You're way overestimating the size needed. Even in drought years there is substantial precipitation, but generally it happens at the wrong time of year.
That flooding is made worse by hardware (meaning roofs, concrete, etc) that prevents infiltration and overwhelms surrounding areas.
It's often cheaper to oversize your catchment area and direct tank overflow to a nearby garden. Generally the available roof area isn't the limiting factor, but I encourage you to verify this by looking up annual rainfall numbers for areas you're interested in. Do you know how much rain falls on your roof each year?
You're way overestimating the size needed. Even in drought years there is substantial precipitation, but generally it happens at the wrong time of year.
Often you'll have flooding in the same year as drought! See this recent example: http://news.nationalgeographic.com/news/2013/09/130913-color...
That flooding is made worse by hardware (meaning roofs, concrete, etc) that prevents infiltration and overwhelms surrounding areas.
It's often cheaper to oversize your catchment area and direct tank overflow to a nearby garden. Generally the available roof area isn't the limiting factor, but I encourage you to verify this by looking up annual rainfall numbers for areas you're interested in. Do you know how much rain falls on your roof each year?