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:

R0.5sm(mon)=R0.5x(mon1)+R0.5x(mon)+R0.5x(mon+1)3R_{0.5sm(mon)}=\frac{R_{0.5x(mon-1)}+R_{0.5x(mon)}+R_{0.5x(mon+1)}}{3} 1:3.2.1

where R0.5sm(mon)R_{0.5sm(mon)} is the smoothed maximum half-hour rainfall for a given month (mm H2_2O) and R0.5xR_{0.5x} is the extreme maximum half-hour rainfall for the specified month (mm H2_2O). Once the smoothed maximum half-hour rainfall is known, the representative half-hour rainfall fraction is calculated using the equation:

α0.5mon=adj0.5α[1exp(R0.5sm(mon)μmonln(0.5yrsdayswet))]\alpha_{0.5mon}=adj_{0.5\alpha}*[1-exp(\frac{R_{0.5sm(mon)}}{{\mu_{mon}}*ln*(\frac{0.5}{yrs*days_{wet}})})] 1:3.2.2

where α0.5mon\alpha_{0.5mon} is the average half-hour rainfall fraction for the month, adj0.5αadj_{0.5\alpha} is an adjustment factor, R0.5smR_{0.5sm} is the smoothed half-hour rainfall amount for the month (mm H2_2O), μmon\mu_{mon} is the mean daily rainfall (mm H2_2O) for the month, yrsyrs is the number of years of rainfall data used to obtain values for monthly extreme half-hour rainfalls, and dayswetdays_{wet} 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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