US Army Corps of Engineers
US Army Corps of Engineers

National Drought Atlas

Example 4

How rare is the drought of record?

This question is asked for several reasons. For planning purposes, it is helpful to know whether the drought of record is a very rare event. When the location of interest is represented by a station with a long record, it is likely, though not certain, that the drought of record will have a recurrence interval of 50 years or more. For stations with short records, the situation is less clear, because a very rare event can occur in the shortest of records.

The calculations to determine the frequency of the drought of record are the following:

- Select the precipitation worksheet for the appropriate region
- Compare the precipitation depth in the historic drought to the precipitation
depths for 2%, 5%, 10% non-exceedance frequency the appropriate starting month
and duration
- If the historic depth is between two values in the table, interpolate between the two
values by plotting the ratios vs. recurrence intervals, and reading the recurrence interval from the plot
- If the historic depth is lower than the 2% depth, the recurrence interval cannot be 
estimated precisely; moderate extensions of the plotted line referred to in the previous step can be used, but the shape of the tail of the distribution cannot be accurately inferred from this plot. 

In those cases, the recurrence interval be reported as "longer than 50 years."

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revised 1 Aug 2006



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