WATER QUALITY MODELLING OF THE MEKONG RIVER BASIN: CLIMATE CHANGE AND SOCIOECONOMICS DRIVE FLOW AND NUTRIENT FLUX CHANGES TO THE MEKONG DELTA

The world's deltas and their upstream river basins are undergoing long term changes from climate change, land use change, population increases, and other industrial and socioeconomic changes. The Mekong delta is recognized as one of the world's most vulnerable mega-deltas, facing many of these issues, which demanded a strong need to assess how the Mekong River basin will be affected in terms of water and nutrient fluxes into the delta system. The INtegrated CAtchment models (INCA) were applied to the whole Mekong River basin to simulate flow and water quality (nitrogen and phosphorus) across 24 river reaches ranging from the Himalayas down to the delta in Vietnam. The UK Met Office PRECIS regionally coupled climate model was used to downscale precipitation and temperature to the Mekong catchment with the Global Circulation Model GFDL-CM providing the boundary conditions under two carbon control strategies, namely representative concentration pathways (RCP) 4.5 and a RCP 8.5 scenario. In addition, a range of socioeconomic scenarios were constructed to investigate the potential impacts of changing population, atmospheric pollution, economic growth and land use change up to the 2050s. Results of INCA simulations indicate increases in mean flows of up to 24%, with flood flows in the monsoon period increasing by up to 27% by the end of the century, but with low flows declining slightly by the 2050s. A shift in the timing of the monsoon is also simulated, with a 4 week advance in the onset of monsoon flows on average. Decreases in nitrogen and phosphorus concentrations occur primarily due to flow dilution, but fluxes of these nutrients also increase by 5%, which reflects the changing flow, land use change and population changes. This is the first time that a process based dynamic model of flow and water quality has been applied to the entire Mekong River basin. The model projections provide valuable knowledge to support planning and strategic decision making in the Mekong River system.