Modelling climate change impacts on the flood pulse in the Lower Mekong floodplains K. Va ¨ stila ¨ , M. Kummu, C. Sangmanee and S. Chinvanno ABSTRACT K. Va ¨ stila ¨ (corresponding author) M. Kummu Water and Development Research Group, Aalto University, P.O. Box 15200, 00076 Aalto, Finland Tel.: +358 94702 3836 Fax: +358 94702 3856 E-mail: kaisa.vastila@tkk.fi C. Sangmanee S. Chinvanno Southeast Asia START Regional Center, Chulalongkorn University, Chulawich Building, Bangkok 10330, Thailand The flood pulse is a key element characterizing the hydrology of the Mekong River and driving the high ecosystem productivity in the Lower Mekong floodplains, both in the Cambodian lowlands and the Mekong Delta in Vietnam. This paper assesses the impacts of climate change, both in terms of changed basin water balance and sea level rise, on the Lower Mekong flood pulse. The impacts were simulated by a three-dimensional hydrodynamic model using the projected changes in sea level and the Mekong mainstream discharge under the influence of climate change as boundary conditions. The model simulations projected that average and maximum water levels and flood duration increase in 2010–2049. The most consistent and notable changes occurred in the average and dry hydrological years. Sea level rise had the greatest effects in the Mekong Delta, whereas the impacts of changed basin water balance were more notable in the upper areas of the Mekong floodplains. The projected impacts were mostly opposite to those resulting from regional water infrastructure development. Higher and longer flooding could cause damage to crops, infrastructure and floodplain vegetation, and decrease the fertile land area. On the other hand, it might boost ecosystem productivity and enhance dry season water availability. Key words | climate change, flood pulse, hydrology, impact assessment, mathematical modelling, the Mekong River ABBREVIATIONS amsl above mean sea level BASIN changed basin water balance scenario DIVA Dynamic Interactive Vulnerability Assessment tool ECHAM4 a general circulation model developed by the Max Planck Institute for Meteorology EIA 3D a three-dimensional hydrodynamic model developed by the Environmental Impact Assessment Centre of Finland Ltd GCM General Circulation Model IPCC Intergovernmental Panel on Climate Change MRCS Mekong River Commission Secretariat POM Princeton Ocean Model PRECIS a regional climate model developed by the Hadley Centre for Climate Prediction and Research RMSE root mean square error SEA sea level rise scenario SEA þ BASIN cumulative impact scenario SEA START RC Southeast Asia START Regional Center SRES Special Report on Emissions Scenarios START Global Change SysTem for Analysis, Research and Training network TKK Helsinki University of Technology VIC Variable Infiltration Capacity model WUP-FIN Lower Mekong Modelling Project doi: 10.2166/wcc.2010.008 67 Q IWA Publishing 2010 Journal of Water and Climate Change | 01.1 | 2010