The 36-month drought at return periods or recurrence intervals of a) 25 years, b) 50 years, c) 100 years, and d) 200 years, expressed as percent of normal (1971-2000) precipitation. Source: The Water Cycle and Water Budgets in Illinois: A Framework for Drought and Water-supply Planning by Derek Winstanley et al., Illinois State Water Survey IEM 2006-02.
