Calculations

Hi! I'm having some difficulties with the calculations, specially with the meaning of each variable in each equation. In equation 1, the 'P' (average rainfall event), should we consider the year average or the day average? The building is located at northeast region from Brazil, here it rains a lot during january to may, but the rest of the year it doesn't rain, so the day average isn't a real data. And in equation 2, what is the "rainfall event interval"? Is it the average duration of the rain or the interval between two different rain events? Thanks!