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Standardized Precipitation Index
Drought results from an imbalance between water supply and water demand. The Standardized Precipitation Index (SPI), one of several drought indices that have been developed over the decades, measures water supply, specifically precipitation. The SPI is computed over several time scales, typically from one month to 24 months, to evaluate both short-term drought and long-term drought. It can also measure precipitation excess. The U.S. Drought Monitor incorporates information from the SPI and many other drought indices, as well as drought impacts, in depicting drought.