Performs a volume frequency analysis on annual statistics from a daily streamflow data set and calculates a statistic based on the provided mean n-days and return period of the statistic, defaults to minimum flows. For example, to determine the 7Q10 of a data set, set the roll_days to 7 and the return_period to 10. Function will calculate using all values in 'Values' column (no grouped analysis), unless specified. Analysis methodology replicates that from HEC-SSP. Returns a tibble with statistics.

compute_frequency_quantile(
data,
dates = Date,
values = Value,
station_number,
roll_days = NA,
roll_align = "right",
return_period = NA,
use_max = FALSE,
use_log = FALSE,
fit_distr = c("PIII", "weibull"),
fit_distr_method = ifelse(fit_distr == "PIII", "MOM", "MLE"),
water_year_start = 1,
start_year,
end_year,
exclude_years,
months = 1:12,
ignore_missing = FALSE
)

## Arguments

data A data frame of data that contains columns of events, flow values, and measures (data type). Name of column in data that contains dates formatted YYYY-MM-DD. Only required if dates column name is not 'Date' (default). Leave blank if using station_number argument. Column in data that contains numeric flow values, in units of cubic metres per second. Default 'Value'. Character string vector of seven digit Water Survey of Canada station numbers (e.g. "08NM116") of which to extract daily streamflow data from a HYDAT database. Requires tidyhydat package and a HYDAT database. Leave blank if using data argument. Numeric value of the number of days to apply a rolling mean. Required. Character string identifying the direction of the rolling mean from the specified date, either by the first ('left'), last ('right'), or middle ('center') day of the rolling n-day group of observations. Default 'right'. Numeric vector of the estimated time interval, in years, between flow events of a similar size, inverse of probability, used to estimate the frequency statistic. Required. Logical value to indicate using maximums rather than the minimums for analysis. Default FALSE. Logical value to indicate log-scale transforming of flow data before analysis. Default FALSE. Character string identifying the distribution to fit annual data, one of 'PIII' (Log Pearson Type III) or 'weibull' (Weibull) distributions. Default 'PIII'. Character string identifying the method used to fit the distribution, one of 'MOM' (method of moments) or 'MLE' (maximum likelihood estimation). Selected as 'MOM' if fit_distr ='PIII' (default) or 'MLE' if fit_distr = 'weibull'. Numeric value indicating the month (1 through 12) of the start of water year for analysis. Default 1. Numeric value of the first year to consider for analysis. Leave blank to use the first year of the source data. Numeric value of the last year to consider for analysis. Leave blank to use the last year of the source data. Numeric vector of years to exclude from analysis. Leave blank to include all years. Numeric vector of months to include in analysis (e.g. 6:8 for Jun-Aug). Leave blank to summarize all months (default 1:12). Logical value indicating whether dates with missing values should be included in the calculation. If TRUE then a statistic will be calculated regardless of missing dates. If FALSE then only those statistics from time periods with no missing dates will be returned. Default FALSE.

## Value

A numeric value of the frequency analysis quantile, given the roll_days and return_period.

compute_frequency_analysis

## Examples

if (FALSE) {

# Working example:

# Compute the annual 7-day flow value with a 1 in 10 year return interval
compute_frequency_quantile(station_number = "08NM116",
roll_days = 7,
return_period = 10)

}