Standardized Precipitation Index
Also known as: SPI
The Standardized Precipitation Index (SPI) is a climate index that quantifies precipitation anomalies relative to historical norms, standardized to account for differences in precipitation climatology across regions. Introduced by McKee, Doesken, and Kleist in 1993, SPI has become a primary tool for drought detection and characterization, adopted by meteorological agencies worldwide for operational drought monitoring and early warning systems.
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When to use it
Use SPI to quantify and monitor drought conditions, particularly for agricultural and water resource management applications. SPI is ideal for detecting onset and intensity of droughts at monthly to inter-annual timescales. Combine SPI with other indices (SPEI, NDVI) for comprehensive drought characterization. SPI is less useful in tropical regions with high-intensity convective rainfall or in areas with snow accumulation where precipitation is not immediately hydrologically effective.
Strengths & limitations
- Data-parsimonious: requires only precipitation data, enabling application in data-sparse regions
- Standardized across regions, allowing direct comparison of drought intensity between different climate zones
- Flexible timescale: SPI can be computed at 1-, 3-, 6-, or 12-month intervals to capture droughts of different durations
- Widely adopted and operationally used, ensuring compatibility with existing drought monitoring networks and decision-making frameworks
- Based solely on precipitation; does not account for temperature, evapotranspiration, or soil moisture, which influence actual water availability
- Probability distribution assumptions (gamma, Pearson III) may not hold at all locations or time periods, particularly in regions with high precipitation variability
- Requires long historical records (30+ years) to establish robust climatology; unreliable in data-short regions or for new weather stations
- Short accumulation periods (1–3 months) can be noisy and may not reflect true hydrological drought conditions
Frequently asked
What is the difference between SPI and SPEI?
SPI uses only precipitation, while SPEI (Standardized Precipitation-Evapotranspiration Index) also incorporates temperature and evapotranspiration. SPEI better captures the combined effect of precipitation deficit and increased evaporative demand, making it more physically relevant for soil moisture and hydrological drought. However, SPEI requires additional temperature data.
How is a SPI value interpreted?
A SPI of 0 represents the long-term mean. Positive values (up to +3 or more) represent wet conditions; negative values (down to -3 or lower) represent dry conditions. A commonly used classification: SPI > 1.5 is extremely wet; 1.5 to 1.0 is very wet; 1.0 to 0.5 is moderately wet; -0.5 to 0.5 is near normal; -0.5 to -1.0 is moderately dry; -1.0 to -1.5 is severe drought; -1.5 to -2.0 is very severe drought; < -2.0 is extreme drought.
Why use multiple timescale SPI values?
Different timescales capture droughts affecting different sectors. 1-month SPI shows rapid changes and is sensitive to short-term rainfall events. 3- and 6-month SPI reflect soil moisture and agricultural drought. 12-month SPI indicates hydrological drought affecting streamflow and groundwater. Using all timescales together provides a complete picture of drought at all relevant timescales.
Can SPI be used for precipitation forecasting?
No. SPI is a retrospective index based on observed precipitation. To forecast future drought, one must first forecast precipitation (using weather or climate models) and then compute SPI from the forecasted precipitation. SPI itself is not predictive.
Sources
- McKee, T. B., Doesken, N. J., & Kleist, J. (1993). The relationship of drought frequency and duration to time scales. Proceedings of the Eighth Conference on Applied Climatology, 179-184. link ↗
- Lloyd-Hughes, B., & Saunders, M. A. (2002). A drought climatology for Europe. International Journal of Climatology, 22(13), 1571-1592. DOI: 10.1002/joc.846 ↗
How to cite this page
ScholarGate. (2026, June 3). Standardized Precipitation Index. ScholarGate. https://scholargate.app/en/geophysics/standardized-precipitation-index
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
- General Circulation ModelGeophysics↔ compare
- NDVIGeophysics↔ compare
- Standardized Precipitation Evapotranspiration IndexGeophysics↔ compare