Simulated changes in water flows of the Mekong River from potential dam development and operations on the Se San and Sre Pok tributaries

Abstract

Rapid regional growth and energy demands have driven plans to build numerous dams along the Mekong’s mainstem and its tributaries prompting significant concern for potential changes in seasonal flow regimes and subsequent impacts on fisheries and biodiversity. In this paper, we demonstrate how multiple dam development and operation in the Se San and Sre Pok tributaries can affect flows in the mainstem of the Mekong. The Se San and Sre Pok tributaries were chosen because they are undergoing rapid dam development, are close to key wetlands, and because they contribute substantial flows to the Mekong. Dam operations along the Se San and Sre Pok Rivers were modelled with the HEC-HMS (Hydrological Modelling System) and HEC-ResSim (Reservoir Simulation) models. Two levels of dam development and three reservoir operation scenarios were modelled. For all operation scenarios of existing and high priority dams, daily changes in water flows and levels occur, but overall seasonal effects are low. However, substantial changes in discharge flows result when large medium priority dams are included and operated to maximize electricity generation. Dry season flows nearly double and wet season peak flows are significantly reduced, translating to changes in the order of 8% of water flows in the receiving Mekong River at Stung Treng. Changes in flows are clearly dependent on the number, size, and operation of reservoirs in the basin. However, simulation results imply that a coordinated management of dams operated under a wider set of rules to minimize changes to natural flow pulses can be a key to maximize total economic return by including the value of downstream ecosystem services. This analysis provides a methodology and basis for future work on studying changes in water levels from dam operations by which strategic options for dam development in the Mekong can be considered.