1:3.2.1 Monthly Maximum Half-Hour Rain
For each month, users provide the maximum half-hour rain observed over the entire period of record. These extreme values are used to calculate representative monthly maximum half-hour rainfall fractions.
Prior to calculating the representative maximum half-hour rainfall fraction for each month, the extreme half-hour rainfall values are smoothed by calculating three month average values:
1:3.2.1
where is the smoothed maximum half-hour rainfall for a given month (mm HO) and is the extreme maximum half-hour rainfall for the specified month (mm HO). Once the smoothed maximum half-hour rainfall is known, the representative half-hour rainfall fraction is calculated using the equation:
1:3.2.2
where is the average half-hour rainfall fraction for the month, is an adjustment factor, is the smoothed half-hour rainfall amount for the month (mm HO), is the mean daily rainfall (mm HO) for the month, is the number of years of rainfall data used to obtain values for monthly extreme half-hour rainfalls, and are the number of wet days in the month. The adjustment factor is included to allow users to modify estimations of half-hour rainfall fractions and peak flow rates for runoff.
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