The Standardized Precipitation Index (SPI) was developed for the purpose
of defining and monitoring drought. Among others, the Colorado Climate
Center, the Western Regional Climate Center, and the National Drought
Mitigation Center use the SPI to monitor current states of drought in
the United States.
For more information on the methodology behind SPI, click on either
a PostScript version
or an Adobe PDF version
of Chapter 3 of Dan Edwards' Masters Thesis. This document details the
mathematics used in SPI calculation.
For source code for a simple SPI program:
Unix tar format
zip format
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