Illinois Drought: Update, Illinois State Water Survey

Illinois Drought Update


Illinois Drought Update, September 7, 2005
DROUGHT RESPONSE TASK FORCE,
Illinois State Water Survey, Department of Natural Resources


SUMMARY. The drought has worsened over much of the northern two thirds of Illinois. Several major watersheds (Green, Kishwaukee, Fox, Vermilion-Illinois, and Little Wabash) are now experiencing 10-year low flows, with the Green River experiencing its all-time record low flow. Water level in the Mississippi River at St. Louis fell almost 7 feet from August 31 to September 7. Without significant amounts of rain over the next week, the region of much-below normal streamflow likely will expand to cover over 60 percent of Illinois. Many reservoirs in central Illinois are 1 to 2 feet below normal. We are entering a drought phase where the cumulative impacts of precipitation deficits on water supplies will start to become most apparent. Although isolated at-risk supply systems may experience problems this fall, the greater potential for drought impacts on water supplies will occur if extended dry conditions extend into winter and spring. We do not know how much precipitation will fall over the months ahead, but we do know that more frequent, severe, and extended droughts have occurred in the past.


1. DROUGHT STATUS. According to the U.S. Drought Monitor (Figure 1), most of northern and western Illinois remains in a severe or extreme drought (categories 2 and 3 in their 4-category drought classification). Much of eastern Illinois remains in the category of abnormally dry. Only far southern Illinois is considered to be clear of drought. The Drought Monitor is updated each Thursday morning at 8am EDT and can be accessed via the Internet at http://www.drought.unl.edu/dm/monitor.html.


2. PRECIPITATION. Statewide precipitation since the last Drought Response Task Force meeting on August 26 has been 0.86 inches, which is 30 percent below normal. Furthermore, most of that precipitation fell in southern Illinois as a result of the passage of hurricane Katrina. The rest of the state received little from the storm. As a result, there has been some worsening of the drought, especially in eastern and northeastern Illinois. Statewide precipitation since March 1 has been 15.55 inches, 7.58 inches below normal, or 68 percent of normal (Figure 2). North of Peoria, the precipitation deficit is more than 10 inches, or less than 60 percent of normal.


3. LAST 100 YEARS. March-August 2005 was the7th driest such period in Illinois since 1895 (Figure 3). 1936 remains the driest March-August on record in Illinois, recording nearly 4 inches less precipitation than was measured in 2005. In 1936, precipitation statewide averaged only 11.62 inches, about 50 percent of normal. Illinois as a whole was drier in 1936 than the driest part of the state in 2005. Overall, these data indicate that spring/summer droughts were more common and severe early in the last century. Of the 10 driest spring/summer periods in Illinois, only 3 have occurred in the last 69 years, while 7 occurred in the first 42 years of record.


March - August 2005 was considerably drier in northern and central Illinois than in southern Illinois (Figure 4). Three climate divisions of the state (northwest, northeast and central) experienced the third driest March-August since 1895, while 4 more divisions were in the top 12 driest for their respective division. Conversely, although the March - June period in Illinois was uniformly dry statewide, heavy rain events in the past two months across southern Illinois have raised March-August totals in the southeastern division to a level not far from normal.


4. SOIL MOISTURE. Figure 5 shows that on September 1, moisture in the top 72 inches of soil was less than 75 percent of normal in most of northern and central Illinois, with less than 10 percent of normal soil moisture at three sites monitored by the Water Survey. Although localized improvements occurred in near-surface soil moisture in western and southern Illinois, the low values measured in central Illinois indicate decreasing soil moisture conditions.


5. GROUNDWATER Statewide, shallow groundwater levels continue to be below normal for the fifth consecutive month. Water levels at ISWS shallow observation wells at Fermi National Laboratory (DuPage County) and Bondville (Champaign County) were at their lowest levels for August since records started in November 1988 and March 1982, respectively. Water levels in these wells fell about 0.9 and 1.3 feet, respectively, during August.


Irrigation demand decreased during the past two weeks in northwestern Illinois. Groundwater levels in the buried, artesian Sankoty aquifer of Lee, Whiteside, Bureau, and Henry counties continued to rise during August. Irrigation decreases allowed recoveries of 10 to 15 feet over a very broad area in this, Illinois' second largest irrigation area. Meanwhile, groundwater levels in the surficial, water-table aquifer (the Tampico aquifer) continue to fall slowly - observations of 0.2 to 0.9 feet decline were common. Water levels in the Tampico typically decline until September-October and lag behind the decline of the deeper Sankoty aquifer, which occurs earlier in the summer.


Very few new reports have been received from well drillers lowering pumps or redrilling wells to greater depths as a result of lowered water levels in aquifers in northern and northwestern Illinois.
In central Illinois, the City of Decatur began pumping (8/31/05) from their emergency well field in response to a below-normal reservoir pool level and low inflow from the Sangamon River. The well field discharges to Friends Creek, which flows into the Sangamon River, ultimately supplementing the city's water supply from Lake Decatur. The Sangamon River is down to almost no-flow conditions at Fisher, and is only 6 cfs and dropping at Monticello. This is only the second time this well field has been pumped since its construction in 1990, to increase lake levels for the city (the first time was the Winter of 1999-2000). Groundwater levels around the well field, as well as Friends Creek and Sangamon River discharges, are being monitored by Water Survey scientists.


