The SPI (Standardised Precipitation Index) transforms accumulated precipitation into a standardised value with mean 0 and standard deviation 1. Values of −1 to −1.5 indicate moderate drought; <−2, extreme drought. It is calculated for multiple scales: SPI-1 (monthly), SPI-3, SPI-6, SPI-12, SPI-24.
Advantages
Being standardised, it allows comparing droughts across regions with very different climates (Galicia vs. Almería). The WMO recommends it as the primary meteorological drought indicator. In Spain, AEMET publishes SPI monthly. A prolonged SPI-12 <−2 (several months) indicates a severe multi-year drought such as those in 2005, 2017, or 2023.