Illinois Drought Update, July 22, 2005
DROUGHT RESPONSE TASK FORCE, July 22, 2005
Illinois State Water Survey, Department of Natural Resources
1. DROUGHT STATUS. According to the US Drought Monitor (July 19), most of Illinois is in a state of moderate or severe drought, with extreme drought in Northern Illinois and extending down the Illinois River Valley (Figure 1). The drought has worsened in Wisconsin and eastern Iowa.
The Illinois State Water Survey defines two categories of drought based on percent of normal precipitation. For a 5-month period, the area of northern Illinois with below 52 % of normal rainfall is classified as severe drought and areas with between 52 and 67 percent as moderate drought. By these criteria, far southern Illinois is no longer considered to be in climatological drought, although drought impacts linger.
2. PRECIPITATION. Precipitation since the July 7 Task Force meeting shows the effect of the remains of Hurricane Dennis and recent rains (Figure 2). Generally, most of southern and east-central Illinois received above-normal rainfall (2 to 4 inches) during this period. Rainfall amounts were below-normal (less than 2 inches) across most of western and northern Illinois, where the drought intensified. In the past few days, only scattered rains have occurred in the severe drought area, but there have been heavy rains (up to 5 ins) in eastern Illinois south of Kankakee and Joliet. Precipitation maps, expressed as both deviations from normal and percent of normal, for the period March 1 through July 21 are shown in Figure 3.
3. LAST 100 YEARS. Precipitation in Illinois from March 1 through July 21 in 2005 ranks as the 5th driest since 1900. 7 out of the 10 driest periods occurred in the first 4 decades of the 20th century. The past half century generally has been relatively drought free in this March-July period, with exceptions in 1988/1989 and 2005.
4. NORMAL SUMMER RAINFALL. Illinois normally receives about one inch of rain per week at this time of year. This amount is needed for the drought not to worsen. More than this amount is needed to reduce the precipitation deficits.
5. LONG-TERM DROUGHTS. Expected state-wide precipitation for selected drought durations and return periods is shown in Figure 4. These numbers illustrate that all parts of Illinois can experience longer-term droughts, which can have serious consequences. Percent of normal precipitation generally increases as drought duration increases (e.g., from 12 months to 60 months), but decreases as the return period lengthens (e.g., from 25 to 200 years). A higher probability of drought than has occurred in the past 50 years should be taken into account in water planning and evaluating future risks. It would be misguided to plan for the future based on the climate record of only the past 50 generally wet years.
6. SOIL MOISTURE. On July 15, moisture in the top 72 inches of soil was less than 75% of normal at most sites within the soil moisture monitoring network operated by the Water Survey, with less than 50% of normal at some locations in central and northern Illinois (Figure 5).
7. GROUNDWATER. Shallow groundwater levels in Illinois remain below average. The Water Survey's two shallow observation wells located at Fermi National Laboratory (DuPage County) and Bondville (Champaign County) are at their lowest levels for July since their records started in Nov-1988 and Mar-1982, respectively. Requests for information regarding groundwater availability have increased and many callers have noted their wells have gone or are going dry. There are continued references to instances of water hauling to supplement low water levels in shallow (usually less than 100 feet deep), bored wells, which are common in southern, central, and western Illinois. Impacts to the state's deeper, principal aquifers are also occurring, due primarily to increased pumpage that results in greater drawdown in local areas. There are reports that some well pumps have needed to be lowered in wells finished in these aquifers. Several of these cases have been reported in northeast Illinois (Kane, Lake, and McHenry Co.), and more than a dozen cases in northwestern Illinois (Ogle and Whiteside Co.) where irrigation systems are applying large quantities of groundwater to crops. Groundwater levels in northwestern Illinois area are at the lowest level that has ever been observed. However, we expect the situation in the irrigated areas to start improving as the irrigation season begins to wind down in the next several weeks allowing recovery of groundwater levels.
