Illinois State Water Survey - Drought Assessment
June 23, 2000
"RECOVERY FROM DROUGHT STATE-WIDE:
CONCERNS REMAIN OVER DEEP SOIL MOISTURE AND WATER SUPPLY IN WEST-CENTRAL ILLINOIS"
1. Precipitation across Illinois during the first 22 days of June has ranged from much above average (more than 6 inches) in northern and southern areas to above average (about 4 inches) across central Illinois (Figure 1).
2. Streamflow levels in Illinois have rebounded in central and southwestern Illinois, with normal flow amounts for June. Increased inflows have brought relief to several water supply reservoirs that were experiencing drought concerns, however selected reservoirs in the area continue to have low water levels (Figure 2). The water supply reservoirs for Springfield, Bloomington, and several smaller communities in the vicinity of Macoupin County continue to be watched.
3. On June 15, soil moisture conditions across most of Illinois were generally normal to above normal in all layers, except across central and southwestern parts of the state, where the deeper soils continue to exhibit below normal to much below normal moisture levels (Figure 3).
4. Shallow ground-water levels in western and southern Illinois at the end of May remained low, but are increasing with the recent rains.
5. The climate outlook 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.
6. Water conservation measures have been enacted in some communities in central and western Illinois, and should be encouraged in other communities where reservoirs are low.
Mid-June rainfall across Illinois averaged 5.10 inches, 81 percent above the long-term average. Early June rainfall has been very heavy in northern and southern Illinois and above average in central Illinois. Regional rainfall was quite variable, ranging from 94 - 152 % above average in the two northernmost and two southernmost climatological district of Illinois to 35 % above average in the west district.
Precipitation in Illinois during March through mid-June has been 7 % above average. The northern and southern districts averaged 8 to 21 % above average while central Illinois averaged 14 % below average to 3 % above average.
Through June 22, there no longer are any districts in Illinois with an accumulated precipitation deficit of 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 west-southwest district is the driest in the state (-9.07).
Streamflow levels in Illinois have rebounded in central and southwestern Illinois, with normal flow amounts for the time of year. Many streams in this area were at levels below normal until the heavy rain event on June 21. Meanwhile, very heavy rainfall has caused flooding in northern Illinois, bringing monthly streamflow amounts to the highest on record when compared to other June flows measured over the last 85 years.
Several reservoirs in the west-central portion of the state continue to experience below-normal to much-below-normal conditions. Recent rainfalls have brought relief to several water-supply reservoirs in the vicinity of Macoupin and Christian Counties, reducing the number and geographic extent of water supply systems having drought concerns. However, several smaller water supply reservoirs in the Macoupin County area are still experiencing below-normal supplies. The two major water supply systems of concern, Springfield and Bloomington, have experienced an improvement in water levels this month, but are still well below normal and could experience significant water supply concerns if dry conditions resume for the remainder of this year.
Soil moisture on June 15 near the surface was above normal across Illinois, but large parts of central and southwestern Illinois continued to show substantial dryness below about 2 feet of depth. Near the surface, there was a general increase in moisture levels during early June, especially in northern areas where amounts exceeded 200 percent of normal. Very heavy rainfall fell across southern Illinois within a couple of days subsequent to the mid-month soil moisture observations and will be reflected in upcoming measurements. Conditions in the deepest observed layer (40 to 72 inches) remain quite dry across central and southwestern Illinois with observed levels up to 75 percent below normal moisture.
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.
Lake Michigan is 20 inches below its long-term average level for June, but has risen 4 inches in the last month in response to heavy rainfall in Wisconsin and other portions of its watershed.
The seasonal U.S. Drought Outlook issued by the Climate Prediction Center on June 16 depicts areas of long-term drought risk in the Midwest. The climate outlook 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 temperatures. 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.
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.
Though many streams in central and southern Illinois are currently at normal levels, there is the potential that these stream levels may drop at rates faster than normal this summer as a result of the cumulative impacts of the long-term precipitation deficit and reduced baseflow levels.
Selected reservoirs in west-southwestern Illinois continue to experience water levels that are substantially below normal. If dry conditions resume, water supply problems could develop later this year for several communities. A number of these communities have enacted voluntary water conservation programs, and are considering enacting greater restrictions on water use. Water supply reservoirs in most other regions of Illinois appear to be in good shape heading into the summer, but can soon be expected to experience drops in water levels as a result of increased seasonal water demand and evaporation.
Good soil moisture in most areas of the state appears to be sufficient for current crop development. Excess moisture may be a problem in some northern and southern areas. 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 in west-central Illinois will be fully recharged this growing season.
Water conservation measures have been enacted in some communities in central and western Illinois, and should be encouraged in other communities where reservoirs are low.