The Standardized Precipitation Index (SPI) is a probability index that was developed to give a better representation of abnormal wetness and dryness than the PSDI. It uses a gamma or a Pearson Type III distribution and can be created for differing periods of 1-to-36 months. The SPI is a value derived from monthly precipitation that can be compared across regions with markedly different climates. The standardization of the SPI allows the index to determine the rarity of a current drought. There is some variation in the methods used to derive the SPI. The WMO recommends, that all national meteorological and hydrological services should use the SPI for monitoring of dry spells.
NCL scripts that generates the SPI are at: http://www.ncl.ucar.edu/Applications/spi.shtml . One script reads ascii (text) for a specific station while the other script reads the Global Precipitation Climatology Project data set. Each script computes the SPI at user specified periods. The scripts can be readily modified to compute the SPI with other monthly data sets. Further, they could be modified to create a netCDF file for other uses.
http://www.wrcc.dri.edu/spi/explanation.html: Technically, the SPI is the number of standard deviations that the observed value would deviate from the long-term mean, for a normally distributed random variable. Since precipitation is not normally distributed, a transformation is first applied so that the transformed precipitation values follow a normal distribution. The Standardized Precipitation Index was designed to explicitly express the fact that it is possible to simultaneously experience wet conditions on one or more time scales, and dry conditions at other time scales, often a difficult concept to convey in simple terms to decision-makers. Consequently, a separate SPI value is calculated for a selection of time scales, covering the last 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 18, 24, 30, 36, 48, 60, and 72 months, and ending on the last day of the latest month.
If you use information or images from this page, please cite:
National Center for Atmospheric Research Staff (Eds). Last modified 03 Apr 2013. "The Climate Data Guide: Standardized Precipitation Index (SPI)." Retrieved from http://climatedataguide.ucar.edu/guidance/standardized-precipitation-index-spi.
Page history: Originally posted on 07 Nov 2011 and last modified 03 Apr 2013.
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, , , and (2013), Climate Data Guide Spurs Discovery and Understanding, Eos Trans. AGU, 94(13), 121. [article]
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