In the calculation section of the guide it says that we need to have the rainfall event interval to calculate the minimum drawdown rate.To what is this event interval refers to? is it the maximum gap within a year or a average gap interval within a year which can be determined by the rainfall data.(we have annual rainfall data).

Further i am unclear of the fact that we need to have the system to address 15% of both AVERAGE YEAR and 2-YEAR,24-HOUR DESIGN STORM..
how to we cater to both of that requirements at once??

please post your ideas..