Rainwater Harvesting

Rainwater harvesting intercepts precipitation before it runs off, storing it for drinking, irrigation, and livestock. Even in moderately arid climates, a well-designed system can supply a family's full water needs.

Key Concepts

• —Catchment area calculation: roof area (in square meters) multiplied by annual rainfall (in meters) multiplied by a runoff coefficient of 0.75-0.85 gives usable yield in cubic meters per year.
• —First-flush diverter: the initial runoff from a roof carries the most contaminants (bird droppings, dust, debris); a first-flush device automatically diverts the first 20-25 liters before storage.
• —Storage sizing: a cistern should hold enough water to bridge the longest expected dry period; in seasonally dry climates, this often means 3-6 months of household storage.
• —Roof material quality: metal and tile roofs produce cleaner runoff than asphalt shingles; treated or painted roofs may leach chemicals and require first-flush diversion and settling.
• —Earthworks catchment: swales, berms, and retention ponds on slopes capture runoff and recharge groundwater, benefiting both wells and spring flows downslope.

Practical Guide

1. 1.Calculate your catchment yield before sizing storage: measure roof footprint area, multiply by local annual rainfall and 0.8 efficiency to get liters per year, then divide by 365 for daily average.
2. 2.Install gutters sloped at least 1 cm per meter toward a downspout; screen all entry points with fine mesh to exclude debris, insects, and small animals.
3. 3.Build or install a first-flush diverter using a vertical standpipe capped at a volume of roughly 1 liter per 10 square meters of roof area; it fills and seals before overflow enters the tank.
4. 4.Construct storage tanks from ferro-cement (5-7 layers of chicken wire plastered with cement mortar) if no commercial tanks are available; a 5,000-liter tank can be built by two people in a week.
5. 5.Cover all storage tanks completely to prevent mosquito breeding, algae growth, and evaporation; a tight-fitting lid reduces evaporative losses by more than 90% in hot climates.
6. 6.Route tank overflow to a garden bed or infiltration trench rather than allowing it to erode pathways or concentrate near foundations.
7. 7.Test stored rainwater annually for bacterial contamination; treat with 2 drops of 5% bleach per liter as a precaution if the tank cannot be inspected and cleaned.