Illinois Drought: Update October 3, 2005, Illinois State Water Survey

Illinois Drought

Comparison of the 2005 Drought with Other Drought Years since 1895

The following data allow for a comparison between 2005 climate conditions in Illinois and those in other drought years since 1895. An important message is that more severe drought conditions have occurred in the past and, hence, can be expected to recur some time in the future. We cannot predict how precipitation in Illinois will evolve over the next 12 months, but examination of precipitation records for 9 other years with March-September drought shows how precipitation evolved in these years. In all 9 years, 18-month precipitation deficits remained through the subsequent summer. Above normal precipitation in 2 years reduced the precipitation deficits considerably. Normal precipitation in some years merely maintained the precipitation deficits. In three years, precipitation deficits increased through the following May. By the following summer, severe drought conditions prevailed in 3 of the 9 years.

State-wide Normal[1] Precipitation: 39.23 inches

  1. Annual Statistics
    1. Statewide

                                                               i.      Worst year: 33% below normal in 1901 (26.32 inches)
                                                             ii.      Worst 12-month period: 43% below normal, starting in March 1930
                                                            iii.      2005 through September: 10% below normal

    1. Climate Divisions
                                                              i.      Worst 12-month period: 44 to 53% below normal
                                                             ii.      2005 through September for northern[2] Illinois: 23% below normal
  1. March-September
    1. Statewide
                                                     i.      Worst on record: 42% below normal in 1988
                                                    ii.      2005: 26% below normal
    1. Climate Divisions
                                                               i.      Worst on record for northern Illinois: 47% below normal
                                                             ii.      2005 for northern Illinois: 37% below normal
  1. Evolution of the northern Illinois drought: For the other nine driest March-September periods in northern Illinois, their precipitation deficits for the next 15 months are applied to the current situation (see figure 1). For example, what will the future precipitation deficit look like if this year performs like the 1988-1989 drought or the 1936-1937 drought?
  1. Worst October September for Illinois: 33% below normal in 1901

Figure 1. Possible evolution of the 2005 drought.

[1] Normal refers to the 1971-2000 average

[2] Northern Illinois is defined as the northwest, northeast, and central climate divisions of the state

Illinois Drought

Illinois State Water Survey

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