R/mc_adjust_transition.R

In HydrosystemsGroup/Weather-Generator: Daily weather generator for performing a climate stress test.

#' Adjust Markov Chain Transition Matrix
#'
#' @param transition markov transition matrix
#' @param dry_spell change factor for dry spell (default=1, no change)
#' @param wet_spell change factor for wet spell (default=1, no change)
#' @export
#' @examples
#' transitions <- mc_fit(x=sample(c('d', 'w', 'e'), size=720, replace=TRUE, prob=c(0.5, 0.3, 0.2)), months=rep(rep(seq(1, 12), each=30), times=2))
#' mc_adjust_transition(transition[[1]], dry_spell=1, wet_spell=1)
#'
mc_adjust_transition <- function(transition, dry_spell=1, wet_spell=1) {
  states <- rownames(transition)

  p <- transition
  p_new <- p

  # adjust dry spell
  p_dw <- (p[states[1], states[2]] + p[states[1], states[3]])/dry_spell - p[states[1], states[3]]
  p_dd <- p[states[1], states[1]] + (p[states[1], states[2]] - p_dw)
  p_new[states[1],states[2]] <- p_dw
  p_new[states[1],states[1]] <- p_dd

  #adjust wet spell
  p_wd <- (p[states[2], states[1]] + p[states[2], states[3]])/wet_spell - p[states[2], states[3]]
  p_ww <- p[states[2], states[2]] + (p[states[2], states[1]] - p_wd)
  p_new[states[2],states[1]] <- p_wd
  p_new[states[2],states[2]] <- p_ww

  stopifnot(all(p_new > 0))

  p_new
}