The paradox
Hydropower burns no fuel, yet a reservoir floods forest and soil. In warm tropical water, that organic matter rots without oxygen and releases methane (CH₄), a gas about 27 times stronger than CO₂ over 100 years (IPCC AR6 GWP-100).
Methane leaves a reservoir by diffusion from the surface, by bubbles rising from the bed (ebullition), and by degassing when deep water passes through turbines and spillways. Emissions peak in the first 10–20 years and then decline as the flooded carbon is used up — so old Kerala dams like Mattupetty or Neyyar emit far less than a newly filled reservoir would.
Green and blue carbon sinks
Green carbon is the CO₂ that forests absorb. Western Ghats evergreen forest takes up roughly 3.2–4.8 tonnes of CO₂e per hectare per year, so a healthy catchment can outweigh its reservoir's methane.
Blue carbon is carbon locked away by water ecosystems — here, organic sediment buried on the reservoir bed (about 0.2–0.6 tCO₂e/ha/yr) plus wetlands and riparian buffers. Protected and restored, these can generate verified carbon credits under schemes like Verra's VCS.
How JalaNet estimates it
Every figure on a dam page is a labelled estimate, never a measurement:
- Surface area ≈ capacity ÷ an assumed 20 m mean depth (Kerala does not publish water-spread areas consistently).
- Gross methane = area × 141 kg CH₄/ha/yr (IPCC 2019 tropical wet) × 27, reduced for reservoirs older than 20 years.
- Green sink = catchment × 60% forest cover × 4.0 tCO₂e/ha/yr.
- Hydro offset = installed MW × 35% capacity factor × CEA grid factor 0.716 kg CO₂/kWh.