R/swsi.R

In jmigueldelgado/hdi:

#' requires lubridate, dplyr
#' read more in http://climate.colostate.edu/pdfs/climo_rpt_91-3.pdf
#' @param df.hist dataframe of historical monthly reservoir volume, streamflow and precipitation of columns rs, sf, pr respectively. First column must be of class POSIXct
#' @param df.i dataframe with same structure of df.hist with values for the month for which we are calculating the index. Can have more than one row, ie more than one month
#' @param coeff is a dataframe of time (posix) and coefficients a, b and c for each month January to December (coefficients may vary along the seasons). 
#' @export
swsi <- function(df.hist,df.i,coeff)
{
    df.i$month <- month(df.i$posix)
    coeff$month <- month(coeff$posix)
    coeff <- coeff[,colnames(coeff)!="posix"]
    df.i <- left_join(df.i,coeff)
    
    ecdf.rs <- ecdf(df.hist$rs)
    ecdf.sf <- ecdf(df.hist$sf)
    ecdf.pr <- ecdf(df.hist$pr)
    swsi <- data.frame(posix=df.i$posix,swsi=100*(df.i$a*ecdf.rs(df.i$rs)+df.i$b*ecdf.sf(df.i$sf)+df.i$c*ecdf.pr(df.i$pr)-0.5)/12)
    return(swsi)
}