The Nagmati Environment and Water Project is a multipurpose water storage scheme in the northeast of the Kathmandu Valley, prepared by the Asian Development Bank with the Government of Nepal. A 95 metre concrete-faced rockfill dam on the Nagmati river creates a reservoir of about 9.5 million cubic metres, regulating monsoon runoff and releasing it through the dry season to augment water supply and environmental flows in the Bagmati river below Kathmandu. The scheme also includes small run-of-river hydropower, watershed management and ecotourism investment in Shivapuri Nagarjun National Park, a biodiversity offset programme, and institutional strengthening for dam safety and integrated water resources management.

With a 100 year design life in a small, steep Himalayan headwater catchment, the safety and value of the scheme depend directly on climate and hydrology. FutureWater carried out the climate risk and adaptation assessment. The historic climate was established from the ERA5-Land reanalysis for 1980 to 2024, and the future climate from the NASA NEX-GDDP-CMIP6 ensemble of 35 downscaled global climate models, bias corrected against the same baseline, under a moderate and a high emissions pathway across near-term, mid-century and late-century horizons.

The catchment receives around 2750 mm of precipitation a year, some four fifths of it in the summer monsoon. The observed record already shows warming of roughly 0.15 °C per decade, and the projections continue that trend to an ensemble median of 2.0 °C by mid-century and 3.3 °C by late century under the high pathway. Annual precipitation rises, but the change is strongly seasonal: the monsoon and post-monsoon months become wetter while mid-winter becomes drier. Heavy rainfall intensifies robustly, with the annual maximum one-day rainfall projected to increase by 16 to 28 percent by late century.

For the scheme this means water becomes more abundant in total but more concentrated in the monsoon and harder to capture, while the dry season becomes more stressed, flood hazard increases and usable storage is gradually eroded by sediment. The assessment rated the dam and reservoir at high risk from extreme rainfall and flooding and from the region’s seismic hazard, and the regulating service the scheme provides at high risk from dry-season water stress. Glacial lake outburst floods, often a prominent Himalayan hazard, were ruled out because there is no glacial cover upstream of the site.

FutureWater set out adaptation measures for each material risk, to be carried forward through the dam safety and operating plans, an adaptive management framework, and review by the project’s Panel of Experts. These included re-deriving the design flood under future climate conditions with conservative spillway capacity and freeboard, managing sediment through catchment protection and flushing capability, and calibrating reservoir operating rules and firm-yield estimates to projected rather than historical inflows. The findings also fed the climate change assessment underpinning the project’s alignment with the Paris Agreement and its climate finance estimate.