The Global SPEI database, SPEIbase, offers long-time, robust information on the drought conditions at the global scale, with a 0.5 degrees spatial resolution and a monthly time resolution. It has a multi-scale character, providing SPEI time-scales between 1 and 48 months. The Standardized Precipitatin-Evapotranspiration Index (SPEI) expresses, as a standardized variate (mean zero and unit variance), the deviations of the current climatic balance (precipitation minus evapotranspiration potential) with respect to the long-term balance. The reference period for the calculation, in the SPEIbase, corresponds to the whole study period. Being a standardized variate means that the SPEI condition can be compared across space and time. Calculation of the evapotranspiration potential in SPEIbase is based on the FAO-56 Penman-Monteith method. Data type: float; units: z-values (standard deviations). No land pixels are assigned a value of 1.0x10^30. In some rare cases it was not possible to achieve a good fit to the log-logistic distribution, resulting in a NAN (not a number) value in the database. Dimensions of the dataset: lon = 720; lat = 360; time = 1356. Resolution of the dataset: lon = 0.5º; lat = 0.5º; time = 1 month. Created in R using the SPEI package (http://cran.r-project.org/web/packages/SPEI).

Global gridded dataset of the Standardized Precipitation-Evapotranspiration Index (SPEI) at time scales between 1 and 48 months. Spatial resolution of 0.5º lat/lon. Temporal coverage between January 1901 and December 2011. This is an update of a previous version of SPEIbase v2 (http://hdl.handle.net/10261/48169). What’s new in version 2.2: 1) Data has been extended to the period 1901-2011 (it was 1901-2009 in v 2.0). 2) The FAO grass reference evapotranspiration from the CRU TS3.2 dataset has been used as PET input for the SPEI (http://badc.nerc.ac.uk/browse/badc/cru/data/cru_ts_3.20). Precipitation is also obtained from the same CRU TS3.2 dataset. 3) Unbiased probability weighted moments (ub-pwm) method has been used for fitting the log-Logistic distribution, instead of the sub-optimal plotting-position pwm method used in version 1.0. For more details on the SPEI visit http://sac.csic.es/spei.

The Global 0.5° gridded SPEI dataset is made available under the Open Database License. Any rights in individual contents of the database are licensed under the Database Contents License. Users of the dataset are free to share, create and adapt under the conditions of attribution and share-alike.

Use of the newest version is recommended. Older versions are still available to allow replicability.