To cite the 'hydroPSO' in publications use:

Zambrano-Bigiarini, M.; R. Rojas (2013), A model-independent Particle Swarm Optimisation software for model calibration, Environmental Modelling & Software, 43, 5-25, doi:10.1016/j.envsoft.2013.01.004
Zambrano-Bigiarini, M., Rojas, R.(2014). hydroPSO: Particle Swarm Optimisation, with focus on Environmental Models. R package version 0.3-4.

Corresponding BibTeX entries:

  @Article{Zambrano-BigiariniRojas2013-hydroPSO_article,
    title = {A model-independent Particle Swarm Optimisation software
      for model calibration},
    journal = {Environmental Modelling & Software},
    author = {{Zambrano-Bigiarini} and {M.} and {Rojas} and {R.}},
    volume = {43},
    pages = {5-25},
    year = {2013},
    url = {http://dx.doi.org/10.1016/j.envsoft.2013.01.004},
  }
  @Manual{Zambrano-BigiariniRojas-hydroPSO_pkg,
    title = {hydroPSO: Particle Swarm Optimisation, with focus on
      Environmental Models},
    author = {Mauricio Zambrano-Bigiarini and Rodrigo Rojas},
    year = {2014},
    note = {R package version 0.3-4 --- For new features, see the
      'NEWS' file (on CRAN, rforge or the package source)},
    url = {http://www.rforge.net/hydroPSO,
      http://cran.r-project.org/web/packages/hydroPSO},
  }