India Rainfall Intensity Map — IMD + CPHEEO Manual
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About rainfall design
Stormwater systems and roof drainage are sized for the design 1-hour intensity (mm/hr) at a chosen return period (typically 5-25 years for residential, 50-100 years for critical infrastructure). The CPHEEO Manual maps annual rainfall to design intensity via depth-duration-frequency curves.
Practical sizing rules
- Roof drainage: 1 cm² of downpipe per 1 m² roof per 50 mm/hr design rate.
- Stormwater drains: Manning's formula with runoff coefficient 0.7-0.9 (paved), 0.2-0.5 (green).
- Rainwater harvesting: storage volume = 0.6 × annual rainfall × catchment area × runoff coeff.
- Foundation: include groundwater rise from monsoon recharge in design water table.
Cross-references
Source IS / IRC / NBC standards
Related maps
Calculators & tools
Articles & guides
Frequently asked questions
What is the design rainfall intensity in Mumbai?
Mumbai's design 1-hour intensity ~75-100 mm/hr (5-year return period). Annual ~2,200 mm. CPHEEO recommends 100 mm/hr for stormwater + 75-200 L/s/ha for sub-catchment design.
What is the wettest place in India?
Mawsynram (Meghalaya) holds the world's annual rainfall record at ~11,872 mm. Nearby Cherrapunji historically averaged ~11,000 mm/yr. Both are in the Khasi Hills, capturing southwest monsoon convergence. Design here is site-specific — outside standard codes.
How do I size rainwater harvesting?
Storage volume = 0.6 × annual rainfall (m) × catchment area (m²) × runoff coefficient (0.8 paved). E.g., 1,000 sqm roof in Mumbai (2.2 m rainfall): 0.6 × 2.2 × 1,000 × 0.8 = 1,056 m³ collectable annually. Storage usually 5-10% of this for monthly carryover.
Annual rainfall values are 1981-2020 IMD long-period averages. Climate variability, microclimate (urban heat island, coastal cooling), and topography (windward vs leeward) cause significant deviation. Use IMD station data for the nearest specific monitoring station for design.