Illinois State Water Survey - Drought Assessment
May 19, 2000
"DROUGHT WORSENS IN CENTRAL AND SOUTHWESTERN ILLINOIS.
CONCERNS GROW OVER WATER SUPPLY SHORTAGES."
1. Drought conditions have worsened in central and southwestern Illinois, while water resources have improved in northern and southern Illinois.
2. Precipitation in early May was below normal across most of central and western Illinois, compounding earlier water deficits (Figure 1). Some heavier regional rain events over parts of northern and southeastern Illinois has narrowed the region of much below normal streamflow levels to just central Illinois.
3. Current reservoir levels in central and southwestern Illinois are well below normal and most streams in the area lack sustainable flows (Figure 2). Shallow ground-water levels in central and southern Illinois are low. Soil moisture below about 1 foot of depth is substantially in deficit across central and southwestern Illinois. Above average precipitation continues to be required to mitigate these impacts.
4. Climate outlooks 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.
5. Water conservation measures have been enacted in Springfield, Illinois, and should be encouraged in other regions of central Illinois.
May precipitation across Illinois through May 19 averaged 2.48 inches, 1 percent below average.
Precipitation was quite variable across the climatological districts of Illinois, ranging from 49 percent above average in the northeast district to 43 percent below average in the west district.
Precipitation totals in Illinois from 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. Precipitation in Illinois during March through May 19 has again showed a deficit in rainfall, being about 19% below normal. The northernmost districts were close to average while central and southern districts ranged from 18% to 35% below normal.
Through May 19, there are three districts in central Illinois with accumulated precipitation deficits of about 10 inches or more since last July. 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 9.04 inches.
Streamflow levels in Illinois for the first half of May have been variable, with normal flows in northern and parts of southern Illinois and much-below normal flows in central Illinois. For most streams, the average flows experienced in the first half of May are very similar to their April 2000 average flows. However, this does not indicate that flow levels are static. To the contrary, flows in most streams have risen in response to periodic rains, but have generally receded quickly in the intervening dry periods. Compared to normal spring conditions, there is relatively little sustained base flow in streams to keep their flow levels up following runoff events.
Reservoirs in the central and southwestern portion of the state continue to experience below-normal to much-below-normal conditions. Recent rains have caused water levels in most smaller reservoirs to increase by 0.5 to 1 foot. Many of these reservoirs are still 3-5 feet below full pool, but do not face immediate water supply concerns. The Paris water supply situation has improved considerably with the East Lake rising by 1.5 feet. However, the Springfield and Bloomington Lakes have seen an decrease in their volume over the past 15 days, and can expect to continue to drop without above-normal rains. Water supply reservoirs in other portions of the state are full or nearly full leading into the summer.
In mid-May, soil moisture is close to normal in northwestern Illinois and the eastern one-third of the state, but large parts of central and southwestern Illinois continue to show substantial dryness, especially below about 1 foot. Near the surface, there was a decrease in moisture levels during early May across central and southern Illinois. The remainder of the state reported little change since the end of April. Conditions in the deeper layer (40 to 72 inches) across central and southwestern Illinois remain quite dry.
Seasonal U.S. Drought Outlooks issued by the Climate Prediction Center depict areas of long-term drought in the Midwest. The area highlighted mirrors the region of severe to extreme drought determined by the latest Palmer Drought Severity Index. Indications of expected persistence is based on current conditions and the latest seasonal and long-lead outlooks. The odds slightly favor subnormal precipitation during June-August, with greater chances of above normal temperatures. Regardless, even if near or slightly above average amounts of precipitation are recorded over the next several weeks, moisture shortages 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 central and southwestern Illinois compared with long-term averages. At the end of April, levels across the state ranged from 8.4 feet below to 1.9 feet above average. One well, Coffman (Pike Co.), is currently at a record low ground-water level for April. Shallow water levels within the southern part of the state continued to decline in response to rainfall deficits, whereas the northern portion of the state has received enough precipitation for levels to move above normal.
Lake Michigan is rising slowly, but at a rate slower than would be expected for this time of year, thus, it's below normal deficit is increasing.
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 supply reserves and typical increased water use during dry conditions.
Lower streamflow levels can be expected over central and southwestern 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.
With some notable exceptions, most water-supply reservoirs appear to be in good shape, having sufficient water to supply their communities at least through the remainder of the year. However, several reservoirs in central Illinois, such as Springfield and Bloomington-Evergreen, have water levels that are substantially below normal for this time of year. IF dry conditions continue, reservoirs in central Illinois and other portions of the state could receive little replenishment from streamflow over the next 8 to 9 months, leading to supply problems in the second half of the year.
Near-surface soil moisture in most areas of the state continues to be sufficient for early crop development. However, timely normal- to- above normal rains are needed to maintain crop development as roots penetrate deeper into the soils. Weather and crop growth have begun to greatly increase evaporation, transpiration, and crop needs, placing a large demand on soil-moisture levels. This will only accelerate in the months to follow. The most critical areas for agriculture are those in central and southwestern Illinois where deeper soils currently have low amounts of moisture. It will take above-normal rains to recharge these dry soils and timely precipitation events throughout the summer to help maintain crops in these areas.
Water conservation measures have been enacted in Springfield, Illinois, and should be encouraged in other regions of central Illinois.