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

The package is centered on the SPEI. For more information on this drought index, please visit https://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 version from GitHub:

library(devtools)
install_github('sbegueria/SPEI')

Or get the latest version on CRAN:

install.packages('SPEI')

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

If you are interested in testing the current development version you can do so by installing the devel branch:

library(devtools)
install_github('sbegueria/SPEI@devel')

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

• S.M. Vicente-Serrano, S. Beguería, J.I. López-Moreno. 2010. A Multi-scalar drought index sensitive to global warming: The Standardized Precipitation Evapotranspiration Index – SPEI. Journal of Climate 23: 1696, DOI: 10.1175/2009JCLI2909.1.

• Beguería S, Vicente-Serrano SM, Reig F, Latorre B. 2014. Standardized precipitation evapotranspiration index (SPEI) revisited: parameter fitting, evapotranspiration models, tools, datasets and drought monitoring. International Journal of Climatology 34(10): 3001-3023.

Other (possibly useful) references:

• Vicente-Serrano, S.M., Beguería, S., López-Moreno, J.I., Angulo, M., El Kenawy, A. 2010. A new global 0.5° gridded dataset (1901-2006) of a multi-scalar drought index: comparison with current drought index datasets based on the Palmer Drought Severity Index. Journal of Hydrometeorology 11: 1033--1043.

• Beguería, S., Vicente-Serrano, S.M. y Angulo, M. 2010. A multi-scalar global drought data set: the SPEIbase: A new gridded product for the analysis of drought variability and impacts. Bulletin of the American Meteorological Society 91: 1351--1354.

• Vicente-Serrano, S.M., Beguería, S. and Juan I. López-Moreno. 2011. Comment on “Characteristics and trends in various forms of the Palmer Drought Severity Index (PDSI) during 1900-2008” by A. Dai. Journal of Geophysical Research-Atmosphere 116: D19112, doi:10.1029/2011JD016410.

• Vicente-Serrano, S.M., Santiago Beguería, Jorge Lorenzo-Lacruz, Jesús Julio Camarero, Juan I. López-Moreno, Cesar Azorín-Molina, Jesús Revuelto, Enrique Morán-Tejeda and Arturo Sánchez-Lorenzo. 2012. Performance of drought indices for ecological, agricultural and hydrological applications. Earth Interactions 16: 1--27.

• Vicente-Serrano, S.M., Célia Gouveia, Jesús Julio Camarero, Santiago Beguería, Ricardo Trigo, Juan I. López-Moreno, César Azorín-Molina, Edmond Pasho, Jorge Lorenzo-Lacruz, Jesús Revuelto, Enrique Morán-Tejeda and Arturo Sanchez-Lorenzo. 2012. The response of vegetation to drought time-scales across global land biomes. Proceedings of the National Academy of Sciences of the United States of America, doi: 10.1073/pnas.1207068110.

• Vicente-Serrano, S.M., Gerard Van der Schrier, Santiago Beguería, Cesar Azorín-Molina, Juan-I. Lopez-Moreno. 2015. Contribution of precipitation and reference evapotranspiration to drought indices under different climates. Journal of Hydrology 426: 42--54.

• Vicente-Serrano, S.M., Beguería, S. 2016. Comment on "Candidate Distributions for Climatological Drought Indices (SPI and SPEI)" by James H. Stagge et al. International Journal of Climatology 36: 2120--213.

1. Solving several minor bugs in <thornthwaite>, <hargreaves>, and <penman> functions (output difference is lower than 0.1% with respect to version 1.7).
2. Solving a bug in <spei> that resulted in bad cumulative data when using a non-rectangular kernel, resulting in incorrect SPEI values.
3. Implementation of more thorough data and options checks, and providing a single error message containing all the errors upon failure.
4. Implementation of verbosity in all functions: now they print information about the options being used. New argument verbosity added, defaulting to TRUE, to override this behavior.
5. All the functions now accept 3-d arrayed data, enabling the possibility of using the functions on gridded climate data.
6. Implementation of different versions of the Penman-Monteith ETo calculation in function <penman>.
7. Implementation of an option to include CO2 concentration data in function <penman>.
8. Implementation of a new option for when no wind data are available in function <penman>.
9. Functions <spei> and <spi> now admit time series of any frequency, and not only monthly (frequency 12) data.
10. Function <plot.spei> completely rewritten based on ggplot2, solving some bugs and enabling more flexibility.

1. Several code optimizations and improvements (by github user @doug-friedman).
2. Added formal unit testing with testthat (@doug-friedman).
3. Move documentation over to roxygen (@doug-friedman).
4. Update <spei> function to allow processing data with frequency different than 12 (by github user @MuDestructor).

1. Corrected an error in <spei> function, which was not working when distribution was Gamma or PeasonIII and using user provided parameters. (Fixed by Emanuele Cordano, [email protected] -- ecor)
2. Added probability of monthly precipitation = 0 (pze) when using user provided parameters. (Fixed by Emanuele Cordano, [email protected] -- ecor)

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

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

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

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

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

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

1. Functions <spei> and <spi> 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 <spei> and <spi>.

First release of the SPEI package.

Feel free to send 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.