How to Calculate Rainwater Harvest
What is Rainwater Harvest?
Rainwater harvesting collects precipitation for irrigation, toilet flushing, or emergency use. It reduces groundwater demand and stormwater runoff.
Formula
Calculate: Annual collection = Catchment area × Annual rainfall × Collection efficiency
Step-by-Step Guide
- 1Calculate: Annual collection = Catchment area × Annual rainfall × Collection efficiency
- 2Roof collection: 90% efficiency, pavement: 70%, storage needed for dry periods
- 31000 m² roof, 1 m rain: 700-900 m³ water available
Worked Examples
Input
Residential roof 100 m², annual rainfall 500 mm
Result
Annual collection ~45 m³ (45,000 L), covers 30-40% irrigation needs
Location dependent
Common Mistakes to Avoid
- ✕Under-sizing tank for dry season needs
- ✕Forgetting overflow management (can cause flooding)
Frequently Asked Questions
Is rainwater safe to drink?
No without treatment. Suitable for irrigation, toilet, washing after basic filtration.
How large must storage tank be?
Rule of thumb: volume for one dry month. Location's dry season determines sizing.
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