high.spells: Calculates high flow spell statistics

In hydrostats: Hydrologic Indices for Daily Time Series Data

Calculates high flow spell statistics

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

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 <[e-mail]>

Examples

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 June 1, 2022, 9:06 a.m.