Palmer drought index
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 also hard-to-calibrate factors including evapotranspiration and recharge rates. Palmer tried to overcome such difficulties by developing an algorithm that approximated them based on the most readily available data, precipitation and temperature.
The index has been most effective in determining long-term drought, a matter of several months, but it is not as good with conditions over a matter of weeks. It uses a 0 as normal, and drought is shown in terms of negative numbers; for example, -2 is moderate drought, -3 is severe drought, and -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.
The Palmer index is widely used operationally, with Palmer maps published weekly by the United States Government's National Oceanic and Atmospheric Administration. It also has been used by climatologists to standardize global long-term drought analysis. Global Palmer data sets have been developed based on instrumental records beginning in the 19th century. In addition, dendrochronology has been estimated Palmer index values for North America for the past 2000 years, allowing analysis of long term drought trends. It has also been used as a means of explaining the late Bronze Age collapse.
In the US, regional Palmer maps are featured on the cable channel Weatherscan.
- Alley, William: The Palmer Drought Severity Index: Limitations and Assumptions, Journal of Climate and Applied Meteorology, Vol. 23, pp. 1100–09, July 1984
- Dai, Aiguo et al.: A Global Dataset of Palmer Drought Severity Index for 1870–2002: Relationship with Soil Moisture and Effects of Surface Warming, Journal of Hydrometeorology, Vol. 5, No. 6, pp. 1117–30, December 2004
- Cook, E.R. et al.: Long-Term Changes in the Western United States, Science, Vol. 306, No. 5698, pp. 1015–18, November 5, 2004