Standardized precipitation and Evapo-transpiration index
The SPEI (Standardized Precipitation Evapotranspiration Index) is a variant of the widely used Standardized Precipitation Index (SPI). In evaluating drought, the SPEI takes into consideration both precipitation and potential evapotranspiration (PET). As a result, unlike the SPI, the SPEI captures the primary effect of rising temperatures on water demand. The SPEI, like the SPI, can be calculated across a range of periods from one to forty-eight months. The SPEI has been demonstrated to correlate with the self-calibrating PDSI over longer durations (>18 months) (sc-PDSI).
PET can be approximated using the simple Thornthwaite approach if only limited data is provided, such as temperature and precipitation.
Variables that can alter PET, such as wind speed, surface humidity, and sun radiation, are not taken into account in this simplified technique. The Standard precipitation and evapotranspiration index (SPEI) fulfils the requirements of a drought index since its multi-scalar character enables it to be used by different scientific disciplines to detect, monitor, and analyze droughts.
Like the sc-PDSI and the SPI, the SPEI can measure drought severity according to its intensity and duration, and can identify the onset and end of drought episodes. The SPEI allows comparison of drought severity through time and space, since it can be calculated over a wide range of climates, as can the SPI. Moreover, Keyantash and Dracup (2002) indicated that drought indices must be statistically robust and easily calculated, and have a clear and comprehensible calculation procedure. All these requirements are met by the SPEI. However, a crucial advantage of the SPEI over other widely used drought indices that consider the effect of PET on drought severity is that its multi-scalar characteristics enable identification of different drought types and impacts in the context of global warming. The SPEI can account for the possible effects of temperature variability and temperature extremes beyond the context of global warming. Therefore, given the minor additional data requirements of the SPEI relative to the SPI, use of the former is preferable for the identification, analysis and monitoring of droughts in any climate region of the world, one of the nice advantages 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,1993).
| Value | SPEI Class |
| >= 2 | Extremely wet |
| 1.5 to 1.99 | Very wet |
| 1 to 1.49 | Moderately wet |
| -0.99 to 0.99 | Normal |
| -1 to -1.49 | Moderately dry |
| -1.5 to -1.99 | Very dry |
| <= -2 | Extremely dry |
