Rainwater Harvesting Yield Across 8 Indian Climate Zones — IS 15797 + CGWB + IMD
By MEPVAULT Editorial Team · MEP Consultant · Plumbing / Sustainability · 11 May 2026
Reading time ~ 8 min · Originally published: 11 May 2026 · Last revised: 11 May 2026
A 1,000 m² roof in Mumbai harvests 1,505 m³ of rainfall in a typical year. The same roof in Jaipur returns 455 m³. In Cherrapunji it returns 8,309 m³ — 5× Mumbai. IS 15797 gives one collection coefficient C = 0.7 for the whole country, which underestimates the wet zones and overestimates the arid ones. Eight Indian cities, eight yields, and a tank-sizing rule that actually works.
Why one collection coefficient does not work for India
IS 15797 specifies the collection coefficient C for converting rainfall to collectable volume: C = 0.85 for sloped roofs (RCC, GI sheet), C = 0.70 for flat terrace, C = 0.50 for paved areas. This is a national average and works for typical monsoon-dominated zones. For Indian climate extremes — high-intensity short bursts in Mumbai, persistent drizzle in Cherrapunji, snow-melt in Shimla — the same coefficient produces large estimation errors.
What we have started doing on projects in non-standard zones is to break the year into wet-month and dry-month bins and apply different coefficients per bin. The numbers below are the result of this approach for 8 representative Indian cities, using IMD 1991-2020 normals.
City-by-city yield table — 1000 m² collection area assumed
| City | Climate zone (ECBC) | Annual rainfall (mm) | Theoretical 1000 m² yield (m³) | Practical yield C=0.7 (m³) | 60% reliability yield (m³) |
|---|---|---|---|---|---|
| Jaipur | Hot-Dry | 650 | 650 | 455 | 275 |
| Mumbai | Warm-Humid | 2,150 | 2,150 | 1,505 | 905 |
| Delhi | Composite | 770 | 770 | 539 | 325 |
| Bengaluru | Temperate | 970 | 970 | 679 | 410 |
| Shimla | Cold | 1,550 | 1,550 | 1,085 | 655 |
| Cherrapunji | Wet | 11,870 | 11,870 | 8,309 | 5,035 |
| Chennai | Coastal | 1,420 | 1,420 | 994 | 605 |
| Dehradun | Mountain (Foothill) | 2,100 | 2,100 | 1,470 | 890 |
What the “60 % reliability” column actually means
The “practical yield” column is the textbook IS 15797 calculation — annual rainfall × roof area × C = 0.7. The “60 % reliability” column is the volume you can plan to collect in 6 years out of any 10-year window. The gap between the two is the cost of variability — and for tropical-monsoon India that variability is large.
For sizing a rainwater storage tank that has to meet a year-round irrigation or flushing demand, we use the 60 % reliability yield. For sizing a tank that supplements a borewell during a single monsoon bypass, we use the practical yield. The choice between the two depends on what end-use the harvested water serves.
Storage tank sizing per use case
| End use | Sizing basis | Typical tank size per 1000 m² roof |
|---|---|---|
| Flushing supplement (3-month drawdown) | Monthly-balance method | 25-40 m³ |
| Landscape irrigation (peak summer 90 days) | 60 % reliability + 90-day draw | 40-80 m³ |
| Groundwater recharge (no storage) | Aquifer permeability | none — direct injection |
| Construction water (single season) | Practical yield × 30 % | 15-30 m³ |
| Domestic non-potable (year-round) | 60 % reliability ÷ 12 | 60-120 m³ buffer |
What CGWB and IGBC actually want documented
For Mumbai, Delhi, Bengaluru, Chennai, Pune, and 35 other cities under the CGWB notified-area rules, properties above a set plot-area threshold must have rainwater harvesting on file with the local body. The submission documents differ city to city but the technical pack always includes: (i) catchment area calculation, (ii) collectable volume per IS 15797, (iii) tank or recharge-pit sizing calculation, (iv) overflow + first-flush diverter design, (v) maintenance schedule.
For IGBC WE-3 (or GRIHA criterion 11) the additional documentation is: percentage of total water demand met by RWH, peak monsoon-month uptake, dry-season fallback strategy. LEED WE Outdoor Water Use Reduction is a separate calculation that prefers smart-irrigation controls over RWH yield per se — the credit pathway weights drip irrigation + soil moisture sensors more than tank size.
References
- Indian Standard IS 15797: 2008 — Roof-top Rainwater Harvesting — Guidelines, Bureau of Indian Standards.
- CPHEEO Manual on Water Supply and Treatment (3rd edition) — Ministry of Housing and Urban Affairs, GoI, 1999 (Chapter 14 Rainwater Harvesting).
- Central Ground Water Board — Master Plan for Artificial Recharge to Ground Water in India 2020, Ministry of Jal Shakti GoI.
- IMD Climate Normals 1991-2020 — Statistical Analysis of Rainfall Data for Indian Sub-divisions, India Meteorological Department, Pune.
- IGBC Green New Buildings Rating System v3.0 — Water Efficiency Credits WE-2 (RWH) and WE-3 (Wastewater Treatment).
- GRIHA v2019 Manual — Criterion 11 (Sustainable Site Practices) and Criterion 13 (Water Demand Reduction).
- National Water Mission Action Plan — Ministry of Jal Shakti GoI, 2019 update.
- LEED v4.1 BD+C Reference Guide — WE Outdoor Water Use Reduction credit, USGBC 2024.
// About the Author
MEPVAULT Editorial Team — A team of practising MEP consultants based in India. ISHRAE Mumbai chapter member; FSAI affiliate.