totals ranged from 10 % below to 40 % above average. Precipitation in Illinois during March through May showed a statewide deficit about 12% below average. The northeastern district was average while the remainder of the state ranged from 8% to 27% below average.
Through June 5, there is one district with an accumulated precipitation deficit of 10 inches or more since last July with 3 more districts within one-half inch of that threshold. A 10-inch precipitation deficit is a threshold used by Water Survey staff to define surface-water drought conditions. The statewide deficit during that period totals 7.93 inches.
Streamflow levels in Illinois for May have been variable, with above-normal flows in northern Illinois and much-below normal flows in west-central Illinois. Many streams in central and southern Illinois experienced much-below-normal flows for the first half of May, but rose for several days following two heavy rainfall events, providing two to three inches of precipitation over many locations in central Illinois during the last week of May. On average, approximately 0.2 to 0.3 inches of runoff was generated. This brought total May flows in these areas into the below-normal to normal range. For many locations in the state, there is still relatively little sustained flow in streams to keep their flow levels up following runoff events. A two-week period of little rainfall would be sufficient to return many streams to much-below normal conditions.
Several reservoirs in the west-central portion of the state continue to experience below-normal to much-below-normal conditions. The 0.2-0.3 inches of runoff with recent rainfalls created greater inflow into many reservoirs in central Illinois. Lakes in areas with heavier rainfall, such as Lake Carlinville, had greater amounts of inflow, while other lakes, such as Lake Springfield, had less inflow. For several lakes, the recent inflows have caused a significant increase in their remaining supply, alleviating concerns of water supply shortages for this calendar year. However, the reservoirs that have presented the greatest water supply concern generally had little to moderate increases in the water supply storage. Water supply reservoirs in other portions of the state are full or nearly full leading into the summer.
Soil moisture near the surface is above normal across Illinois, but large parts of central and southwestern Illinois continue to show substantial dryness below about 2 feet of depth. Near the surface, there was a general increase in moisture levels during early May. Conditions in the deeper layer (40 to 72 inches) remain dry across central and southwestern Illinois with observed levels up to 75 percent below normal moisture.
Seasonal U.S. Drought Outlook issued by the Climate Prediction Center depict areas of long-term drought risk in the Midwest. The odds slightly favor below normal precipitation and above normal temperatures during June-August. Regardless, even if near or slightly above average amounts of precipitation are recorded over the next several weeks in these areas of long-term drought risk, drought impacts would persist or worsen because of the depleting effects of agricultural, evaporative, and human water demands during summer.
Shallow ground-water levels continue to be low in western and southern Illinois compared with long-term averages. At the end of May, levels across the state averaged 1.1 feet below average and ranged from 6.3 feet below to 5.3 feet above average. Levels within the very dry southwestern portion of the state have received some recharge from local precipitation events. However, levels continue to trend downward in response to the precipitation deficit in this part of the state. One well at Greenfield (Greene County) is currently at its record low level for May.
Lake Michigan is 15 inches below its long-term average level for May and is rising at a rate consistent with what would be expected for this time of year.
Deep-aquifer wells that supply water to many communities typically are unaffected by short-term climate variations. Elevated pumpage due to increased water use during dry conditions will decrease water levels within these systems. However, as yet, no noticeable trends beyond normal seasonable fluctuations have been observed.
Lower streamflow levels can be expected over west-central Illinois in the upcoming summer months unless above-average rainfall occurs. The primary impact of the current low streamflows in central Illinois is their inability to refill water supply reservoirs due in part to seasonal increases in water demand and evaporation.
Water-supply reservoirs in most regions of Illinois appear to be in good shape heading into the summer. However, several reservoirs in central and west-central Illinois have water levels that are substantially below normal. If dry conditions continue, water supply problems could develop later this year for several communities. A number of these communities have enacted voluntary water conservation programs, and several of these are considering to enact greater restrictions on water use.
Near-surface soil moisture in most areas of the state appears to be sufficient for current crop development. Weather and crop growth have begun to greatly increase evaporation, transpiration, and crop needs, placing a large demand on soil-moisture levels, which will only accelerate during June and remain at high seasonal levels during the balance of the summer. Even with normal summer rainfall, it is unlikely that the moisture in the lower soil layers will be fully recharged in west-central Illinois.
Water conservation measures have been enacted in some communities in central and western Illinois, and should be encouraged in other communities in this region.