To allow for short periods of intense rain, the minimum available ‘empty’ storage volume should be close to the average daily rainwater harvest. The exact available empty volume required on any given day should take into account the accumulation of rainwater versus use. The volume Vol(n) which would be accumulated on any given day (n) in a limitless volume storage tank is the RWH of that day plus the sum of the previous days (n-1) collected volume minus the Rain Water Use (RWU) of that same day (n-1).