Increased rainfall-runoff drives flood hazard intensification in Central Himalayan river systems

Area of Science:

• Hydrology
• Climate Science
• Environmental Modeling

Background:

• The Central Himalayan region faces significant flood risks, yet climate change impacts on fluvial floods remain poorly understood.
• Accurate flood projections are crucial for developing effective local mitigation strategies in this vulnerable area.

Purpose of the Study:

• To conduct a large-ensemble, regional climate change impact assessment of design floods for the Karnali River.
• To quantify climatic, hydrological, and statistical uncertainties in flood projections for Nepal and China.

Main Methods:

• Utilized high-resolution modeling for climate change impact assessment.
• Employed a large ensemble approach to capture uncertainties.
• Analyzed projected changes in flood magnitude, specifically the 1% annual exceedance probability flood.

Main Results:

• Simulations project substantial increases in 1% annual exceedance probability flood magnitude by 2060-2099: +40% (medium-emissions) and +79% (high-emissions).
• Rainfall-runoff is identified as the primary contributor, accounting for ≥90% of additional flood water.
• Uncertainty in projections is linked to the variability between extreme and common flood events.

Conclusions:

• Climate change poses a significant threat of increased fluvial flood magnitudes in the Central Himalayas.
• Incorporating both atmospheric general circulation models and earth system models is essential for capturing the full range of uncertainties in flood event projections.