Illinois Drought Update, August 3, 2005
DROUGHT RESPONSE TASK FORCE, August 3, 2005
Illinois State Water Survey, Department of Natural Resources
1. DROUGHT STATUS. According to the US Drought Monitor (July 26), most of Illinois remains in a state of moderate to extreme drought, with extreme drought in northern and northwestern Illinois extending into northeastern Missouri (Figure 1) .
2. PRECIPITATION. Figure 2 shows precipitation since the July 22 Task Force meeting. 1-2 inches have fallen in northern Illinois and less than 1 inch in southern Illinois. Rainfall amounts were somewhat above normal in most of the north and below normal in the south. Statewide, precipitation in July was 93% of normal, but this average figure hides some large regional differences: July precipitation in the west was 45% below normal, but more than 30% above normal in the east and south.
3. LAST 100 YEARS. Figure 3 shows that precipitation in Illinois since March 1 ranks as the 6th driest since 1895. 8 out of the 10 driest March-July periods occurred in the first 42 years of the record from 1895 to 1936. In the almost 70 years since 1936, only 1988 and 2005 are on the top 10 list. These data show that March-July droughts occurred on average about once every 5 years at the start of the 20th century, but only once every 35 years since the 1930s. This change in the frequency of drought is part of a long-term change in natural climatic conditions. We do not know whether drought will remain a relatively rare event in the years ahead, or whether drought again will occur more frequently.
Figure 4 shows that in the northwest, northeast, west, central, and west-southwest Climate Divisions March-July ranked the 3-4 driest on record. Other Climate Divisions experienced the 7-13th driest March-July on record. 2005 is on par with 1988 and 1914 with precipitation in 5 Climate Divisions rankings in the top four driest ever. In 1936, 8 Climate Divisions ranked in the top 4 driest on record.
Figure 5 shows the departures from normal precipitation across Illinois since March 1, 2005. An area straddling the Illinois River has a deficit of more than 9 inches of precipitation, and most of the area north and west of a line from St. Louis to Chicago has more than 40%, and in some areas more than 50% below normal precipitation (less than 60% and 50% of normal, respectively).
4. SOIL MOISTURE. Figure 6 shows that on August 1, 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 in most areas from northeast Illinois across to St. Louis. Although some localized improvements in near-surface soil moisture occurred, outside of southern Illinois soil moisture down to 72 inches generally decreased.
5. GROUNDWATER. Shallow groundwater levels in Illinois remain below average. The latest data for July 31 from the ISWS shallow observation well network shows that water levels fell approximately 1 foot since June and remain about 1.7 feet below the 15-year mean. 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 November1988 and March 1982, respectively. Anecdotal evidence based on telephone calls received in the Groundwater Information Office indicates that some private wells have gone dry in northern and northwestern Illinois (Kane, DeKalb, McHenry Counties). Water levels collected last week in selected dolomite bedrock wells within the heavily irrigated region of Kankakee and Iroquois Counties show that water levels are ~9 to15 feet above levels experienced in late July of the drought year 1988, suggesting conditions in that part of the state are not as severe as 1988. However, ISWS continues to receive reports of domestic well pumps being lowered in Whiteside County due to widespread irrigation withdrawals.
6. ILLINOIS STREAMFLOWS. Figure 7 shows that streamflows in Illinois represent contrasting conditions. Flows in the north-central portion of the State are at record low levels for this time of year. The Kishwaukee River experienced its lowest total July flows on record (dating to 1940), and is currently experiencing its lowest flows (regardless of season) since 1964. The Green River is currently experiencing its lowest flows (regardless of season) since 1976. The Little Wabash River, in southeastern Illinois, also had its lowest total July flows on record (dating to 1915). Streamflows over roughly one-third of Illinois are still in the much below normal category, including much of the north-central, west-central, and southeastern portions of the State. In contrast, heavy rainfall in east-central Illinois has caused streams in that area to rise to normal or above-normal levels. Flow levels in parts of southern Illinois are still relatively high following the rainfall from Hurricane Dennis. It would take weeks of continued hot and dry weather to return the flows in these two areas to below-normal conditions, such as they had one month ago.
