Rainwater harvesting in India is no longer a green-building gesture — it is a CGWA mandate for every plot ≥ 100 m² in notified blocks (which now covers most of urban India). This article walks through the yield calculation, storage vs recharge decision, recharge-pit sizing per IS 6295, and IGBC + GRIHA point capture.
CGWA mandate scope
The Central Ground Water Authority’s 2020 notification requires rainwater harvesting for:
- All plots ≥ 100 m² in notified blocks (state-by-state list, growing)
- All commercial/institutional/industrial plots ≥ 300 m² regardless of block notification
- All plots that draw groundwater via tubewell/borewell (independent of plot size)
Penalty for non-compliance: revocation of NOC + up to ₹10 lakh fine.
States with the most aggressive enforcement: Rajasthan, Gujarat, Karnataka, Tamil Nadu, Maharashtra, Delhi NCR. Other states are catching up.
The basic yield equation
V_annual (kL) = Catchment area × Annual rainfall × Runoff coefficient × Filtration efficiency × 0.001
For a 1500 m² RCC roof in Mumbai:
V = 1500 × 2200 × 0.85 × 0.90 × 0.001 = 2,524 kL/year
That’s 2.5 million litres/year of recoverable water — enough to meet 350 days of a 7,000 L/day non-potable demand, or recharge enough to refill a 1,500 m² aquifer column in one monsoon.
Runoff coefficients per surface (NBC + IS 6295)
| Surface | Coefficient C |
|---|---|
| RCC roof | 0.85 |
| Metal / GI roof | 0.95 |
| Tile roof | 0.80 |
| Asphalt paving | 0.70 |
| Concrete paving | 0.80 |
| Paver block | 0.55 |
| Lawn / garden | 0.15 |
| Gravel surface | 0.30 |
Apply per-surface to compute total runoff. Mixed catchments (RCC roof + paving + landscape) are typical.
Storage vs recharge — the strategic choice
CGWA preferred strategy: recharge to aquifer rather than store for use. This is because:
- Indian groundwater is in critical/over-exploited status in 1,063 blocks (CGWA 2024 assessment)
- Surface storage requires treatment + disinfection if reused, raising opex
- Recharge keeps water in the local hydrological cycle
Many sites end up at storage + recharge with overflow — first-flush diversion → filter → storage tank for 3-day non-potable demand → overflow to recharge pit.
The decision flowchart:
1. Plot ≥ 100 m² → CGWA mandate triggered → recharge pit minimum
2. Daily non-potable demand exists (landscape, flush, AC make-up)? → storage tank for 3-7 days
3. Storage potable use intent? → membrane filtration (UF + RO + UV) + IS 10500 weekly testing
4. Roof + paving combined catchment > 1000 m²? → multiple smaller pits, not one large
Peak storm flow + recharge pit sizing
Peak intensity (rational method):
q_peak (L/s) = C × i_peak (mm/h) × A (m²) / 3600
For 1500 m² RCC roof in Mumbai (i_peak = 100 mm/h):
q_peak = 0.85 × 100 × 1500 / 3600 = 35.4 L/s
Recharge pit must absorb peak with 30-min residence time:
V_pit = q_peak × 30 × 60 / 1000 = 35.4 × 1800 / 1000 = 63.7 m³
Adjust for void ratio (typical pit fill = boulder + gravel + sand at void ratio 0.4):
V_pit_actual = 63.7 / 0.4 = 159 m³
At 3 m depth: pit footprint = 53 m². Practical: 8 m × 7 m (or split into 2 × 4 m × 7 m pits).
First-flush diversion
The first 2 mm of rainfall washes the catchment of dust, leaves, bird droppings, atmospheric deposition. NBC Pt 9 + CPCB Manual require first-flush diversion to drain (not to harvested storage):
V_first_flush = 2 mm × A
For 1500 m² roof: 3,000 L diverted per storm event.
A first-flush diverter (mechanical or electronic) is mandatory under CGWA notification.
Filtration system tiers
| Tier | Filter | Efficiency | Use case |
|---|---|---|---|
| 1 | First-flush only | 0.85 | Recharge only |
| 2 | Sand + charcoal | 0.90 | Storage + recharge for non-potable |
| 3 | Multimedia + UF | 0.93 | Storage for landscape + non-potable |
| 4 | Integrated (sed + UF + UV) | 0.95 | Storage for treated non-potable |
For potable-grade (rare in commercial), additional RO + monthly IS 10500 testing required.
IGBC + GRIHA credit capture
IGBC v3 — Sustainable Sites credit (Rainwater Harvesting):
- 1 point: ≥ 25 % of plot rainfall harvested or recharged
- 2 points: ≥ 50 % harvested or recharged
GRIHA Criterion 28 — Rainwater Harvesting:
- 1-2 points based on % of catchment harvested + reuse ratio
- Documentation required: catchment plan, RWH-tank-size calc, water balance
MEPVAULT RWH Calculator outputs IGBC point estimate + GRIHA Criterion 28 alignment automatically.
From the Field — Engineer’s Notebook
On a 5-acre IT campus in Hyderabad (2023), the architect’s initial RWH design provided one large 200 m³ recharge pit at the lowest contour. The 30-min storm calc showed that during a 100 mm/h Hyderabad event, the pit overflowed within 20 min — water spilled to adjacent plots. We split into 4 smaller pits across the site (50 m³ each, 6 m × 4 m × 2.5 m), each fed by a separate roof + paving catchment zone. Result: peak storm absorbed without overflow + 12 % better recharge distribution across the site. Lesson: a single large pit fails on Indian peak-intensity events; multiple smaller pits distributed across catchment zones is the right architecture.
5 common mistakes
1. No first-flush diversion. CGWA mandatory; design without it = compliance fail.
2. Single large pit on a multi-zone catchment. Storm intensity outpaces single-pit infiltration; split into multiple smaller pits.
3. Storage tank for potable use without IS 10500 testing protocol. Liability exposure + health risk.
4. No CPCB-listed filter for storage path. CPCB maintains a list of empanelled filter manufacturers; use these for non-controversy compliance.
5. Ignoring overflow path. Every pit needs a designed overflow to a backup pit or municipal drain. Otherwise plot-boundary spillover → neighbour disputes.
Designer’s checklist
- [ ] Plot ≥ 100 m² → CGWA notification status checked
- [ ] Catchment plan drawn (roof + paving + landscape with C-values)
- [ ] Annual yield calc (V = A × R × C × η × 0.001)
- [ ] Peak storm intensity calc per local IDF curve
- [ ] First-flush diversion sized at 2 mm of catchment
- [ ] Filter tier chosen based on end-use (recharge / non-potable / potable)
- [ ] Recharge pit sized for 30-min peak residence + void ratio adjustment
- [ ] Multiple smaller pits for catchment > 1000 m²
- [ ] Storage tank sized for 3-7 day non-potable demand if surface storage required
- [ ] Overflow path designed to backup pit or municipal drain
- [ ] IGBC RWH credit + GRIHA Criterion 28 documentation prepared
- [ ] CPCB-empanelled filter manufacturer specified
Pairs with: RWH Yield Calculator, Greywater Recycling per IGBC + GRIHA, Research Paper 024 — RWH Yield 8 Indian Climate Zones