The Standardized Precipitation Index (SPI) quantifies precipitation anomalies at multiple time scales, providing a versatile measure of drought and wet conditions. It is the World Meteorological Organization's recommended drought index.
The Standardized Precipitation Index (SPI) was developed by McKee, Doesken, and Kleist in 1993 as a simple, versatile drought index based solely on precipitation data. The SPI transforms precipitation data to fit a probability distribution (typically gamma), then converts to a standard normal distribution, producing values with a mean of zero and standard deviation of one. Negative SPI values indicate drier-than-normal conditions and positive values indicate wetter-than-normal conditions, with values below -2 representing extreme drought and above +2 representing extremely wet conditions. A key advantage of the SPI is its ability to be calculated at multiple time scales (1, 3, 6, 9, 12, 24, and 48 months), allowing it to monitor different types of drought: short time scales (1-3 months) capture meteorological drought, medium scales (3-12 months) reflect agricultural drought, and long time scales (12-48 months) relate to hydrological drought and reservoir levels. The World Meteorological Organization recommended the SPI as the primary meteorological drought index in 2009 because it requires only precipitation data (widely available), is spatially consistent, and enables comparison across different climatic regions. Limitations include its reliance on precipitation alone without considering temperature or evaporative demand.