Coupling process analysis of suspended sediment fluctuations and soil erosion in Southeast Asia

The sediment dynamics of large river systems are being profoundly altered by climate change and intensive human activities, posing challenges to watershed ecology and coastal stability. This PhD thesis aims to develop an assessment framework integrating multi-source satellite remote sensing, process-based models, and hydrological observations to reveal sediment transport patterns from source to estuary under strong anthropogenic interference. The first two research chapters (Chapters 2 and 3) focus on analyzing the spatiotemporal dynamics of suspended sediment concentration (SSC) in the downstream parts of river systems, the main channels and estuaries. Using the Pearl River as a case study, Chapter 2 develops and validates a high-accuracy, basin-scale SSC retrieval model, revealing the sediment trapping by cascade reservoirs and complex longitudinal variations that traditional station data cannot capture. Chapter 3 expands this perspective to a macro-regional scale, analyzing SSC trends in major estuaries of the Western Pacific from 1990 to 2020 using long-term Landsat data. It finds a widespread significant long-term decline, directly linking upstream dam sediment interception to regional coastal sediment decline. The latter two chapters (Chapters 4 and 5) shift the focus upstream to the sediment sources, aiming to quantitatively connect hillslope erosion processes with observed downstream sediment flux. Chapter 4 develops a monthly-scale soil erosion model for the Yangtze River Basin, demonstrating that high-frequency heavy precipitation events are the dominant driver of seasonal erosion. A key finding is the model's systematic overestimation of sediment yield at stations downstream of the Three Gorges Project, thereby inversely quantifying the massive sediment trapping efficiency of the dam system. Finally, Chapter 5 constructs a complete cascade model integrating erosion, reservoir trapping efficiency, and riverine sediment transport. Applied to the Jialing River, it reveals how water conservancy projects disrupt natural sediment transport pathways. In summary, this thesis provides an integrated analytical framework and scientific basis for understanding and managing large river sediment systems heavily impacted by human activities.