SpeI

Standardized Precipitation-Evapotranspiration Index (SPEI) is a drought index that measures moisture balance anomalies by combining precipitation and potential evapotranspiration (PET) over multiple time scales. It was introduced to extend the Standardized Precipitation Index (SPI) by explicitly incorporating atmospheric demand through PET, making it more responsive to heat and drought risk in changing climates.

Calculation and interpretation: For a chosen accumulation period (for example 1, 3, 6, or 12 months), researchers compute the climatic water balance as D_t = P_t − PET_t. The series of balances is aggregated to the selected time scale, then fitted to a suitable probability distribution. The distribution is transformed to a standard normal distribution, yielding SPEI values with a mean around zero and unit variance. Negative SPEI values indicate drier-than-average conditions (drought), while positive values indicate wetter-than-average conditions. The magnitude reflects drought intensity, and the sign indicates moisture surplus or deficit.

Features and applications: SPEI is multiscalar, allowing assessment of short-term to long-term droughts and wet spells. PET can be estimated by methods such as Thornthwaite or Penman-Monteith, with the choice affecting results. It is widely used in hydrological and climate studies for drought monitoring, impact assessment, water resources planning, and risk analysis, using data from meteorological stations or gridded climate datasets.

Limitations: Results depend on the PET method and the chosen probability distribution, and non-stationarity in climate can influence interpretation. SPEI is often used alongside other indices such as SPI and PDSI for a fuller drought picture.