Long-term discharges, water levels, salinity concentrations, and precipitation in the Mekong River basin

Hydropower dams have been increasingly and planned in the Mekong River basin, and questions about the impacts of the largest hdyropower dams in the Lancang cascade remain largely unknown. To quantify the impacts of existing hydropower dams in the Mekong River basin on flow regimes on the lower Mekong River and associated impacts on salinity intrusion in the Vietnamese Mekong Delta, we collected and analyzed long-term observed discharges, water levels, salinity concentrations, and precipitation along the entire lower Mekong River, from Chiang Saen, Thailand to the Vietnamese Mekong Delta. Then, we applied various hydrological indicator and trend test methods to interpret the time series data. Discharges and water levels are available over 36 years (1980-2015) at Chiang Saen (Thailand), Kratie (Cambodia), Tan Chau, Chau Doc, and My Thuan (Vietnam), and salinity concentrations are available during the period 1990-2016 at Tra Vinh and Cau Quan in the Vietnamese Mekong Delta. These data were provided by the Vietnamese National Hydro-meteorological Data Center and Mekong River Commission. Annual precipitation at seven rain gauges in the Mekong River basin (Lincang, Simao, Lancang, Jinhong, Chiang Saen, Pakse, and Nong Khai) were collected over the period 1954-2015 for the analysis. Results from the analysis show that climate conditions controlled flow regimes, while hydropower dams in the Mekong River basin, as the main driver, intensified the impacts during the period 1993-2008. The operations of the hydropower dams in the Mekong River basin were the main drivers of hydrologic alterations during the period 2009-2015. Mainstream dams controlled the maximum and high-flow discharges, while tributary dams controlled the minimum and low-flow discharges in the lower Mekong River. The cumulative effects of the six cascading dams were significant in the wet season, while tributary dams had a dominant influence in the dry season. Resulting from reduced low-flow water levels, postdam salinity intrusions in the Vietnamese Mekong Delta have increased in magnitude and have shifted to occur approximately 1-2 months earlier than the no-dam period