Illinois Water Supply Planning


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Illinois is a water-rich state with abundant precipitation in solid and liquid forms, the ultimate source for Illinois’ water resources. Precipitation is heaviest in southern Illinois and lightest in northern Illinois.

The pattern of average annual precipitation at selected Illinois locations (Figure 1) shows less than 32 inches in the northeastern corner of Illinois and more than 46 inches across the Shawnee Hills of southern Illinois, a north-south difference of 14 inches. May–August are the wettest months in northern Illinois, with little precipitation in winter months. Precipitation is more evenly distributed throughout the year and relatively heavy from March–September in central Illinois. January–June is the wettest period in southern Illinois (see graph for Cairo).

Highest and lowest annual precipitation expected to occur once in 5 years and once in 50 years in Illinois are shown (Figure 2). For example, highs expected once in 50 years range from more than 70 inches (southern Illinois) to 46 inches (northern Illinois). Lows expected once in 50 years range from 22 inches (northeastern Illinois) to 28 inches (southern Illinois).

Snowfall occurs regularly in all parts of the state and contributes 10 to 15 percent of the annual precipitation in northern Illinois but only 3 to 5 percent in extreme southern Illinois. Snowfall and snow cover are much more frequent in the far north than in the far south so there is a large north-to-south gradient.

Snow is a very important aspect of Illinois’ climate, causing significant socioeconomic and environmental impacts. Snowfall contributes directly to the water resources of the state: snow melt recharges soil moisture. Melting also can exacerbate flooding due to heavy rainfall; rapid melting of deep snow cover even has caused flooding in the absence of any rainfall. Snow is a good insulator, and snow cover significantly slows the penetration of frost into the ground. Deep snow and persistent snow cover can lead to structural damage of homes and other buildings. Snowfall also can disrupt transportation systems.

Distribution of the average annual snowfall (Figure 3) ranges from less than 12 inches (far south) to 30 inches (central Illinois) to 40 inches (Chicago metropolitan area). Occasional episodes of lake-effect snow increase snowfall in the Chicago area as cold easterly to northeasterly winds moving across warmer waters of Lake Michigan pick up moisture that is deposited as snow along the shoreline.

The societal impacts of snow are in part determined by the frequency of snow events. Even small amounts of snow can disrupt transportation. The average number of days per year when snowfall equals or exceeds 1 inch (Figure 4) varies from less than 4 days per year (far south) to 12 days (far northern Illinois).

Severe damages often develop when snowfall equals or exceeds 6 inches (Figure 5), an infrequent event throughout the state. This occurs about once a year (far north), once every two years (much of northern and central Illinois), and less than once every three years (far south).

The average annual number of days when snow cover (or snow depth) equals or exceeds 1 inch (Figure 6) ranges from less than 10 days (far south) to more than 50 days (north). Snow cover data better illustrate the great north-to-south differences in Illinois’ snow climatology than snowfall data. Due to warmer temperatures in southern Illinois, snow usually melts rapidly after it falls. By contrast, average winter temperatures are well below freezing in northern Illinois, and lengthy periods with continuous snow cover are a normal feature of the climate.


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