#' Function to return the DL16 and DL17 hydrologic indicator statistics for a given data frame
#'
#' This function accepts a data frame that contains a column named "discharge" and
#' calculates the variability and median average pulse duration for each year of flow below the 25th percentile for the entire record
#'
#' @param qfiletempf data frame containing a "discharge" column containing daily flow values
#' @return dl16.17 list containing the median duration of low pulse flow and variability for the given data frame
#' @export
#' @examples
#' load_data<-paste(system.file(package="HITHATStats"),"/data/obs_data.csv",sep="")
#' qfiletempf<-read.csv(load_data)
#' dl16.17(qfiletempf)
dl16.17 <- function(qfiletempf) {
isolateq <- qfiletempf$discharge
sortq <- sort(isolateq)
frank <- floor(findrank(length(sortq), 0.75))
lfcrit <- sortq[frank]
noyears <- aggregate(qfiletempf$discharge, list(qfiletempf$wy_val),
FUN = median, na.rm=TRUE)
colnames(noyears) <- c("Year", "momax")
noyrs <- length(noyears$Year)
dur <- data.frame(Year = rep(0,nrow(qfiletempf)), dur = rep(1,nrow(qfiletempf)))
nevents <- 0
for (i in 1:noyrs) {
subsetyr <- subset(qfiletempf, as.numeric(qfiletempf$wy_val) == noyears$Year[i])
flag <- 0
for (j in 1:nrow(subsetyr)) {
if (subsetyr$discharge[j]<lfcrit) {
flag <- flag+1
nevents <- ifelse(flag==1,nevents+1,nevents)
dur$Year[nevents] <- subsetyr$wy_val[j]
dur$dur[nevents] <- dur$dur[nevents]+1
} else {flag <- 0}
}
}
subset_dur <- dur[1:nevents ,]
meanbyyr <- aggregate(subset_dur$dur, list(subset_dur$Year), mean)
dl16 <- median(meanbyyr$x)
dl17 <- sd(meanbyyr$x)*100/mean(meanbyyr$x)
dl16.17 <- list(dl16=dl16,dl17=dl17)
return(dl16.17)
}
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