Illinois State Water Survey - Drought Assessment
July 7, 2000
"CONTINUED RECOVERY FROM DROUGHT STATE-WIDE"
SUMMARY
1. June was the third wettest since 1895. Precipitation across Illinois during June was above average in central Illinois and well above average in northern and southern Illinois. Regional rainfall during the month averaged about 6 inches in the central and eastern climatological districts to more than 8 inches across northern and southern districts. Early July rainfall has been well above average across all but the two southern-most districts of the state.
2. Streamflow levels have rebounded in central and southwestern Illinois with normal to well above normal flow amounts compared to typical levels for the end of June (Figure 1). Increased inflows have filled or nearly filled most reservoirs that were previously experiencing drought concerns. The only water supply lakes that are at less than 90 percent of capacity belong to the Bloomington public water supply system.3. At the end of June, soil moisture conditions across most of Illinois were generally normal to above normal in all layers, except at Peoria and Belleville (St. Clair County), where deeper soils continue to exhibit below normal to much below normal moisture levels (Figure 2). However, these soils are beginning to moisten as well.
4. Shallow ground-water levels are average or above average across most of the state. Levels at a few locations in west-southwestern Illinois are currently below average, but well above their record lows.
5. The climate outlook of June 16 produced by the National Climate Prediction Center shows an above-average (about 40%) chance of below normal rainfall and an above-average (about 40%) chance of above normal temperatures in July through September. This also means a 60% chance of normal or above normal rainfall and a 60% chance of normal or below normal temperature.
CURRENT STATUS
June rainfall across Illinois averaged 7.56 inches, 101 percent above the long-term average, and was the third wettest since 1895. June rainfall was very heavy in northern and southern Illinois and above average in central areas. Regional rainfall was quite variable, ranging from 61 % above average in the central climatological district of Illinois to 142 % above average in the southwest district.
Early July rainfall (through July 6) has been well above average as well, averaging 1.5 to 2.5 inches within all but the two southern-most districts where about 0.75 inches has fallen.
Precipitation in Illinois during March through July 6, roughly the first half of the growing season, was 19 % above average. District averages ranged from 7% to 30 % above average.
From the start of the drought period in July 1999, through July 6, 2000, there were no districts in Illinois with an accumulated precipitation deficit in excess of 6 inches. Water Survey staff use a 10-inch precipitation deficit threshold to define surface-water drought conditions. Spring and early summer rainfall has removed all districts from near that threshold. Driest districts are located roughly in the central half of the state where precipitation totals over the last year are generally about 15 % below average.
Streamflow levels have rebounded in central and southwestern Illinois with normal to well above normal flow amounts for June. Many streams in this area reported below normal levels early in the month until heavy rains occurred on June 20-24. Some of the very heavy rainfall also caused flooding in northern and southern Illinois, bringing monthly streamflow amounts in these areas to the highest on record when compared to other June flows measured over the last 85 years.
Recent heavy rainfalls have filled or nearly filled water-supply reservoirs in Sangamon, Macoupin, and Christian Counties, previously the area of greatest drought concern, and have essentially removed all drought conditions at this time. The only water supply reservoir system that continues to experience drought impact is the Bloomington system, which currently measures roughly 70 percent of storage capacity, but with slightly over one year's supply of water in its lakes. No other water supplies in the state appear to be threatened by drought conditions at this time.
Soil moisture conditions at the end of June were above to well above normal across Illinois in all layers, except for continued dryness over parts of central and southwestern Illinois below about 2 feet of depth. Heavy rainfall fell across most areas of Illinois during several rain events in June, raising near-surface moisture levels far above concerns of drought. However, due to just average June rainfall at Peoria and typically high seasonal levels of evaporation and plant water use, conditions in the deepest observed layer (40 to 72 inches) remain dry across parts of central and southwestern Illinois. The moisture deficits at Peoria in this layer remain at 75 percent below normal.
Shallow ground-water levels averaged above levels typical for the end of June. Collectively, levels were 1.6 feet above average and ranged from 1.9 feet below to 7.3 feet above average. The area of greatest concern for dry conditions remained in west-southwestern Illinois at Greenville (Greene Co.), which continued below average. However, levels here rose significantly during the month as well and are well above the record low for this time of year.
Lake Michigan is 19 inches below its long-term average level for June, but has risen 3 inches in the last month. The water level in the lake is expected to begin normal seasonal declines during late summer and autumn.
The climate outlook issued by the Climate Prediction Center on June 16 showed an above-average (about 40%) chance of below normal rainfall and an above-average (about 40%) chance of above normal temperatures in July through September. This also means a 60% chance of normal or above normal rainfall and a 60% chance of normal or below normal temperatures.
IMPACTS
Currently, there are no drought impacts in the state.