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What are the limitations of SPI (standardized precipitation index)?

The standardized precipitation index (McKee et al. 1993) has been developed for defining and monitoring droughts. The Standardized Precipitation Index (SPI) is widely used as drought meteorological index, to identify the duration and/or severity of a drought.The Standardized Precipitation Index, known as SPI, seems to be the most popular among the existing simple indices for the estimation of drought.The Standardized Precipitation Index (SPI) is widely used for defining and monitoring meteorological droughts.

The SPI is based on the conversion of the precipitation data to probabilities based on long-term precipitation records computed on different time scales. Probabilities are transformed to standardized series with an average of 0 and a standard deviation of 1. The main advantage of the SPI as compared with the Palmer indices is that the former allows analyzing drought impacts at different temporal scales while the latter does not (Edwards and McKee 1997).

However according to the scientific papers (Wu et al., 2005; Mishra and Singh, 2010), the first limitation is the length of precipitation record that it has a significant impact on the SPI values, it means that with different lengths of precipitation records of data we can observe different results of answers of SPI. The second limitation is the probability distributions. There are different probability distributions that has been used in SPI calculations, namely gamma and Pearson Type III distribution, lognormal, extreme value, and exponential distributions have been widely applied to simulations of precipitation distributions (Wu et al., 2007). another disadvantage is: when applying the SPI at short time scales (1, 2, or 3 months) to regions of low seasonal precipitation, misleadingly large positive or negative SPI values may result.

McKee TB, Doesken NJ, Kleist J. 1993. The relationship of drought frequency and duration to time scales. In Proceedings of the 8th Conference on Applied Climatology, Anaheim, CL, 17-22 January, 179-183.

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