Agricultural and Meteorological Software

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What is SPI (Standardized Precipitation Index)?

The SPI is the most popular drought index (Karabulut, 2015) and is a widely recognized index for characterizing meteorological droughts(Hayes et al., 1999; Deo, 2011). McKee et al. (1993, 1995) defined SPI suitable for different timescales (1, 3, 6, 12, 24 and 48 months), and the output values ranged from -2.0 to 2.0. Because precipitation data may be fitted by a gamma distribution, the SPI is calculated as following using a probability density function of the gamma distribution: AgMerra Drought paper

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