Intensity-Frequency-Duration Analysis
If rainfall data from a recording raingauge is available for long periods such as 25 years or more, the frequency of occurrence of a given intensity can also be determined. Then we obtain the intensity-frequency-duration relationships. Such relationships may be established for different parts of the year, e.g. a month, a season or the full year. The procedure to obtain such relationships for the year is described in this section. The method for parts of the year is similar.
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When the intensity-frequency-duration analysis is carried out for a number of locations in a region, the relationships may be given in the form of equation (3) with a different set of regression coefficients for each location. Alternatively, they may be presented in the form of maps (with each map depicting maximum rainfall depths for different combinations of one return period and one duration) which can be more conveniently used especially when one is dealing with large areas. Such maps are called isopluvial maps.
Annual maximum and annual exceedance series
In the procedure presented above annual maximum series of rainfall intensities were considered. For frequency analysis a distinction is to be made between annual maximum and annual exceedance series. The latter is derived from a partial duration series, which is defined as a series of data above a threshold. The maximum values between each upcrossing and the next downcrossing are considered in a partial duration series. The threshold should be taken high enough to make successive maximums serially independent or a time horizon is to be considered around the local maximum to eliminate lower maximums exceeding the threshold but which are within the time horizon. If the threshold is taken such that the number of values in the partial duration series becomes equal to the number of years selected then the partial duration series is called annual exceedance series.
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In HYMOS annual maximum series are used in the development of intensity-duration-frequency curves, which are fitted by a Gumbel distribution. Equation (4) is used to transform T into TE for T < 20 years. Results can either be presented for distinct values of T or of TE .
Data Requirements
Series codes
Series can be selected by clicking the series in the series codes list-box. Only one series may be selected at a time. Preferably the series should not contain missing values, missing values in the analysis period will be treated as zero values and can influence greatly the results.
Select intensity interval
The analysis timesteps must always be equal or greater than the selected series timestep. The maximum intensities are computed for the durations entered in this spreadsheet. The values entered in the first column are the replicators of the input timestep. The durations in the second column are computed by HYMOS. If the timestep of the input series is for example hour then a value 1 means that the first computed duration is one hour, a value 2 means a duration of 2 hours, etc..
Sub period
In the sub-period textboxes the start and end date of the sub-period can be entered. Computation of input data for the month of July requires a start date of "01-07" and an end date of "01-08". For the analysis of complete years the start date of the sub-period must be "01-01" and the end date of the sub-period must also be "01-01".
Return Periods
For each duration at distinct return periods the intensities intensity-duration curves will be made. The return periods can be entered in this spreadsheet.
Constant in plotting formula
The plotting of ordered data is done according to a general plotting position function:
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Constant 'a' is an input variable and is default set to 0.44, according to Gringorton. This is the best plotting position function for the Gumbel distribution.
Output Options
A selection can be made to make a computation of:
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