Secular trend in water discharge transport in the Lower Mekong River-delta: Effects of multiple anthropogenic stressors, rainfall, and tropical cyclones

The Mekong River is one of the world's most important continental rivers, and provides invaluable water re-sources for the survival and social development of nearly 70 million inhabitants of Southeast Asia. However, little work has been carried out on the multiforcing-induced hydrological behavior of the Lower Mekong River -delta (LMR) over the past 100 years. Here, the secular variations in water discharge were analyzed for seven gauging stations along the main stream of the LMR from the 1920s-2019, to identify the riverine hydrological mechanisms under different dynamic conditions. The results show that the water discharge delivered by the LMR to the delta decreased on both 50-year and centennial scales. Meanwhile, the daily water discharge of the LMR-delta has become more evenly distributed intra-annually, leading to "no flood in the flood season, no drought in the dry season" in this reach. While regional rainfall dominates the long-term variations in the water discharge in the LMR, episodic tropical cyclones can induce a gradual increase with enlarged spatial divergences in water discharge from the mountainous region to the delta by bringing heavy rainfall. Moreover, although the operation of the mega dams in the Upper Mekong Basin has significantly influenced the distribution of the downstream water discharge in the dry season, minor impacts can also be found in the wet season. The water consumption for irrigation was increased from the upstream to the downstream basin, which may contribute to the severe reduction in the downstream water discharge, even though typhoons induced much more runoff along the downstream reach. Considering future climate change in Southeast Asia with potential upward trends in anthropogenic stressors, it can be expected that the water discharge of the LMR-delta could worsen, and urgent countermeasures that address water resource management should be strongly considered by decision-makers in the affected countries of the Mekong River-delta.