8. ILLINOIS STREAMFLOWS. Rainfall from the remnants of Hurricane Dennis and the more recent storms have had variable impacts on streamflow levels in Illinois. In general, these rains have not altered low-flow concerns significantly. Streamflow levels in many regions have become less responsive to normal rainfall amounts, such that moderate rains may produce little runoff. Average flow levels for roughly half of Illinois still fall in the much-below normal (lowest 10th percentile) category for the past 14 days (Figure 6). Several watersheds in southernmost Illinois, southwestern Illinois near St. Louis, and east-central Illinois had sufficient rainfall to lift flow volumes into the average or near-average range. On the other hand, there was relatively little recovery in other nearby watersheds in southern and central Illinois. Streamflow levels in northern and western Illinois in particular continue to fall at a steady pace. Several major streams in northern Illinois, specifically the Kishwaukee and Green Rivers, are near 10-year low flows, with the possibility for continued decline. The Kishwaukee River, located east of Rockford, is experiencing its lowest July flows on record (since 1940).
9. WATER LEVELS AT PUBLIC WATER SUPPLY (PWS) RESERVOIRS. PWS reservoir levels in Illinois typically do not respond quickly to short droughts. Rather, substantial declines in reservoir levels usually occur following extended periods of below normal precipitation and streamflow. Thus, if flows continue in the much-below normal range, we will see reservoir levels fall in upcoming months. Current water levels at most of the PWS reservoirs monitored by the Water Survey are only slightly below normal for this time of year, but are reasonably reflective of conditions at the beginning of a hydrologic drought. Although conditions are right in many areas for continued water level decline, we do not expect there to be noticable water supply concerns in Illinois reservoirs for many months. Many reservoirs are designed to store water over droughts that last multiple years and their communities may potentially not have acute concerns in a continuing drought condition until the end of the calendar year, should the drought continue.
10. FEDERAL RESERVOIRS. Carlyle Lake and Lake Shelbyville are about 0.6 and 0.5 feet below full pool, which is below their normal condition, but not extraordinarily low. In contrast, the average July 2005 water level for Rend Lake, at 405.7 feet, is near its lowest July levels on record - dating back to the lake’s construction in the early 1970s. Water supplies from Rend Lake are not adversely affected.
12. MISSISSIPPI RIVER. Flow levels in the Mississippi River have dropped substantially over the past 14 days, and are at below-normal flow levels. The flow amount at St. Louis is currently about 96,000 cubic feet per second (cfs), having fallen from double that amount in just two weeks. In past drought years, problems for commercial navigation on the river have typically developed when the flow approaches around 70,000 cfs.
12. OHIO RIVER. Flow levels in the Ohio River, which were below normal 14 days ago, have increased as a result of the large amounts of rain associated with Hurricane Dennis and subsequent rains. The Ohio River currently is at a normal level for this time of year.
13. ILLINOIS RIVER. The Illinois River is experiencing its lowest July flows on record. Although the locks and dams have maintained their pools at normal operating levels, water levels immediately downstream of the dams are 1-3 feet below normal because of the low flow amounts.
14. LAKE MICHIGAN. The level of Lake Michigan has remained unchanged since mid-May, showing an unusual lack of recharge for this time of year. Water supplies from Lake Michigan are not affected by the lake level.
15. OUTLOOK. After July 22, the only chance of rain for the next 5 days, according to the National Weather Service, appears to be limited to a frontal boundary entering western Illinois around July 26. The event is currently forecast to be a light rain producer. Very high temperatures, with daily maxima in the upper 90s and lower 100s are forecast for the period through July 26, after which temperatures are forecast to become more seasonal (80s). High temperatures will lead to higher rates of moisture loss from the soil and vegetation. The 6-10 day forecast for July 27-31 indicates above normal precipitation and above normal temperature in the southern half of Illinois. The 8-14 day forecast also maintains a slightly above normal chance of above normal precipitation for southern Illinois, and no change in normal probabilities for temperatures. Little significant drought relief is evident in the National Weather Service predictions out to 14 days.
The Climate Prediction Center monthly forecast for August and the seasonal forecast for August-October indicate no changes in the normal range of probabilities for either precipitation or temperature in Illinois during those time periods.
For more drought information please go to http://www.sws.uiuc.edu/.
Derek Winstanley: tel 217-244-5459; e-mail dwinstan@uiuc.edu