Downstream Hydrological Impacts of Hydropower Development in the Upper Mekong Basin

The Mekong River Basin in Southeast Asia is experiencing extensive hydropower development. Concerns have been raised about the consequences of the development for the ecosystems, livelihoods and food security in the region. The largest planned hydropower dam cascade in the basin, the Lancang‐Jiang cascade, is currently under construction and already partly built into the Upper Mekong Basin, China. In this paper we assess the impact of the Lancang‐Jiang cascade on downstream hydrology by using a combination of a hydrological model and a reservoir cascade optimization model. The hydrological changes were quantified in detail at the Chiang Saen gauging station in Thailand, the first gauge station downstream from the cascade, and in lesser detail at four other downstream locations in the Mekong mainstream. We found that on average the Lancang‐Jiang cascade increased the December–May discharge by 34–155 % and decreased the July–September discharge by 29–36 % at Chiang Saen. Furthermore, the Lancang‐Jiang cascade reduced (increased) the range of hydrological variability during the wet season (dry season) months. The dry season hydrological changes were significant also in all downstream gauging stations, even as far as Kratie in Cambodia. Thus the Mekong’s hydrological regime has been significantly altered by the Lancang‐Jiang cascade, but what the consequences are for ecosystems and livelihoods, needs further study.