PALMER INDEX DATA
The National Drought Atlas statistical analysis of Palmer Index Data is
not available on-line.
The Palmer Drought Severity Index (Palmer, 1965) is calculated by a soil moisture accounting
procedure based on precipitation and temperature for estimating evapotranspiration by the
Thornthwaite method. Ordinarily, the Index is calculated for state climatic divisions. The National Climatic Data Center recalculated the Palmer Drought Severity Index for the National
Drought Atlas on a station-by-station basis rather than a climatic division basis for the period of record of
1,035 stations in the Historical Climatology Network.
Unadjusted precipitation totals from this data set were used (see Precipitation section for a discussion of adjustments). Time series go back from 1990
(except from 1988 or 1989 for a few sites) to at least 1930 after allowing for an initialization period of four
years. Computations require not only serially complete monthly temperature and precipitation data, but
also soil water capacities, coefficients for computing potential
evapotranspiration, and weighting factors.
The values of these latter constants and factors that were used for the at-site computations were the
values on file at the National Climatic Data Center that are representative of the climatic division in which
the site is located. Although not examined, the use of climatic division values was not thought to
significantly impact the at-site modified Palmer Index computations. The resulting estimates
should be superior to other estimates which have been calculated to date.
In order to calculate the Palmer Index, missing data had to be estimated. In addition, some of the
monthly precipitation totals are the sum of daily values for less than a full month, i.e., some of the monthly
data are partial totals. Because of the localized nature of precipitation, there is no generalized, accepted
method of obtaining meteorologically reliable estimates of missing data.
A simple precipitation estimation procedure was used that is based on the assumption that, even
though precipitation is a localized phenomenon, an average monthly total over a small area should be
reasonably representative of any site within the area. A missing total for a given month and
year was estimated by averaging the totals for the month and year for all stations in the National Climatic Data
Center's Summary of the Month digital data base that are within 60 miles of the station with the missing
data. The number of nearby stations whose totals were averaged varied from 1 to 87. The number of
stations depends on the density of the station network (most dense in the Northeast and least dense in the
Rocky Mountains) and the year (fewer stations in the earlier years of the period of record). If digital data
were not available, Climatological Data publications were examined, and the published recorded or
estimated values for the month and year in question at the Historical Climatology Network station were
used. The publications were also used to obtain a small number of values to extend the length of record
of the temperature records (the length of record of the temperature data at a site is often shorter than that
of precipitation data).
If a station had a partial precipitation total for a given month and year, the average of the totals for
the nearly stations was computed and compared to the partial total. The higher of the partial total or
nearby comparisons were made between a nearby station average and a partial total. The nearby station
average was higher than the partial total for 60 percent of the comparisons and lower for 40 percent.
These percentages subjectively confirm the difficulty in estimating monthly precipitation amounts over a
small area and add credibility to the decision to use the unadjusted data from the Historical Climatology
Network for the precipitation analysis.
Note that the number of stations for which the drought index has been computed is less than the
number of stations for which precipitation probabilities have been computed. It was necessary to eliminate
some stations from the drought computations because of inadequate data. The
inadequacies resulted from the requirement that both temperature and precipitation data had to be serially complete from 1926
onward. If either the temperature or precipitation data were not available and missing data could not be
estimated because of lack of digital or published data from nearby stations, then the Palmer Index could
not be computed.
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