7. WATER LEVELS AT PUBLIC WATER SUPPLY (PWS) RESERVOIRS. The Water Survey records monthly level readings on 35 public water supply (PWS) reservoirs in Illinois, or roughly one-third of all PWS reservoirs in Illinois. Of the 31 reservoirs reporting this month, 28 were below normal (full) pool, with an average level of 1.3 feet below normal pool. Twenty-six of these reservoirs are below their average July level. Water levels have declined over the past month at 24 reservoirs, with an average drop of 0.5 foot. Four of the 31 reservoirs are more than 2 feet below normal; these reservoirs are located in various parts of central Illinois (Lake Bloomington, Canton Lake, Altamont Lake, and Lake Paradise near Mattoon). None of these reservoirs have water levels that yet pose a water supply concern, nor do we expect to see any such concerns in Illinois reservoirs for several months.
8. FEDERAL RESERVOIRS. Carlyle Lake and Lake Shelbyville are roughly 0.8 and 0.9 feet below full pool, respectively, which is below their normal condition but not extraordinarily low. The month-end July 2005 water level for Rend Lake was at an elevation of 405.4 feet, its third-lowest July level on record - dating back to the lake’s construction in the early 1970s. The low levels on Rend Lake do not affect water supplies.
9. MISSISSIPPI AND OHIO RIVERS. The flow level in the Mississippi River (100,000 cfs at St.Louis) is not substantially different than it was two weeks ago, having received a bounce in its level over a week ago but thereafter receding. In past drought years, problems for commercial navigation on the river have typically developed when the flow approaches around 70,000 cfs. The Ohio River has been at a normal to slightly below-normal level for this time of year.
10. ILLINOIS RIVER. The Illinois River experienced its lowest total July flow on record. Even though flow levels have rebounded in the past week, they still remain at record low levels for this time of year due to a combination of drought and the reduction in recent years of water originating from the Lake Michigan diversion. As reported two weeks ago, water levels immediately downstream of the dams are 1-3 feet below normal because of the low-flow amounts.
11. LAKE MICHIGAN. The level of Lake Michigan has remained unchanged since mid-May at an elevation of 578.1 feet. Lake Michigan is now 1.4 feet below normal (having been only 0.7 foot below normal in February), and is 1.4 feet higher than its record low level for August, set in 1964.
12. STATEWIDE SUMMARY. Figure 8 summarizes the evolution of statewide-average temperature, precipitation, soil moisture, streamflow, and groundwater levels from July 2004 to July 2005.
13. NORMAL AUGUST PRECIPITATION. Illinois normally receives about one inch of precipitation per week in August. This amount is needed for the drought not to worsen. More than this amount is needed to reduce the precipitation deficits.
14. CLIMATE OUTLOOK (prepared August 2, 2005). The period begins with a National Weather Service (NWS) forecast of well above normal temperatures for Illinois on August 3-4. On August 4, a cold front will bring a chance of precipitation. Present indications are that the precipitation will be scattered with most areas receiving less than 0.25 inches. The Climate Prediction Center (CPC) 6-10 day forecast for August 8-12 indicates a tendency toward warmer than normal conditions statewide, and a slight tendency towards an above normal chance of precipitation in the extreme northern portion of the state. The forecast model indicates less organized precipitation opportunities during this period, likely to be in the form of scattered thunderstorms, so some locations are likely to get some rain, and others will miss. The CPC 8-14 day forecast for August 10-16 indicates a continued warmer than normal tendency, and below normal precipitation probabilities for southern Illinois. The CPC monthly forecast for August, updated on July 31, calls for normal probabilities for temperature and precipitation.
For more drought information please go to http://www.sws.uiuc.edu/.
Derek Winstanley: tel 217-244-5459; e-mail dwinstan@uiuc.edu