Standardized Precipitation Index

The Standardized Precipitation Index (SPI; McKee et al. 1993) is the number of standard deviations that observed cumulative precipitation deviates from the climatological average. The index is based entirely on monthly precipitation accumuluations and its values can be compared across different climatic and geographic regions. These characteristics of the SPI have contributed to its popularity for application towards drought and water resource monitoring.

Studies have suggested that a minimum of 50 years of precipitation data be used to calculate SPI values. It should therefore be noted that the UEA New and TS2.1 datasets are the only precipitation datasets in the current set of analyses that meet that recommendation. Extreme SPI values in the other three datasets (CAMS OPI, CMAP, GPCP) may be suspect as they are based on roughly half of the recommended amount of data.

More information about all four precipitation datasets can be found at the following locations:
1. NASA GPCP V2: Documentation, Data
2. NOAA NCEP CPC CAMS_OPI: Documentation, Data
3. NOAA NCEP CPC Merged_Analyis (CMAP): Documentation, Data
4. UEA CRU New CRU05: Documentation, Data
5. UEA CRU TS2p1: Documentation, Data

The analyses in this dataset are based on a Pearson Type III (i.e., 3-parameter gamma) distribution as suggested by Guttman (1999). Fortran 77 source code made available in that reference was used to create the SPI analyses.

Guttman, N. B., 1999: Accepting the Standandardized Precipitation Index: a calculation algorithm. J. Amer. Water Resour. Assoc., 35(2), 311-322.

McKee, T.B., N. J. Doesken, and J. Kliest, 1993: The relationship of drought frequency and duration to time scales. In Proceedings of the 8th Conference of Applied Climatology, 17-22 January, Anaheim, CA. American Meterological Society, Boston, MA. 179-184.

McKee, T.B., N.J. Doesken, and J. Kliest, 1995: Drought monitoring with multiple time scales. In Proceedings of the 9th Conference of Applied Climatology, 15-20 January, Dallas TX. American Meteorological Society, Boston, MA. 233-236.

The SPI analyses can be found here.