Reservoir operation impacts on streamflow and sediment dynamics in the transboundary river basin, Vietnam

Human interventions activities around the world, particularly reservoir operation, have dramatically altered hydrological and sediment regimes in most of the major river basins. In the Mekong River, specifically the Upper Srepok River Basin (USRB) which is a main tributary of the river basin connected to the Mekong Delta's rice bowl and the Tonle Sap Lake's top inland fisheries, there are increasing concerns about the impacts of cascade reservoir operations on downstream streamflow and sediment budgets. Previous studies estimating impacts either relied solely on observed data or did not verify simulations of regulated streamflow. Using a process-based hydrological model calibrated and validated for both natural and regulated streamflow in the USRB, it was found that the monthly hydrological changes were up to & PLUSMN;20% compared to pre-dam periods at the most downstream station bordering between Vietnam and Cambodia (i.e., Ban Don station). The basin also experienced a slight decrease (less than 2%) in annual streamflow. Additionally, average and peak suspended sediment concentration decreased significantly in both of the annual and seasonal periods. At the Ban Don station, sediment loads were reduced 140 thousand tons/year (i.e., 15%) compared to pre-dam period. Most of the changes in streamflow and sediment budgets in the basin were driven by the Buon Tua Srah reservoir, which had the highest degree of regulation in the basin. Therefore, integrated and transboundary water and sediment management, particularly at Buon Tua Srah reservoir, needs to be developed for the sustainability of the river basin. The Srepok_HYPE model can provide useful information to support the decision-making process for water and sediment management in the basin.For seasonal effects on stream flows, the reservoirs within the USRB increased the low flows in the dry season and decreased the high flows in the wet season up to & PLUSMN;20% at Ban Don.Regarding the impacts of reservoir operations on SSC, average and maximum SSC decreased dramatically in both of the annual and seasonal periods.image