dflowR

A R package to find the design flow based on methodology used in U.S. EPA's DFLOW program with a modification to account for missing flow data or days with zero flow. If there are missing flow data in any particular water year, all data from that water year is not used. This approach is recommended by EPA (although not implemented in DFLOW 3.1) and applied in USGS's SWSTAT program.

The methodology is based on the equations contained in the DFLOW user manual as presented by Rossman (1999). Design flow is computed from the sample of lowest m-day average flow for each defined year, where "m" is the user supplied flow averaging period. The arithmetic averaging is used to calculate the m-day average flows. A log Pearson Type III probability distribution is fitted to the annual minimum m-day flows. The design flow is the value from the distribution whose probability of not being exceeded is 1/r where r is the user-supplied return period.

This function produces values consistent with DFLOW 3.1 (in cases without missing data), but there still may be areas of the code that need improvement. Use at your own risk.

References:

Rossman, L A. 1990. DFLOW USER'S MANUAL. U.S. Environmental Protection Agency, Washington, DC, EPA/600/8-90/051 (NTIS 90-225616) https://nepis.epa.gov/Exe/ZyPDF.cgi/30001JEH.PDF?Dockey=30001JEH.PDF

Install

devtools::install_github("OR-Dept-Environmental-Quality/dflowR"", 
                         host = "https://api.github.com", 
                         dependencies = TRUE, force = TRUE, upgrade = "never")

Usage

dflow(x, m, r, yearstart, yearend, wystart, wyend)

x = A data frame object where, col 1 = POSIXct date, col 2 = daily mean flow (numeric format)

m = Flow averaging period in days

r = Return period in years

yearstart = Optional. The starting year of the calculation period. Should be in integer format as yyyy. If not specified the default will be the year of the minimum date in x.

yearend = Optional. The ending year of the calculation period. Should be in integer format as yyyy. If not specified the default will be the year of the maximum date in x.

wystart = Optional. The water year beginning date (excluding year). The date should be in character format "mm-dd". If not specified the default is "10-01".

wyend = Optional. The water year ending date (excluding year). The date should be in character format "mm-dd". If not specified the default is "09-30".

If a particular season is desired, set wystart and wyend arguments to correspond to the beginning and ending dates of the season. The season does not have to span an entire year. Only flow records that fall within the season will be used in the calculations.

Example

See dflow_example.R for more examples.

library(dataRetrieval)
library(dflowr)

# -- Example using USGS text files included w/ DFLOW 3.1 download ----

q.df  <- read.table(file= "C:/Program Files (x86)/DFLOW 3.1b/usgs02072000.txt",
                    header = TRUE, sep = "\t", skip = 29, 
                    stringsAsFactors = FALSE )
q <- q.df[, c(3,4)]
colnames(q) <- c("date", "flow")
q$date <- as.POSIXct(q$date, format = "%m/%d/%Y")

dflow(x=q, m=7, r=10, yearstart=NA, yearend=NA, wystart="10-01", wyend="09-30")

# -- USGS web download example ----

# download flow data
q.df <- readNWISdv(siteNumbers = "14174000",
                   parameterCd = "00060",
                   startDate = "1970-10-01",
                   endDate = "2016-09-30",
                   statCd = "00003")

# Just get columns 3 and 4 (date and flow)
q <- q.df[, c(3,4)]
colnames(q) <- c("date", "flow")
q$date <- as.POSIXct(q$date, format = "%Y-%m-%d", tz = "UTC")

dflow(x = q, m = 7, r = 10, yearstart = NA, yearend = NA, 
wystart = "10-01", wyend = "09-30")