Hydrodynamic and water quality 3D modelling of the Nam Theun 2 Reservoir (Lao PDR): predictions and results of scenarios related to reservoir management, hydrometeorology and nutrient input

A 3D water quality model has been applied to predict medium term evolution of the water quality in the Nam Theun 2 Reservoir and also to quantify the effect of various scenarios. 15-year simulations show that the oxygen concentration will continue to increase in the water column although the hypolimnion will remain anoxic in some areas of the Reservoir. In parallel, the concentration of reduced compounds will decrease with time. The significance of the hydrodynamics in water quality evolution is pointed out with two scenarios in which natural or human forcings have been modified. The comparison of simulations made for years with contrasted hydrometeorological conditions shows that and duration of major hydrometeorological related events (rainfall, flood and air temperature drop) have a major influence on the seasonal evolution of water quality in the whole Reservoir. Simulations also show that the physico-chemical quality of the water released downstream of the power house would have been different if the commissioning had been carried out immediately after the impoundment. Finally, the model has been used to quantify the impact of an increase of the NO3 - and PO4 3- incoming flux consecutive to potential changes in the watershed land use. The fluxes have been multiplied by a factor two separately (2 scenarios) and together. While the additional load of NO3 - has almost no impact on physico-chemistry and phytoplankton activity, the increase of PO4 3- leads to a larger increase of chlorophyll-a close to the Intake than close to the dam.