Drought

What it is

The Standardised Precipitation Evapotranspiration Index is published by the Spanish National Research Council (Consejo Superior de Investigaciones Científicas). It uses the monthly difference between precipitation and potential evapotranspiration, the water the air could draw from the ground and plants. This simple climatic water balance is calculated at different time scales. The global database has a 0.5 degree grid and monthly values from January 1901 to December 2024, with time scales from 1 to 48 months, and it covers land only. It is built on monthly climate data from the Climatic Research Unit at the University of East Anglia.

Why it matters

Because the index counts evaporation as well as rainfall, it responds to heat as well as to missing rain. The different time scales suit different parts of the water system, from soil moisture over a few months to rivers and groundwater over a year or more.

How to read it

Each square shows the 12-month value for December 2024, which covers January to December 2024. The value is a standard score for that place. Zero is a typical year, negative values are drier than usual and shown in brown, and positive values are wetter than usual and shown in teal. The darker the colour, the further from usual.

Limits

The current version of the database ends in December 2024. Only land with data is coloured, from about 56 degrees south to about 84 degrees north; ocean and the rest of the globe are light grey-blue. Evaporation demand is calculated from climate data. Each colour compares a place with its own history, so equal colours in a wet region and in a dry region mean different amounts of water.

Sources

• Spanish National Research Council: Data base, SPEI, The Standardised Precipitation-Evapotranspiration Index (SPEIbase)
• Spanish National Research Council: Information, SPEI, The Standardised Precipitation-Evapotranspiration Index
• Vicente-Serrano, S. M., Beguería, S. and López-Moreno, J. I. (2010): A Multiscalar Global Drought Dataset: The SPEIbase, American Meteorological Society, Bulletin of the American Meteorological Society
• Open Data Commons: Open Database License (ODbL) v1.0

License

Map: Fix Planet, from the global 0.5 degree drought index database of the Spanish National Research Council, 12-month scale, December 2024. The database is licensed under the Open Database Licence 1.0 and its contents under the Database Contents Licence. Boundaries: Natural Earth, public domain.

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