Deficit Precipitation over Northern Illinois Continues

Researcher: Bob Scott, Climatoligist, (217) 333-4966, rwscott1@uiuc.edu, By: Eva Kingston, (217) 244-7270, eva@sws.uiuc.edu

With precipitation totals six inches below typical amounts over the last seven months, 69 percent of average, Illinois has experienced the seventh driest September–March period since 1895, and the driest such period over the last 46 years, according to the Illinois State Water Survey (http://www.sws.uiuc.edu), a division of the Illinois Department of Natural Resources.

Regional precipitation conditions in Illinois for September–March also have been very dry. The northwest and northeast Crop Reporting Districts (CRDs) were the driest on record (50 and 52 percent of average, respectively). The west and central CRDs were the second driest (57 and 53 percent of average, respectively). The east CRD was fourth driest (57 percent of average). With 73 and 77 percent of average, respectively, the west-southwest and east-southeast CRDs were the 15th driest. Precipitation has been near average to above average in the southwest and southeast CRDs, and they are not currently experiencing a deficit.

Technically, these data place the northern half of Illinois in a climatological precipitation drought, and impacts are beginning to be observed in other water resources of the state. Surprisingly, soil moisture conditions within the uppermost six inches of soil are relatively near average across Illinois, but sub-soil moisture conditions vary considerably. Observations from the southern third of the state indicate above average moisture levels between 40 and 72 inches below the surface, but conditions in parts of central and northern Illinois in this layer are only 25 percent of normal.

Without a large snowmelt this year, combined with the low precipitation, streamflows along selected rivers and streams are showing below to much below normal flows for this time of year in all but extreme southern Illinois. Overall, current statewide flows are just 37 percent of the median flow rates for the end of March and have been below median rates since last October.

Shallow groundwater water table levels in observation wells away from pumping stations are also well below average levels and have been below the statewide average for the last seven months. Overall, shallow groundwater levels were below average levels for March by 2.2 feet. In northern Illinois, the well at Mt. Morris (Ogle County), where readings began in 1961, reported a record low for March, and the well at Fermi Laboratory (DuPage County) reported the lowest monthly level in the 15 years of observation at that site.

Reservoir data have shown only slight impacts from low precipitation totals, primarily because there are no public water reservoirs in the northern third of the state. Some northernmost reservoirs are below normal levels by a few feet, not considered a concern at this time, but these conditions should be monitored closely for those reservoirs not filled by the end of April.

Reduced precipitation is not only present in Illinois, but also over much of the Midwest. Consequently, the water level in Lake Michigan this spring is at its lowest since 1964. The lake is two feet below the normal for April and only eight inches above the record low level observed in 1964.

Fortunately, the precipitation deficit has occurred during a period when monthly rainfall totals in northern and central Illinois are typically low, crops are not using water, and evaporation rates are considerably lower than in warmer seasons. Thus, the primary potential impact of the precipitation deficit currently is on agriculture. Above average rainfall over the next several months is needed to recharge sub-soil moisture in dry areas as crop roots begin to tap into these depths later in the summer. However, without a return to average precipitation, rapid reductions in other water resources within the northern half of Illinois may soon follow.

If history is a guide, only two years of the remaining years in the top 10 driest September–March periods had slightly above average statewide precipitation amounts in the April–June period that followed (14 and 18 percent above average). Six of the years observed slightly below average rainfall (6 –16 percent below average), and precipitation in Illinois within the following three months was less than 50 percent of average in 1934.

For more information on current precipitation conditions in Illinois see the following Web sites: http://www.sws.uiuc.edu/warm (more specific data on soil moisture), and http://www.drought.unl.edu/dm/monitor.html (U.S. drought monitor).