high.spells: Calculates high flow spell statistics

Description Usage Arguments Value Author(s) Examples

Description

Calculates a suite of statistics describing flood characteristics, such as the timing, frequency and duration of events. The event threshold can be defined as a flow quantile (e.g. upper 90th percentile [default]) or a specific threshold volume (e.g. ML/day).

For the purpose of deriving annual flood statistics, the function can also be applied based on the hydrologic year. This is advisable where the high flow season spans years, such that prolonged spells may span years. Setting the parameter hydro.year=TRUE uses the hydro.year function to determine the appropriate hydrologic year for each record, which is then used for deriving annual spell characteristics.

It is possible for there to be multiple days with the same annual maximum flow value (although less likely than for low flows). In estimating the average timing (and sd of timing) of minimum flows, the function calculates the average day of year (DOY) of minimum flows in each year first, before calculating the average across years. Circular functions are used to address the proximity between days toward the beginning and end of the year.

Missing values are allowed for convenience (NA's are removed and the time-series is concatenated before functions are applied), but may lead to biased results. For the purpose of the annual statistics years with fewer than 350 days of available record are ignored.

When used with ddply to compute outputs for multiple gauges or time periods simultaneously, results, icluding graphs are produced for each factor level, including graphs. Note the funtion will return warnings if annual stats are calculated when year is used as a factor.

Usage

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high.spells(flow.ts, quant = 0.9, threshold = NULL,
ind.days = 5, duration = TRUE, volume = TRUE, plot = TRUE, ignore.zeros = FALSE,
ctf.threshold = 0.1, ann.stats = TRUE, ann.stats.only = FALSE, inter.flood = FALSE, 
hydro.year=FALSE)

Arguments

flow.ts

Dataframe with date and discharge data in columns named "Date" and "Q" respectively. Date must be in POSIX format (see ts.format).

If a third column exists then this is assumed to provide a vector of years for the purpose of calculating annual spell statistics based on a predetermined hydrologic year.

quant

Percentile/quantile to use for defining event magnitude (default 0.9). A value of 0.9 is the upper 90th percentile (i.e. a volume exceeded 10% of the time), and corresponds to Q90.

threshold

A user supplied threshold for defining spells. This would typically be derived from hydraulic models or similar knowledge pertaining to a gauge site.

ind.days

Periods between spells of less than ind.days (default 5) are considered to be 'in spell' for the purpose of further calculations. A value of 0 means spells 1 day apart are considered indpedendent.

duration

logical. Should statistics describing spell duration be returned?

volume

logical. Should statistics describing spell volumes be returned? Note that for days considered 'in-spell', the returned values have the threshold volume subtracted first, and hence reflect the amount of water that was flowing past the gauge above threshold. This is most useful in water planning scenarios.

plot

logical. Should the time-series be plotted? Data points considered 'within spell' are identifed using red circles and the threshold is identified with a horizontal line.

ignore.zeros

logical. If TRUE, days below a user defined cease-to-flow threshold (default 0.1) will be excluded when estimating the spell threshold for a given percentile. This is primarily of interest in highly ephemeral rivers, where flow may only occur for a small fraction of the time. In such cases, the inclusion of zeros will skew estimates of high flow events downwards, which may be undesirable.

ctf.threshold

values below this threshold are treated as zero for the purpose of percentile based calculations (see ignore zeros).

ann.stats

logical. If TRUE, the function returns results describing the annual maximum series (i.e. that describing the characteristics of the largest flood event in each year of the time-series). The duration of each annual high.spell is defined as the number of days above the smallest annual maximum for the largest (and longest) high.spell event in each year.

ann.stats.only

logical. If TRUE, statistics describing the annual series only are returned.

inter.flood

logical. If TRUE, statistics describing inter-flood spell characteristics are reported.

hydro.year

logical. If TRUE, each record is first assigned to a hydrologic year based on the timing of minimum flows. See hydro.year for further details.

Value

A dataframe with the following columns.

flood indices

high.spell.threshold

The high spell threshold applied in the analysis)

n.events

The number of events in the series greater than or equal to the high.spell.threshold

spell.freq

The frequency of spell events (no. per year)

ari

Average Recurrence Interval of events in years (1/spell.freq)

min.high.spell.dur

Minimum duration of spell events

avg.high.spell.dur

Average duration of spell events

med.high.spell.dur

Median duration of spell events

max.high.spell.dur

Maximum duration of spell events

avg.spell.volume

Average spell volume (volumes above the threshold only)

avg.spell.peak

Average spell peak

sd.spell.peak

Standard deviation of spell speaks

avg.rise

Average absolute rate of daily rise during spell events

avg.fall

Average absolute rate of daily fall during spell events

interflood indices

average.interval

The average time between spells (years)

min.interval

The mininum time between spells (years)

max.interval

The maximum time between spells (years)

Annual flood statistics

avg.max.ann

The average annual maximum flow

cv.max.ann

The coefficient of variation of annual maximum flows

flood.skewness

The average annual maximum / mean daily flow

ann.max.timing

The average day of the year (0-366) on which maximum flows occur

ann.max.timing.sd

circular standard deviation of the average timing of annual maximum flows

ann.max.min.dur

Minimum duration of the annual maximum spells (always equal to 1)

ann.max.avg.dur

Average duration of the annual maximum spells

ann.max.max.dur

Maximum duration of the annual maximum spells

ann.max.cv.dur

The coefficient of variation of the duration of annual maximum spells

Author(s)

Nick Bond <n.bond@latrobe.edu.au>

Examples

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data(Cooper)
Cooper<-ts.format(Cooper)

high.spells(Cooper, quant=0.9)

high.spells(Cooper, quant=0.9, ann.stats=FALSE, plot=FALSE)

high.spells(Cooper, quant=0.9, ann.stats=FALSE, ignore.zeros=TRUE)

high.spells(Cooper, quant=0.9, ann.stats=FALSE, ignore.zeros=TRUE, hydro.year=TRUE)


require(plyr)
Cooper$year<-strftime(Cooper$Date, format="%Y")
ddply(Cooper, .(year), function(x) high.spells(x, ann.stats=FALSE))


Cooper$time.period <- ifelse(Cooper$year<1980,"pre_1980","post_1980")

ddply(Cooper, .(time.period), function(x) high.spells(x, ann.stats=FALSE))

hydrostats documentation built on May 6, 2019, 1:02 a.m.