Odds of Drought Years Repeating in Illinois

I am in the middle of my winter farm talk tour so I have not had much time to post on the blog. However, in coming days I will try to cover some recurring themes from my talks.

One of those themes is the odds of repeating drought in Illinois. Or in other words, does the drought of 2012 put us at increased risk for drought in 2013 in Illinois? If you look at the 10 driest years in Illinois since 1895, none of them were back to back to each other. That is comforting news.

A follow-up question I got from a farmer was: what does happen in the year after the big droughts? The table below shows the 10 driest years, their annual total, their departure from the 1981-2010 average, along with the next year, the annual total and departure. Since 2012 was in the list, we are left with nine cases. Of those nine, four cases were above and five were below average. However, being 1-3 inches below average would not be a big concern in my opinion. So we could say that about 6 out of 9 cases were close to average on precipitation.

However, the two pairs that stand out to me are 1963-64 and 1988-89. While the second year in both cases were not as severe in their precipitation deficits, and none of them made it into the top ten, the numbers are worrisome. In fact, 1964 ended up as the 14th driest and 1989 as the 16th driest on record. Meanwhile, the pair of 1953-54 was not as severe. However, it was followed by moderately dry conditions in 1955 (34.81 inches, down 5.14 inches) and more severe conditions in 1956 (30.76 inches, down 9.61 inches).

In conclusion, on one hand you could say that historically there has been a 44 percent chance  (4 out of 9 cases) of having above-average precipitation in the year after a “top ten” drought year. Or you could say that there has been a 67 percent chance (6 out of 9 cases) of being within 10 percent of the long-term average precipitation (39.95 inches) in the year after a “top ten” drought year.

On the other hand, you could say that there was a 33 percent chance (3 out of 9 cases) of having continued drought trouble in the second  year of a “top ten” drought year.

Year Annual Depart Next Year Annual Depart
1901 26.34 -13.61 1902 42.26 2.31
1930 27.88 -12.07 1931 38.17 -1.78
1963 28.00 -11.95 1964 31.94 -8.01
1953 28.05 -11.90 1954 35.44 -4.51
1914 28.58 -11.37 1915 42.28 2.33
1976 28.84 -11.11 1977 41.80 1.85
1940 29.33 -10.62 1941 40.64 0.69
1988 29.71 -10.24 1989 32.59 -7.36
1936 30.24 -9.71 1937 37.25 -2.70
2012 30.34 -9.61 ? ? ?

Note: this analysis was based on annual statewide precipitation for Illinois. The conclusions reflect overall conditions and may not pertain to a particular area of the state where more severe conditions may have occurred.


One thought on “Odds of Drought Years Repeating in Illinois

  1. Seeing as we are still seeing SST cooling and La Nina conditions, I would hope the pattern holds for wetter than average weather this year, but then 1988 and 1964 were both La Nina years.

Comments are closed.