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Standardized Precipitation Index (SPI-3)

Droughts affect millions of people in the world each year and have long-lasting socioeconomic impacts. They can occur over most parts of the world, even in wet and humid regions, and can profoundly impact agriculture, basic household welfare, tourism, ecosystems and the services they provide. The Standardized Precipitation Index (SPI) is the most commonly used indicator worldwide for detecting and characterizing meteorological droughts, which are prolonged periods of less than average rainfall in a given region. It measures precipitation anomalies at a given location, based on a comparison of observed total precipitation amounts for an accumulation period of interest (in this case, 3 months), with the long-term rainfall record for that period. SPI values below ‒1.0 indicate rainfall deficits (drier than normal – yellow to red), while SPI values above 1.0 indicate excess rainfall (wetter than normal – purple to blue). The lower the SPI, the more intense is the drought. The layer show the SPI-3 from the month second to last and is updated monthly.

McKee, T.B., N.J. Doesken and J. Kleist. 1993. The relationship of drought frequency and duration to time scale. In: Proceedingsof the Eighth Conference on Applied Climatology, Anaheim, California, 17–22 January 1993. Boston, American Meteorological Society, 179–184.…

Second to last full month

The exact relationship between the accumulation period and the drought impact depends on the natural environment (e.g., geology, soils) and the human interference (e.g., existence of irrigation schemes). In order to get a full picture of the potential impacts of a drought, the SPI should be calculated and compared for different accumulation periods. A comparison with other drought indicators is also needed, in order to evaluate the actual impacts on the vegetation cover and different economic sectors. SPI is based only on precipitations and does not address the effects of high temperatures on drought conditions. The SPI should be interpreted with care in arid regions (high probability of zero rainfall) where the concept of drought needs to be adapted, or analysed with alternative drought indicators.

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