An R package incorporating a set of functions for computing potential or reference evapotranspiration and several widely used drought indices, currently including the Standardized Precipitation-Evapotranspiration Index (SPEI) and the Standardized Precipitation Index (SPE).

The package is centered on the SPEI. For more information on this drought index, please visit spei.csic.es.


Functions <spei> and <spi> are the workhorse of the SPEI library. Other functions such as <kern>, <cdfglo> or <pglo> are auxiliary low-level functions and they will not be used directly by the typical user. Functions for computing potential evapotranspiration are provided, too, for helping computing the SPEI. These are: <thornthwaite>, <hargreaves> and <penman>.


Install the latest stable development version from GitHub:

install_github('sbegueria', 'SPEI')

Or get it from the CRAN repository:


Please, note that the CRAN version may not be the very latest version of the package.


You can cite this references if you use the SPEI library on your work:

Other (possibly useful) references:

Version history

Version 1.7, November 2016 (current).

  1. Corrected a bug in the function which resulted in a multiplicative kernel instead of an additive one such the one expected in the and functions.
  2. Some small corrections to the plotting function and to the examples.

Version 1.6, September 2013.

  1. Corrected an error in the function which resulted in wrong potential evapotranspiration estimates when a multivariate time series was used as input.
  2. Corrected an error in the function which resulted in wrong handling of zero precipitation months when using the Gamma or PearsonIII distribution.
  3. Minor fixes to the and functions to correctly handle objects when they result from a call to .
  4. Modification to the function, which now yields kernel coefficients averaging one.
  5. Corrected an error in the functions and which resulted in ub-pwm method being used irrespective of the value of the parameter used, when using the ‘Gamma’ or ‘PearsonIII’ distributions.
  6. Minor changes to the documentation.

Version 1.5, May 2013.

  1. Optimization of function , now using embed() for accumulating the data at the desired time scale.

Version 1.4, May 2013.

  1. Minor fixes to functions and .
  2. Documentation of the penman function defined by mistake ed as the saturation vapour pressure, while it should read ‘actual vapour pressure’.
  3. Function zzz.R added to display basic information about the SPEI package at startup.
  4. Function added to display the NEWS file.

Version 1.3, March 2013.

  1. Minor fixes to functions and .
  2. Added new option for user-supplied SPEI parameters in the function. This overrides the fitting of a probability function to the data.
  3. Added new dataset from Allen et al. (1998).

Version 1.2, October 2012.

  1. Fixed a bug causing several functions to fail when a time series not belonging to matrix class was provided.
  2. Function now distinguises between calls to spei and spi and labels the axis accordingly.

Version 1.1, March 2012.

  1. Functions and now yield an object of class “spei”.
  2. New functions for summarizing and plotting “spei” objects are provided.
  3. An option to establish a reference period for the computation of the indices has been implemented in functions and .

Version 1.0, January 2012.

First release of the SPEI package.

To do list (work in progress)

Any problems?

Feel free to write an issue if you have any questions or problems.

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version.