- The Standardized Precipitation Index (SPI) indicates how unusual the amount of accumulated precipitation is, compared to the historical record over a given time scale.
- The Standardized Precipitation-Evapotranspiration Index (SPEI) indicates how unusual a combination of precipitation and evapotranspiration (the atmospheric demand for water) is, compared to the historical record over a given time scale.
- The Palmer Drought Severity Index represents drought severity, framed from the perspective of soil moisture or dryness. The Palmer metrics are computed based upon a location’s recent precipitation, recent atmospheric evaporative demands, and its soil characteristics.
- Coastal Salinity Index (CSI) uses salinity data to characterize saline (drought) and freshwater (wet) conditions in coastal surface waters.
For each of the three indices described above, negative scores indicate drier conditions, and positive scores indicate wetter conditions.