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 (Standardized Precipitation Index) 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 (Standardized Precipitation Index) is calculated as following using a probability density function of the gamma distribution: AgMerra Drought paper.

These different timescales are designed to reflect the impacts of precipitation deficits on different water resources. For instance, soil moisture conditions respond to precipitation anomalies on a relatively short scale, whereas groundwater, streamflow, and reservoir storage reflect longer term precipitation anomalies.

The standardized precipitation index (SPI) for any location is calculated, based on the long-term precipitation record for a desired period. This long-term record is fitted to a probability distribution, which is then transformed to a normal distribution so that the mean SPI for the location and desired period is zero (McKee et al., 1993; Edwards and McKee, 1997).

The Standardized Precipitation Index (SPI) reflects soil moisture and precipitation conditions. The SPI (Standardized Precipitation Index) values for the time periods are equivalent to the precipitation total's number of standard deviations from the normal for that time period. The SPI (Standardized Precipitation Index) computed on shorter scales (e.g. 3 or 6 months) describes drought events that afect agricultural practices, while on the longer ones (e.g. 12, 24 or 48 months), the efects of a precipitation defcit on diferent water resource components are given (stream fow, soil moisture, groundwater and reservoir storage).

In the SPI (Standardized Precipitation Index)values, greater than median precipitation is denoted by positive SPI values whereas negative values indicate less than median precipitation. More specifcally, the "drought" part of the SPI (Standardized Precipitation Index) range is arbitrary divided in four categories; mildly dry (0>SPI>-0.99), moderately dry (-1.0>SPI>-1.49), severely dry (-1.5>SPI>-1.99) and extremely dry conditions (SPI less then -2.0). A drought event is considered to start when SPI value reaches -1.0 and ends when SPI becomes positive again.

The SPI (Standardized Precipitation Index)can clearly show situations that are simultaneously in excess and deficit on different timescales (e.g., short wet episodes within long dry periods, or vice versa) and highlights rather than overlooks such common behavior. To date, this greatest strength of the SPI (Standardized Precipitation Index) has been its least exploited.