Illinois State Water Survey Drought Assessment
May 5, 2000
"Moderate to Severe Drought in Illinois"
Summary
- A moderate precipitation drought exists in Illinois, with a moderate soil-moisture drought and a severe streamflow drought in central and southwestern Illinois.
- Water conservation should be encouraged.
- April precipitation was below normal across central and southern Illinois, compounding earlier water deficits. Streamflows and soil moisture continue to be below normal, and shallow ground-water levels have moved toward a greater deficit. Despite some heavy rain events over much of Illinois during the month, streamflow levels remain low to very low in most districts. Across much of Illinois, streamflows are nearly as low, or lower, than in any previous April, including those during all other major droughts. The last ten months in Illinois rank as the eighth driest July-April period since 1895.
- The May-July climate outlook shows an above average risk (about 40 percent, compared to 33 percent) of below normal rainfall.
- If above average precipitation does not occur over the next several months, all Illinois water resources will begin to reflect the impacts of drought.
Current Status
April precipitation across the state averaged 2.99 inches, 21 percent below average. Precipitation was below average in all climatological districts of central and southern Illinois, ranging from 17 to 43 percent below average. Rainfall averaged 10 percent above average in the northwestern and northeastern district.
Precipitation totals in Illinois from July through November 1999, averaged 45 percent below normal (i.e., 55 percent of normal), with the driest conditions in southern Illinois. Some recovery occurred from December through February, with precipitation averaging 111 percent of average; districts totals ranged from 10 percent below to 40 percent above normal. However, the precipitation did little to recharge water resources. Precipitation in Illinois during March and April again showed a deficit in rainfall (about 16 percent below normal). The northernmost districts were about 18 percent below normal while central and southern districts ranged from 24 to 33 percent below normal.
At the end of April, there were three districts with accumulated precipitation deficits of 10 inches or more since last July. A 10-inch precipitation deficit is a threshold used by Illinois State Water Survey staff to define surface water drought conditions. The last nine months in Illinois rank as the eighth driest July-April period since 1895. The statewide precipitation total during that period was 21.31 inches compared with a normal of 30.05 inches. The driest such period on record was July 1930-April 1931, when only 18.83 inches of precipitation fell.
Statewide, streamflows have been largely unchanged since February, remaining below to very much below normal for most of the state. Most of northern Illinois was rated as below normal in streamflow, and most of central and southern Illinois as much below normal. At the end of April, the Mackinaw River at Congerville and the Kaskaskia River at Vandalia were at record-low flows for April. Periods of record at these sites are 50 and 29 years, respectively. Most rivers and streams in central and southwestern Illinois are experiencing the lowest or second lowest April flows on record.
Many water-supply lakes continue to be 2 to 3 feet below full pool at a time of the year when they are normally overflowing or filling rapidly. Surface water levels at the end of March were below normal pool at 27 of 37 reporting reservoirs. Eighteen of the 37 reservoirs were a foot or more below normal pool. Reservoirs serving Altamont, Bloomington, Carlinville, Pana, Paris, Sorento, and Springfield are 3 feet or more below normal pool (spillway elevation). Recent rains have only slightly increased the levels in most of these reservoirs. Levels at Lakes Bloomington and Evergreen are now almost 9 feet and about 7 feet below normal, respectively. Levels at Lake Springfield and Sorento are 5 feet below normal.
In April, soil moisture in the first 40 inches of soil increased slightly across the state, but large parts of central and southwestern Illinois continue to show substantial dryness below a depth of 20 inches. Near the surface, there was a increase in moisture levels during April due to some heavy rain events during the month. However, deeper layers (40 to 72 inches) across central and southwestern Illinois remain quite dry.
Preliminary reports indicate that shallow ground-water levels continue to be low statewide, compared with April long-term averages.
Lake Michigan is rising slowly, but at a rate slower than would be expected for this time of year. Thus, its below normal deficit is increasing.
Impacts
Deep-aquifer wells that supply water to many communities are largely unaffected by short-term climate variations. Any effect would be from increased pumpage due to the decreases in surface water supplies.
Most water-supply reservoirs appear to be in good shape, with most having sufficient water to supply their communities at least through the remainder of the year. However, only with above normal rainfall will many of these reservoirs refill this spring. If dry conditions continue, reservoirs throughout the state would receive little replenishment from streamflow over the next 9 to 10 months. Low streamflows, combined with seasonal increases in water demand and evaporation, could cause reservoir levels to decline rapidly in summer, leading to supply concerns in the second half of the year.
Near surface soil moisture in most areas of the state is sufficient for early crop development. However, timely normal to above normal rains are needed to maintain crop development as plant roots dig deeper into the soils. Weather and crop growth will begin to greatly increase evaporation, transpiration, and crop needs, placing a large demand on soil-moisture levels beginning in May and the months that follow. The most critical areas for agriculture are those where deeper soils currently have low amounts of moisture. It will take above normal rains to recharge these dry soils and timely precipitation events to help maintain crops in these areas.
Water conservation should be encouraged.
If above average precipitation does not occur over the next several months, the impacts of drought will grow further in all water resources of Illinois.