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Standardized precipitation evapotranspiration index (SPEI)

CL Changhong Liu
CY Cuiping Yang
QY Qi Yang
JW Jiao Wang
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Standardized precipitation evapotranspiration index (SPEI) replaces the monthly rainfall in SPI with the difference between monthly rainfall and monthly potential evapotranspiration, and takes into account the temperature factor, and introduces the influence of surface evaporation changes, which is more sensitive to the drought reaction caused by global temperature rise.

In order to estimate the value of SPEI, the difference of the water balance is normalized as log-logistic probability distribution. The following equation expresses the probability density function:

where parameters α, β, and γ represent scale, shape and origin, respectively. Therefore, the probability distribution function can be expressed as:

Vicente–Serrano25 calculated the SPEI as follow:

When P ≤ 0.5, W=-2ln(P), and when P > 0.5, W=-2ln(1-P), C0 = 2.5155,C1 = 0.8028, C2 = 0.0203, d1 = 1.4327, d2 = 0.1892, d3 = 0.0013. The categorization of drought classified by the SPI and SPEI is show in Table ​Table11.

Climatic moisture categories for the SPI and SPEI2.

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