Impacts of climate change and human activities on the flow regime of the dammed Lancang River in Southwest China

The Lancang River (LR) in China (the upper portion of the Mekong River which is among the world’s most important transboundary rivers) originates on the Tibetan Plateau and provides important freshwater resources for living, agriculture, industry, and hydropower generation for millions of people downstream. The natural flow regime of the LR is critical to sustain native biodiversity and ecosystem integrity; however, it has been changing due to the combined effect of climate change and human activities. Accurate quantification of the impacts of climate change and human activities on changes in the flow regime is a prerequisite for water resources and hydropower exploitation and environmental protection. This study aims to evaluate climate- and human-induced impacts on the LR during 1980–2014. A distributed hydrological model CREST-snow combined with remote sensing and streamflow data and the Budyko framework were jointly used to address this scientific question during three time windows determined by the Mann–Kendall test and the history of dam construction. Results show that compared with the baseline period (1980–1986) when no dam was constructed, significant changes (∼−6%) in mean annual streamflow occurred during 1987–2014, particularly after 2008 when the construction of the largest hydropower plant (Nuozhadu) in the Mekong River basin began. Climatic change contributed ∼57% to streamflow changes during the transition period (1987–2007), whereas human activities contributed ∼95% during the post-impact period (2008–2014). At the seasonal scale, climatic variation plays a more significant role during the dry season (December–May), with precipitation the most important factor among climate variables, whereas human activities contributed more during the wet season (June–November), benefiting the downstream areas through mitigating floods. Among human activities, reservoir construction is a dominant factor affecting streamflow over agricultural, industrial, and domestic water uses. The findings of this study enhance our understanding of hydrological changes in the LR basin that may impact the Lower Mekong River, serve as a basis for water resources and hydropower exploitation and environmental protection, and highlight the need for considering reservoir operation strategies in streamflow projections in similar basins globally under climate change scenarios.