Whereas water usage tends to be relatively constant from day to day, rainfall is very unevenly distributed over time. A rainwater harvest system must take into account yearly totals that range widely. Over the past 34 years, annual rainfall has been as much as 53.1" in 1992 and as little as 14.6" in 2008.
Likewise, rainfall amounts are variable from month to month in any given year. May - June and September - October tend to be the peak months, with considerably less during the rest of the year.
Another factor that must be taken into account is the interval between rainfalls which can be quite long even when the annual total is high.