freqAnal: Frequency Analysis

In USGS-R/DVstats: Functions to manipulate daily-values data

Frequency Analysis

Description

Compute an extreme-value frequency analysis. Three distributions are fit: log-Pearson type III, 3-parameter log-normal, and the log-generalized extreme value. The analyses are used primarily for streamflow data.

Usage

freqAnal(x, id, desc = "", STAID = "Unknown", method = "Auto")

Arguments

x	the observed data. Missing values are removed before computing the descriptive statistics. See Details.
id	an optional identifier for each value in x. If supplied, must match length of x.
desc	a description of the statistic.
STAID	the station identifier
method	a character string indicating the method to use for computing the final estimates. See Details.

Details

Zero values are removed from x and the statistics are computed on the conditional adjustment for the zero values. If there are fewer than six non-zero values or all non-zero values are co-equal, then no distributions are fit and the empirical distribution is used. Any value in x less than zero generates an error.

The choice for method must be one of "Auto," "lp3," "ln3," "lgev," or "PPCC" only the first two characters are required. The methods "lp3," "ln3," and "lgev" select the specific frequency analysis for the method to use. The method "PPCC" selects the maximum PPCC to determine the method. The default method, "Auto," Preferentially selects the log-Pearson type III method if the PPCC of that method is greater than 0.985 and if that fails, selects the 3-parametter log-normal if the PPCC of that method is greater than 0.985 and if that fails then the maximum PPCC is used to select the method.

Value

An object of class "freqAnal" that can be used to compute specific estimates.

Note

The 3-parameter lognormal and log-generalized extreme value will occasionally fail to fit the observed data and no estimate can be made. If either of these are selected by the user, then the selected method will default to the log-Pearson type III method.

The log-Pearson type III method will occasionally fail to fit the observed data, but an estimate can be made. No warning is created, but the printed report indicates that observed values exceed the range of the distribution.

The 3-parameter lognormal can estimate values less than zero if lambda is less than zero. No warning is created, but the printed report will include a note if estimated values correponding to observed values are less than 0. Any predicted values less than zero are converted to zero.

Examples

## Not run: 
library(USGSwsData)
data(ConecuhFlows)
with(ConecuhFlows, freqAnal(Flow, Year, desc="Annual Flows", 
 STAID="02371500"))

## End(Not run)

USGS-R/DVstats documentation built on Oct. 11, 2022, 6:03 a.m.