6. ILLINOIS STREAMFLOWS. Streamflow levels in the northern two thirds of Illinois have fallen steadily. Much of the west-central and north-central portions of the State are experiencing much-below normal streamflows (lowest 10th percentile) for this time of year (Figure 6). Without significant amounts of rain over the next week, the region of much-below normal flow likely will expand to cover over 60 percent of Illinois. Several major watersheds (Green, Kishwaukee, Fox, Vermilion-Illinois, and Little Wabash) are now experiencing 10-year low flows, with the Green River experiencing its all-time record low flow. The only regions of Illinois that are not currently experiencing below-average flow conditions are in the southernmost parts of the State that received rainfall from Hurricane Katrina, and the Kankakee River, which flows into northeastern Illinois from Indiana. Streamflow levels throughout Illinois typically reach their annual minima during the upcoming months of September, October, and November.


7. WATER LEVELS AT PUBLIC WATER SUPPLY (PWS) RESERVOIRS. The Water Survey records month-end water levels for 35 public water supply (PWS) reservoirs in Illinois, with records generally extending back to the mid-1980s to early-1990s. During the month of August, reservoir levels in the southern third and the eastern fringes of the State increased or stayed level in response to above-normal rainfall. In contrast, most reservoirs in central Illinois declined by 0.5 foot or more, and many of these are currently 1 to 2 feet below normal for this time of year. The level of Lake Decatur dropped 1 foot in August, and, as discussed earlier in this update, the City is now pumping groundwater to supplement inflow into the lake. Other reservoirs, such as at Altamont and Canton, are at their lowest August month-end levels since the early 1990s. Lake levels typically experience a steady decline through the fall season, and we are entering a drought phase where the cumulative impacts of precipitation deficits on lake levels will start to become most apparent. Although isolated at-risk supply systems may experience problems this fall, the greater potential for drought impacts on water supplies will occur if extended dry conditions extend into winter and spring.


8. FEDERAL RESERVOIRS. At the end of August, Carlyle Lake and Lake Shelbyville were roughly 1.2 and 1.5 feet below full pool, respectively. These are the lowest August levels experienced in these reservoirs since the 1988 drought. In contrast, with August rains the water level for Rend Lake rose 0.2 foot to an elevation of 405.5 feet. These low water levels on the federal reservoirs generally do not affect water supplies.


9. MISSISSIPPI AND OHIO RIVERS. Water level in the Mississippi River at St. Louis fell almost 7 feet from August 31 to September 7 and a substantial additional drop may create local navigation problems. In general, the entire length of the Mississippi River bordering Illinois is experiencing below average flows, but not extraordinarily low for this time of year. Flows in the Ohio River are above normal for this time of year as a result of rainfall from Hurricane Katrina.


10. ILLINOIS RIVER. Water level in the Illinois River has fallen since the last significant rains occurred in mid-August and is once again at near-record low flow, similar to the record lows observed in the early 1960s. As discussed in previous updates, the near-record low flows on the Illinois are the results of both this summer’s dry conditions and the general reduction in the Lake Michigan diversion caused by water use conservation and the reduction in leakage through the Chicago locks.


11. LAKE MICHIGAN. The level of Lake Michigan is at 577.8 feet, having dropped 0.3 foot over the past month. The level in the lake can be expected to continue dropping until mid-winter as part of its normal seasonal cycle. Lake Michigan is 1.4 feet below normal, and is 1.2 feet higher than its record low level for September, set in 1964.


12. STATEWIDE SUMMARY. Figure 7 summarizes the evolution of statewide-average temperature, precipitation, soil moisture, streamflow, and groundwater levels from August 2004 to August 2005.


13. OUTLOOK. Precipitation during the first 5 days of the period, September 7-11, is likely to be unevenly distributed across Illinois. A cold front will sag into the region and then stall near

I-80 on September 7-8. Modest rainfall amounts averaging 0.25 to 0.50 inches or more are possible in the northern third of the state, where more favorable upper air support will be located. The central portion of Illinois has only a moderate chance of rain, while the southern third of Illinois has little chance of rain from September 7 to 11. The long range forecast for September 12-16 indicates an above normal chance for above normal rainfall amounts in northern Illinois, while the forecast for September 14-20 expands this wet forecast to the whole of Illinois. Maximum daytime temperatures are expected to be above normal. Summers with below-normal rainfall have falls that are 42% likely to be dry, 25% likely to be normal, and 33% likely to be wet.


For more drought information please go to http://www.sws.uiuc.edu/.


Derek Winstanley

Chief

Illinois State Water Survey

Illinois Department of Natural Resources

Champaign

Tel. 217-244-5459; e-mail dwinstan@illinois.edu

Illinois Drought

Illinois State Water Survey

2204 Griffith Dr
Champaign, IL 61820-7463
217-244-5459
info@isws.illinois.edu

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