The standardized precipitation index (McKee et al. 1993) has been developed for defining and monitoring droughts, 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).
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.
Name: Sahara