The National Drought Atlas is a unique source of information about the frequency, severity and duration of drought as reflected by precipitation depths and streamflows. The Atlas can be used in setting policy, making short and long term plans for water use, and for making certain operational decisions. Because the information needed for decisions regarding drought includes all parts of the frequency distribution -- low, middle, and high -- the Atlas can serve not only as a guide to information about droughts but also about long duration wet periods. The information on precipitation and streamflow is now available on-line in the form of Excel spreadsheets. ("Maps to Select Data", left).
The need for extensive frequency analysis was recognized early in the conduct of the National Study of Water Management During Drought. Few such analyses existed, and none had been done for the entire United States. Drought tended to be described in terms of how much difference there was between present precipitation or streamflow and normal conditions. "Normal" was almost invariably taken to be the 30-year mean condition for precipitation. For streamflow, "normal" usually meant the mean flow for the period of record.
The characterization of drought as a departure from normal is of limited usefulness if the probability of the departure is unknown. A local water manager may indeed have some understanding of how rare an event is. It is less common for people who aren't water resource professionals to have such an understanding, and even less common for persons with responsibility for regional and national water planning and management to be able to compare the frequency of a precipitation or streamflow deficiency in one part of a region or state with the frequency at other locations. Without knowing the probability that droughts of various magnitudes and durations will occur, it is impossible to gage the risks of a specific response to drought, whether for a long term investment plan or for a response to an ongoing drought.
revised 1 Aug 2006
Maps to Select Data
How to Use the Atlas
Example 1 - How rare is the current drought?
Example 2 - How likely is it that the drought will end in X months?
Example 3 - How large a drought should we plan for?
Example 4 - How rare is the drought of record?
Palmer Drought Index
(not available online)
HCN Precip Data
HCDN Streamflow Data (or access the data from Atlas spreadsheets for each USGS Region; also available from USGS)
Today's Streamflows USGS