Team Organic Mandya ·
Rooftop Rainwater Harvesting for Farms: Collection and Storage Guide
A standard farm building with a 100 sq m tiled or sheet-metal roof harvests 60,000–80,000 litres of rainwater per year in a district like Mandya (700mm annual rainfall) — enough to supply drip irrigation for 10–15 raised beds through one dry month without touching the borewell. Rooftop rainwater is generally cleaner than pond or check dam water (no silt, no algae, low EC) and can be used directly for crop irrigation or directed into the borewell for aquifer recharge. The system is simple: gutters + first-flush diverter + storage tank + outlet pipe. Total cost for a 200 sq m farm building: ₹8,000–20,000, recovering its cost in the first season by reducing borewell pumping.
60,000–80,000 L
Annual harvest from 100 sq m roof in a 700mm rainfall area — enough for 1–2 dry months of drip irrigation
First-flush diverter
Critical component — discards the first 25–50 litres of contaminated roof runoff before clean water enters the tank
₹8,000–20,000
Total system cost including gutters, tank, and filtration for a 100–200 sq m farm building
0.8 × rainfall × roof area
Formula for harvestable volume (in litres) = 0.8 × annual rainfall mm × roof area sq m
How Do You Calculate Harvestable Volume?
Formula: Harvestable volume (litres/year) = Rainfall (mm) × Roof area (sq m) × Runoff coefficient
Runoff coefficients by roof type:
- Tiled roof: 0.80–0.85
- Corrugated metal sheet (GI/colour-coated): 0.85–0.90
- Concrete flat roof: 0.75–0.80
- Thatch: 0.60–0.70
Examples for Mandya district (700mm annual rainfall):
| Roof Size | Roof Type | Harvestable Volume |
|---|---|---|
| 50 sq m | Metal sheet | 50 × 700 × 0.87 = 30,450 litres |
| 100 sq m | Tiled | 100 × 700 × 0.82 = 57,400 litres |
| 200 sq m | Metal sheet | 200 × 700 × 0.87 = 1,21,800 litres |
| 300 sq m (farm house + shed) | Mixed | 300 × 700 × 0.82 = 1,72,200 litres |
What Are the Components of a Rooftop Harvesting System?
1. Gutters and downpipes:
- PVC or GI half-round gutters (4–6 inch diameter) attached to roof edge
- Slope: minimum 1cm per metre fall toward the downpipe collection point
- Downpipe: 3–4 inch PVC; connects gutter to first-flush diverter
- Cost: ₹80–150 per running metre installed
2. First-flush diverter (mandatory):
- Vertical PVC pipe (same diameter as downpipe) 2–3 metres long, capped at bottom with a slow-drain pinhole
- First 25–50 litres of runoff (contaminated with bird droppings, dust, leaves from roof) flows down into this pipe and is discarded
- Once first-flush pipe fills, cleaner water overflows through a bend into the main collection pipe
- Essential: roof water without first-flush diversion contains high bacteria counts and organic contamination
- Cost: ₹500–1,500 in PVC fittings
3. Pre-tank filter:
- Simple 3-chamber gravel + sand + charcoal filter (same as grey water filter)
- Or a commercially available cartridge filter housing (₹800–2,000)
- Removes suspended particles before water enters the tank
4. Storage tank:
- Concrete sump (underground): best quality water; cool storage; long life; cost ₹15,000–40,000 for 10,000 litres
- HDPE overhead tank: ₹5,000–12,000 for 5,000 litres; provides gravity pressure for irrigation without pump
- Plastic underground tank: ₹8,000–20,000 for 10,000 litres; easy to install, no construction required
- Brick-and-lime traditional tank: traditional option; low cost if local mason is available
Get organic seeds, bio-inputs & farm supplies from our shop — trusted by 12,000+ farmers.
Visit Our Shop →How Can Rooftop Water Be Used for Borewell Recharge?
Rather than storing all rooftop water in a tank, part or all of it can be directed into the borewell for direct aquifer recharge:
Borewell recharge system:
- After the first-flush diverter, route filtered water to a 1.5m deep percolation pit (gravel-filled) adjacent to the borewell
- Alternatively: run a 2-inch PVC pipe directly into the borewell casing — water enters 1–2 metres into the casing; fine filter is critical to prevent silt entering the aquifer
- Install a ball valve between the collection point and the recharge outlet — so you can direct water to storage OR recharge as needed
When to recharge vs store:
- Monsoon (June–September): recharge the borewell; you have ample surface water for crops from rainfall
- October–January: split — some to tank for dry-month buffer, some to borewell
- February–May: use stored tank water for crops; redirect any late-season rain to borewell recharge
What Are the Water Quality Considerations for Crop Use?
| Use Case | Treatment Needed | Safe Without Treatment? |
|---|---|---|
| Drip irrigation (root zone) | First-flush diversion + basic filter | Yes, with first-flush diverter |
| Foliar spray on vegetables | First-flush diversion + sediment filter + UV or chlorination if very concerned | Avoid without good filtration — bacterial contamination possible |
| Domestic drinking water | First-flush diversion + multi-stage filter + UV purification | No — must be properly treated before drinking |
| Borewell recharge | First-flush diversion + gravel-sand filter before entering aquifer | Yes, with pre-filtration — aquifer acts as natural filter |
| Drip fertigation (with Jeevamrutha) | First-flush + sediment filter; Jeevamrutha adds beneficial microbes anyway | Yes — compatible with organic fermented inputs |
Size Your Tank for 2 Dry Months, Not Just One Monsoon
The mistake most farmers make in sizing a rooftop harvesting tank is calculating harvest capacity (how much rain the roof collects) and making the tank the same size. The right approach is to calculate your peak dry-season daily irrigation need and size the tank to cover 45–60 days of that need without any rain. If your farm needs 5,000 litres per day in April–May, you need a 2,25,000–3,00,000 litre tank — or a combination of rooftop tank (for domestic and supplemental) plus farm pond (for the bulk irrigation buffer). A 5,000-litre tank from a 200 sq m roof sounds impressive but covers only one day of irrigation. Plan for months, not days.
Ready to start your organic farming journey?
Get everything you need from our store — seeds, bio-inputs, and farm tools.
Shop Organic Mandya →Last updated: March 2026
Organic Mandya Training
Earn ₹1 Lakh/Month on 1 Acre — Live Online Workshop
Related Guides
Last updated: March 2026