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For every 1mm of rainfall on 1 square metre of roof, you can collect 1 litre of rainwater, so you will want to ensure that you have connected as much roof area as is practical to your tank.
If you experience seasonal rainfall or your peak rainfall and water use times differ, you’ll want bigger or multiple tanks to capture enough water for drier times.
Rain falls more frequently than most people realise. Even in places with very seasonal rainfall patterns, rain fall event frequency can be surprisingly regular. Consult your national or local weather services to determine the rainfall frequency for your area.
Not having enough rainwater can be caused by a number of factors. Surprisingly, low rainfall is often not the key culprit.
Instead, miscalculations (not connecting enough roof area, under-specifying your tank size, overestimating your predicted rainfall), changes in requirements (using your rainwater for more activities than originally planned), compromised water quality and system inefficiencies are often the main cause of volume shortages.
Inefficiencies around your Rain Harvesting system can reduce the volume of water you collect. These inefficiencies may include partially or fully blocked gutters and downpipes, leaking pipes or “water bounce” at your tank inlets, which occurs when fast flowing water splashes off your tank screen rather than going through into your tank.