The Palmer Drought Index is based on a Supply-and-demand model of Soil Moisture . Supply is comparatively straightforward to calculate, but demand is more complicated as it depends on many factors - not just temperature and the amount of moisture in the soil but hard-to-calibrate factors including Evapotranspiration and recharge rates. Palmer tried to overcome these difficulties by developing an algorithm that approximated them based on the most readily available data — precipitation and temperature.
The index has proven most effective in determining long-term Drought — a matter of several months — and not as good with conditions over a matter of weeks. It uses a 0 as normal, and drought is shown in terms of minus numbers; for example, minus 2 is moderate drought, minus 3 is severe drought, and minus 4 is extreme drought. Palmer's algorithm also is used to describe wet spells, using corresponding positive numbers. Palmer also developed a formula for standardizing drought calculations for each individual location based on the variability of precipitation and temperature at that location. The Palmer index can therefore be applied to any site for which sufficient precipitation and temperature data is available.
Critics have complained that the utility of the Palmer index is weakened by the arbitrary nature of Palmer's algorithms, including the technique used for standardization. The Palmer index's inability to account for snow and frozen ground also is cited as a weakness.Alley, William: The Palmer Drought Severity Index: Limitations and Assumptions, Journal of Climate and Applied Meteorology, Vol. 23, pp. 1100 - 1109, July 1984
The Palmer index is nevertheless widely used operationally, with Palmer maps published weekly by the United States Government's has been used to generate estimated Palmer index values for North America for the past 2000 years, allowing analysis of long term drought trends.Cook, E.R. et al.: Long-Term Aridity Changes in the Western United States, Science, Vol. 306, No. 5698, pp. 1015-1018, 5 November 2004
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