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How It Works
Rainwater harvesting captures precipitation from a roof or paved surface and stores it for later use. The harvestable volume depends on three factors: catchment area, depth of rainfall, and collection efficiency (which accounts for losses from evaporation, splashing, and first-flush diversion).
Metal roofs typically achieve 80–95% efficiency, asphalt shingles 75–85%, and green roofs just 20–40%.
Example Problem
A metal roof has an area of 200 m², annual rainfall is 0.9 m, and the collection efficiency is 85%. How much water can be harvested per year?
- H = 200 × 0.9 × 85/100 = 200 × 0.9 × 0.85
- H = 153 m³ (153,000 liters)
That is enough to supply a household's non-potable needs (toilets, laundry, irrigation) for much of the year.
Frequently Asked Questions
How much rainwater can I collect from my roof?
A rough estimate is 1 mm of rain on 1 m² of roof yields about 1 liter. A 150 m² roof receiving 800 mm of annual rainfall at 85% efficiency can harvest roughly 102,000 liters per year.
What is collection efficiency in rainwater harvesting?
Collection efficiency is the fraction of rain that actually reaches your storage tank after accounting for evaporation, splash loss, gutter overflow, and first-flush diversion. It typically ranges from 75% to 95% depending on roof material and system design.
Can harvested rainwater be used for drinking?
Rainwater can be treated to potable standards with filtration and disinfection, but it is more commonly used for irrigation, toilet flushing, and laundry. Local regulations vary -- many jurisdictions require treatment before potable use.
Related Calculators
- Swimming Pool Calculator -- manage pool water volume and turnover rates.
- Solid Waste Calculator -- assess rainfall percolation through landfill cover soil.
- Permeameter Calculator -- measure soil permeability for drainage design.
- French Drain Calculator — design subsurface drainage for overflow management.
- Gutter Design Calculator — size gutters that feed the collection system.
- Volume Unit Converter — convert between gallons, liters, and cubic meters for tank sizing.