What are the exact classifications values for the SPEI (Standardized Precipitation Evapotranspiration Index)?

The SPEI is a multiscalar drought index based on climatic data. SPEI was developed combining the sensitivity of the Palmer Drought Severity Index (PDSI) to changes in evaporation demand and robustness of the multi-temporal nature of the SPI. The Standardized Precipitation Evapotranspiration Index (SPEI) is an extension of the widely used Standardized Precipitation Index (SPI). SPEI has an intensity scale in which both positive and negative values are calculated, identifying wet and dry events. It is defined as an index of water balance (precipitation minus PET). The Penman-Monteith equation and the Thornthwaite equation are two representative parameterizations using the underlying physical principles (changes in available energy, humidity, and wind speed) and air temperature, respectively, for PET calculation.

According to the classifications of McKee et al. (1993) and Paulo et al. (2012) for the SPEI drought classes, we have 5 classes, namely: 1- Non-Drought (in this class the value of SPEI greater than -0.5), 2- Mild (the value of SPEI is between -0.5 and -1), 3- Moderate (SPEI is between -1.5 and -1), 4- Severe (SPEI is between -2 and -1.5), and 5- Extreme (Less than -2).

One of the nice advantage of SPEI is: Statistically based index that requires only climatological information without assumptions about the characteristics of the underlying system. It has few limitations, such as: SPEI needs more data requirements than the precipitation SPI, also SPEI sensitive to the method to calculate potential evapotranspiration (PET).

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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