Illinois State Water Survey - Drought Assessment
April 14, 2000
"MODERATE TO SEVERE DROUGHT IN ILLINOIS"
1. A moderate precipitation drought exists in Illinois, with a moderate soil-moisture drought and a severe stream-flow drought in central and southwestern Illinois.
2.Water conservation should be encouraged.
3. Precipitation in March and early April was below normal across Illinois, compounding earlier water deficits. Stream flows and soil moisture continue to be below normal, and shallow ground-water levels moved toward a greater deficit. Despite rainfall over much of Illinois in the past week, stream-flow levels are falling in most districts. Across most of Illinois, stream flows in mid-April are nearly as low, or lower, than any previous April, including all other major droughts.. The last nine months in Illinois rank as the eighth driest July - March period since 1895.
4. The climate outlook shows an above-average (about 40%, compared with 33%) risk of below normal rainfall in May through July.
5. If above average precipitation does not occur over the next several months, the impacts of drought will begin to be felt in all water resources of Illinois.
March precipitation across the state averaged 2.29 inches, 32 percent below average. Precipitation was below average in every climatological district, ranging from 19 to 57 percent below average. April rainfall (1st-13th) is running 50% below average.
Statewide, stream flows were 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 stream flow, and most of central and southern Illinois as much-below normal. At the end of March, three streams: the Mackinaw River at Congerville, the Vermilion River at Pontiac, and the Kaskaskia River at Vandalia were at record-low flows for March. Period-of-records at these sites are 50, 55, and 29 years, respectively. Most rivers and streams in central and southwestern Illinois are experiencing in mid-April flows that are the lowest or second lowest on record for April. Roughly 70 percent of the state is experiencing flows that are much-below-normal (the lowest 10 percent of all April flows on record).
All other portions of the state are experiencing below-normal flows, with the exception of a small portion of southeastern Illinois, centered on the Little Wabash River basin south and east of Effingham. This area received over 2 inches of rainfall in the last week and is now experiencing flows in the "normal" range for this time of year.
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 26 of 38 reporting reservoirs. Eighteen of the 38 reservoirs were one or more feet below normal pool. The reservoirs serving Altamont, Bloomington, Carlinville, Mt. Olive, Pana, Paris, Sorento, and Springfield are 3 or more feet below normal pool (spillway elevation). Recent rains have only slightly increased the levels in most of these reservoirs. For example, Paris East Lake has risen only 1.5 inches since the beginning of April, despite receiving almost 0.75 inches of rainfall across its watershed. Lakes Bloomington and Evergreen have fallen since the beginning of the month and are now almost 10 feet and about 6 feet below normal respectively. Lake Springfield is almost 6 feet below normal.
In March, soil moisture in the first 40 inches of soil remained fairly constant across the state, with large parts of western Illinois continuing to show substantial dryness below 20 inches. Near the surface, there was a 10 to 25 percent drop in moisture levels during March, while soils moistened by 15 to 20 percent in one of the dry, deeper layers (20 to 40 inches) across central and southwestern Illinois from rains in earlier months.
Ground-water levels dropped about a foot statewide, compared with March long-term averages. One site in southern Illinois (Saline County) is at a record low level for March. Water levels in other shallow wells are declining.
Lake Michigan is rising slowly, but at a rate slower than would be expected for this time of year. The lake is now 21 inches below normal.
Precipitation totals in Illinois fom July through November, 1999, averaged 45 % below normal (i.e., 55 % of normal), with the driest conditions in southern Illinois. December through February saw some recovery with precipitation averaging 111 percent of average; districts totals ranged from 10 % below to 40 % above normal. However, the precipitation did little to recharge water resources.
As of mid-April, there are districts with accumulated precipitation deficits of about 10 inches since last July. A 10-inch precipitation deficit is a threshold used by Water Survey staff to define surface-water drought conditions. The last nine months in Illinois rank as the eighth driest July - March period since 1895. The statewide total during that period was 19.53 inches compared with a normal of 27.64 inches. The driest July-March period on record was July 1930 - March 1931, when only 16.12 inches of precipitation fell.
Deep-aquifer wells that supply water to many communities are largely unaffected by short-term climate variations. Any effect on them 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 stream flow over the next 9 to 10 months. Low stream flows, 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.
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 roots dig deeper into the soils. Seasonal increases in evaporation, crop needs, and transpiration will place a large demand on soil-moisture levels in subsequent months. 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.
Water conservation should be encouraged.
If above average precipitation does not occur over the next several months, the impacts of drought will begin to be felt in all water resources of Illinois.