The average length of record at each site is 85 years. Frequency analysis of the precipitation data used two recently developed techniques, regionalization and L-moments. Regionalization is the use of data from several sites, rather than just one, to estimate drought frequencies at a site. The information on precipitation frequency contained in the atlas is equivalent to 468 versions of Figure 1 for different durations, starting months, and recurrence intervals. Annual precipitation totals, as a percentage of the median annual precipitation, for a drought with recurrence interval of 50 years.
Percentages apply to each site in a region.
Gaging sites are indicated by lines joining the site location to the center of the region. A number of states rely on it for initiating and discontinuing drought emergencies. However, a close look at the index showed it to be misleading in some respects. In preparing the National Drought Atlas, PDSI values were recalculated at monthly intervals for 1036 sites. These calculations revealed that the PDSI index overstates the severity and variability of droughts within a given region.