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The SPEI combines precipitation and temperature data in a water balance framework (see Vicente‐Serrano et al., 2010a,b). The SPEI used here was estimated based on a water balance approach as the difference between precipitation (P) and PET (potential evapotranspiration), i.e., δ = P‐PET. As detailed by Vicente‐Serrano et al. (2010b), the computed values of δ are cumulated on different time scales,

(5.1)

where k is the cumulated time scale and n is the calculation number. This cumulated time series are thereafter fitted with a log‐logistic probability distribution function. The SPEI drought characterization here follows the thresholds defined by McKee et al. (1993), in which a drought condition is assumed to occur when the SPEI is consistently negative and reaches a value of –1. On a 12‐month cumulation, this threshold supports hydrological drought characterization in the Congo Basin.