ADO Factsheet: Standardized Precipitation-Evapotranspiration Index (SPEI)

The Standardised Precipitation-Evapotranspiration Index (SPEI) is a multiscalar drought index based on precipitation and potential evapotranspiration. It can be used for determining the onset, duration and magnitude of drought conditions with respect to normal conditions in a variety of natural and managed ecosystems such as agricultural ecosystems, forests, rivers, etc. The SPEI shows the anomalies (deviations from the long-term average) of the observed total surface water balance for any given location and accumulation period of interest. The name of the index is usually modified to include the accumulation period. SPEI-3, for example, refers to accumulation period of three months (Vicente-Serrano and Beguería, 2022).