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The 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. 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 (Standardised Precipitation Index). Moreover, (1) 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 (potential evapotranspiration) 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 combines the sensitivity to changes in evaporation demand (caused by temperature fluctuations and trends) with the simplicity of calculation and the multi-temporal nature of the SPI.

Computation of the SPEI

The SPEI is really simple to calculate, and is based on the original SPI calculation procedure. The SPI is calculated using monthly (or weekly) precipitation as the input data. The SPEI uses the monthly (or weekly) difference between precipitation and PET. This represents a simple climatic water balance which is calculated at different time scales to obtain the SPEI.

Climatic water balance (precipitation minus evapotranspiration)

A number of equations exist to model PET based on available data (e.g. the Thornthwaite equation, the Penman-Monteith equation, the Hargreaves equation, etc), and the SPEI is not linked to any particular one.

In the original version of the SPEI it were used the Thornthwaite equation ((2) Thornthwaite, 1948), wich was applied to obtain the SPEIbase v1.0. In the SPEIbase v2.0 it were used the FAO-56 (3)Penman–Monteith equation (Allen et al. 1998. ).

With a value for PET, the difference between the precipitation (P) and PET for the month i is calculated:

which provides a simple measure of the water surplus or deficit for the analyzed month.

The calculated Di values are aggregated at different time scales, following the same procedure as for the SPI.

For further details and examples see About the SPEI

References

(1) Keyantash, J. and J. Dracup., 2002: The quantification of drought: an evaluation of drought indices. Bulletin of the American Meteorological Society 83, 1167-1180.2

(2) Thornthwaite, C.W., 1948: An approach toward a rational classification of climate. Geographical Review 38, 55-94.

(3) Allen RG, Pereira LS, Raes D, Smith M. (1998): Crop Evapotranspiration: Guidelines for Computing Crop Requirements, Irrigation and Drainage Paper 56. FAO: Roma, Italia.

• Vicente-Serrano S.M., Santiago Beguería, Juan I. López-Moreno, (2010) A Multi-scalar drought index sensitive to global warming: The Standardized Precipitation Evapotranspiration Index – SPEI. Journal of Climate 23: 1696-1718.
• Beguería, S., Vicente-Serrano, S.M. y Angulo, M., (2010): A multi-scalar global drought data set: the SPEIbase: A new gridded product for the analysis of drought variability and impacts. Bulletin of the American Meteorological Society. 91, 1351-1354
• Vicente-Serrano, S.M., Beguería, S., López-Moreno, J.I., Angulo, M., El Kenawy, A. (2010): A new global 0.5° gridded dataset (1901-2006) of a multiscalar drought index: comparison with current drought index datasets based on the Palmer Drought Severity Index. Journal of Hydrometeorology. 11: 1033-1043
• Vicente-Serrano, S.M., Beguería, S. and Juan I. López-Moreno (2011). Comment on “Characteristics and trends in various forms of the Palmer Drought Severity Index (PDSI) during 1900-2008” by A. Dai. Journal of Geophysical Research-Atmosphere. 116, D19112, doi:10.1029/2011JD016410
• Vicente-Serrano, S.M., Santiago Beguería, Jorge Lorenzo-Lacruz, Jesús Julio Camarero, Juan I. López-Moreno, Cesar Azorin-Molina, Jesús Revuelto, Enrique Morán-Tejeda and Arturo Sánchez-Lorenzo. (2012) Performance of drought índices for ecological, agricultural and hydrological applications. Earth Interactions 16, 1-27.
• Beguería, S., Vicente-Serrano, S.M., Fergus Reig, Borja Latorre. Standardized Precipitation Evapotranspiration Index (SPEI) revisited (2014): parameter fitting, evapotranspiration models, kernel weighting, tools, datasets and drought monitoring. International Journal of Climatology, 34: 3001-3023
• Vicente-Serrano, S.M., Gerard Van der Schrier, Santiago Beguería, Cesar Azorin-Molina, Juan-I. Lopez-Moreno (2015). Contribution of precipitation and reference evapotranspiration to drought indices under different climates. Journal of Hydrology 426: 42-54.
• Vicente-Serrano, S.M., Beguería, S., (2016) Comment on “Candidate Distributions for Climatological Drought Indices (SPI and SPEI)” by James H. Stagge et al. International Journal of Climatology.36: 2120-213

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