Drought Help: Standardized Precipitation Index (SPI)

The Standardized Precipitation Index (SPI) is a drought index based only on precipitation. The SPI can be used to monitor conditions on a variety of time scales. This temporal flexibility allows the SPI to be useful in both short-term agricultural and long-term hydrological applications. The SPI was developed in 1993 by T.B. McKee, N.J. Doesken, and J. Kleist.

The SPI value represents an estimation of the unusualness of a precipitation total, based on the climate division's history. Larger absolute values (both above and below zero) represent increasingly unusual events. A thorough description of the SPI is available from the National Drought Mitigation Center.

The following table gives a brief description of the significance of various SPI values, including an estimate of how likely it is to exceed SPI values.