#' Function to return the DH4 hydrologic indicator statistic for a given data frame
#'
#' This function accepts a data frame that contains a column named "discharge" and
#' calculates the mean of the annual maximum 30-day moving average flow for the entire record
#'
#' @param qfiletempf data frame containing a "discharge" column containing daily flow values
#' @param pref string containing a "mean" or "median" preference
#' @return dh4 numeric containing the mean of the annual maximum 30-day moving average flow for the given data frame
#' @export
#' @examples
#' load_data<-paste(system.file(package="HITHATStats"),"/data/obs_data.csv",sep="")
#' qfiletempf<-read.csv(load_data)
#' dh4(qfiletempf)
dh4 <- function(qfiletempf, pref = "mean") {
day30mean <- rollmean(qfiletempf$discharge, 30, align = "right",
na.pad = TRUE)
day30rollingavg <- data.frame(qfiletempf, day30mean)
rollingavgs30day <- subset(day30rollingavg, day30rollingavg$day30mean !=
"NA")
max30daybyyear <- aggregate(rollingavgs30day$day30mean,
list(rollingavgs30day$wy_val), max, na.rm=TRUE)
if (pref == "median") {
dh4 <- median(max30daybyyear$x)
}
else {
dh4 <- mean(max30daybyyear$x)
}
return(dh4)
}
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