Drought

To calculate Drought Risk, we use the variable [Copernicus Climate Data Store (CDS)]:

• Standardized precipitation index for 6 months cumulation period (SPI6): monthly index that compares precipitation accumulated for 6 months with the precipitation distribution in the 1971-2010 reference period for the same geographical location and accumulation period. This indicator is widely used for drought event analysis.

The method described for Drought Risk assessment is a global standard and its primary source is the scientific literature that introduced and validated the Standardized Precipitation Index (SPI).

Our risk classification (from A to F) corresponds to standard drought categories defined by the index creators and adopted by major world meteorological organizations.

• The Relationship of Drought Frequency and Duration to Time Scales This is the publication that first introduced the SPI index in 1993, describing its calculation methodology and classification categories. The authors proposed the classification that associates specific index values with drought categories (moderate, severe, extreme), which have become the de facto global standard. Our scale from -0.5 to -2.5 is a direct application of their findings.
• Standardized Precipitation Index User Guide The World Meteorological Organization (WMO), a specialized agency of the United Nations, formally recommended SPI as the universal reference index for meteorological drought monitoring. This manual provides comprehensive and authoritative guidance on how to calculate and interpret SPI. It reaffirms and internationally standardizes the drought classification thresholds proposed by McKee et al., confirming the validity of your risk system. The document also explains the utility of different time scales (such as SPI-6 for medium-term agricultural and hydrological monitoring).

In summary, our Drought Risk classification method represents the direct application of the global standard defined by McKee et al. (1993) and recommended by the World Meteorological Organization. The provided sources are the cardinal references for this methodology.