1. Define intended use
Irrigation, toilet flushing, washing and potable uses have different quality and regulatory implications. Confirm what is permitted and how rainwater must be separated from safe piped water.
2. Measure catchment area
Use the horizontal plan area that actually drains to the tank, not the sloping roof surface. Confirm gutters and downpipes can convey the relevant rainfall intensity.
3. Use local rainfall
Collect representative monthly or daily records and test dry, typical and wet years. Annual averages hide timing.
4. Account for losses
Runoff coefficient, first flush, gutter overflow, leakage and collection efficiency reduce gross area-times-rainfall yield. Keep each assumption visible.
5. Control debris and first flush
Screens and source controls reduce debris. Size and maintain any first-flush device to applicable guidance; diverted water reduces available yield.
6. Simulate storage
Step inflow, overflow and demand through time. Compare tank capacities using reliability, unmet demand, overflow and empty periods rather than yield alone.
7. Protect the tank
Provide safe covers, screened vents and overflows, mosquito control, access, drainage, foundations and maintenance. Prevent floodwater and contaminants entering.
8. Pump, treat and separate
Check required pressure and pipe losses. Treatment must address measured hazards and intended use; rainwater must not backflow into treated-water piping.
9. Operate and verify
Inspect roof and controls, clean screens and diverters, manage sediment and test water as required. Revisit demand and rainfall assumptions after operation.
Worked example
A 150 m² roof receiving 80 mm can intercept 12,000 L gross. After 0.9 runoff, 0.85 efficiency and 100 L fixed loss, about 9,080 L reaches storage—but tank overflow and later dry days require a time-step simulation.
Sources
- U.S. CDC — Collecting Rainwater and Your Health
- U.S. Department of Energy — Rainwater Harvesting Tool Help Guide
- Australian Government — YourHome Rainwater Guidance