To cite package 'Rothermel' in publications use:

  Vacchiano G, Ascoli D (2015). “An implementation of the Rothermel
  fire spread model in the R programming language.” _Fire Technology_,
  *51*, 523-535. doi:10.1007/s10694-014-0405-6
  <https://doi.org/10.1007/s10694-014-0405-6>. R package version 1.4.1.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {An implementation of the Rothermel fire spread model in the R programming language},
    author = {Giorgio Vacchiano and Davide Ascoli},
    journal = {Fire Technology},
    volume = {51},
    pages = {523-535},
    year = {2015},
    doi = {10.1007/s10694-014-0405-6},
    note = {R package version 1.4.1},
  }