To cite package ‘SPEI’ in publications use:

Beguería S, Vicente-Serrano SM (2025). SPEI: Calculation of the Standardized Precipitation-Evapotranspiration Index. R package version 1.8.1, https://github.com/sbegueria/spei.

Corresponding BibTeX entry:

  @Manual{,
    title = {SPEI: Calculation of the Standardized
      Precipitation-Evapotranspiration Index},
    author = {Santiago Beguería and Sergio M. Vicente-Serrano},
    year = {2025},
    note = {R package version 1.8.1},
    url = {https://github.com/sbegueria/spei},
  }