To cite npphen in publications use:

Chávez RO, Estay SA, Riquelme CG (2017). npphen: An R package for estimating annual phenological cycle.

Corresponding BibTeX entry:

  @Manual{,
    title = {{npphen}: An R package for estimating annual phenological
      cycle},
    author = {Roberto O. Chávez and Sergio A. Estay and Carlos G.
      Riquelme},
    year = {2017},
  }