R Create function to add water year column

Question

I want to be able to create a water year column for a time series. The US water year is from Oct-Sept and is considered the year it ends on. For example the 2014 water yea

Answer 1

I had a similar problem a while back but dealing with fiscal years that started in October. I found this function which also computes the quarters within the year. For one part, I only wanted it to output the fiscal year, so I edited a tiny part of the function to do that. There is surely a much cleaner/efficient way of doing it, but this should work for smaller data sets. Here is the edited function:

getYearQuarter <- function(x,
        firstMonth=7,
        fy.prefix='FY',
        quarter.prefix='Q',
        sep='-',
        level.range=c(min(x), max(x)) ) {
if(level.range[1] > min(x) | level.range[2] < max(x)) {
        warning(paste0('The range of x is greater than level.range. Values ',
            'outside level.range will be returned as NA.'))
}
quarterString <- function(d) {
        year <- as.integer(format(d, format='%Y'))
        month <- as.integer(format(d, format='%m'))
        y <- ifelse(firstMonth > 1 & month >= firstMonth, year+1, year)
        q <- cut( (month - firstMonth) %% 12, breaks=c(-Inf,2,5,8,Inf),
            labels=paste0(quarter.prefix, 1:4))
        return(paste0(fy.prefix, substring(y,3,4)))
}
vals <- quarterString(x)
levels <- unique(quarterString(seq(
        as.Date(format(level.range[1], '%Y-%m-01')),
        as.Date(format(level.range[2], '%Y-%m-28')), by='month')))
return(factor(vals, levels=levels, ordered=TRUE))
} 

Your input vector should be type Date, and then specify the start month. Assuming you have a data frame(df) with the 'date' column as in your question, this should do the trick.

df$wtr_yr <- getYearQuarter(df$date, firstMonth=10)

Answer 2

We can use POSIXlt to come up with an answer.

wtr_yr <- function(dates, start_month=9) {
  # Convert dates into POSIXlt
  dates.posix = as.POSIXlt(dates)
  # Year offset
  offset = ifelse(dates.posix$mon >= start_month - 1, 1, 0)
  # Water year
  adj.year = dates.posix$year + 1900 + offset
  # Return the water year
  adj.year
}

Let's now use this function in an example.

# Sample input vector
dates = c("2008-01-01 00:00:00",
"2008-02-01 00:00:00",
"2008-03-01 00:00:00",
"2008-04-01 00:00:00",
"2009-01-01 00:00:00",
"2009-02-01 00:00:00",
"2009-03-01 00:00:00",
"2009-04-01 00:00:00")

# Display the function output
wtr_yr(dates, 2)

# Combine the input and output vectors in a dataframe
df = data.frame(dates, wtr_yr=wtr_yr(dates, 2))