# Calculate Low-Flow Quantiles for given Return Periods

### Description

Fits an extreme value distribution using `L`-moments to the dry spells of a time series of discharges and subsequently estimates quantiles (the so called `T`-years event) for given return periods. In the presence of zero flow observations a mixed distribution is fitted.

### Usage

1 2 3 4 5 6 7 8 9 10 11 | ```
tyearsS(lfobj, event = 1/probs, probs = 0.01, pooling = NULL,
dist = "wei", check = TRUE, zeta = NULL,
plot = TRUE, col = 1, log = TRUE, legend = TRUE,
rp.axis = "bottom", rp.lab = "Return period", freq.axis = TRUE,
freq.lab = expression(paste("Frequency " *(italic(F)),
" = Non-Exceedance Probability P ",
(italic(X) <= italic(x)))),
xlab = expression("Reduced variate, " * -log(-log(italic(F)))),
ylab = "Quantile",
variable = c("volume", "duration"), aggr = "max",
hyearstart = hyear_start(lfobj), ...)
``` |

### Arguments

`lfobj` |
An object of class lfobj or an object which can be coerced to class xts. Either with a single column or with a column named 'discharge'. |

`event` |
numeric vector specifying the return periods. E.g. |

`probs` |
Alternate way to specify the return period of the event. |

`pooling` |
a pooling function, see |

`dist` |
A character vector of distributions to fit. Basically all distributions provided by Hosking's |

`check` |
logical, should |

`zeta` |
numeric vector of length one for manually setting a lower bound. Only a few distributions allow for a lower bound, namely |

`plot` |
logical. If |

`col` |
numeric or character vector of length one or as long as |

`log` |
logical. If |

`legend` |
logical, should a legend be added to the plot? |

`rp.axis` |
vector of length one, specifying if and how an additional scale bar for the return periods is drawn. Possible choices are |

`rp.lab` |
character vector, text above the scale bar for return periods |

`freq.axis` |
logical, should an additional abscissa showing the probabilities be drawn on top of the plot? |

`freq.lab` |
character vector, text above the probability axis |

`xlab` |
character vector, a label for the x axis |

`ylab` |
character vector, a label for the y axis |

`variable` |
character vector of length one. Either |

`aggr` |
function like |

`hyearstart` |
vector of length one, providing the start of the hydrological year. This is evaluated by |

`...` |
arguments passed on to |

### Details

This function is vectorised over `dist`

and `event`

.

According to paragraph 7.4.2 of the WMO manual, special care has to be taken in the presence of zero flow observations. A cdf called G(x) is fitted to the non-zero values of the original time series

If a distribution is fitted which allows for finite lower bound (`zeta`

), and `zeta`

is estimated being negative, estimation is repeated constraining `zeta = 0`

. If this behavior is not desired, the parameter `zeta`

has to be set explicitly.

### Value

An object of class evfit, see `evfit`

.

### Author(s)

Gregor Laaha

### References

Gustard, A. & Demuth, S. (2009) (Eds) Manual on Low-flow Estimation and Prediction. Operational Hydrology Report No. 50, WMO-No. 1029, 136p.

### See Also

There are methods for printing summarizing objects of class evfit.

`evfit`

### Examples

1 2 3 4 5 6 |