Lake Dynamics in the Yangtze Basin Downstream of Three Gorges Dam Driven by Natural Determinants and Human Activities
In the era of "Anthropocene", lakes as essential stocks of terrestrial water resources are subject to increasing vulnerability from both climate change and human activities. There has been a widespread recognition of the need for further enhancing our monitoring and understanding of lake dynamics under complex human-environment interactions, particularly in populated and rapidly developing regions. To address this pressing need, this dissertation highlights the Yangtze River Basin downstream of China's Three Gorges Dam (TGD), one of the world's most populous areas and a critical eco-region, which hosts the largest cluster of freshwater lakes in East Asia. The TGD, thus far the world's largest hydroelectric project, initiated water impoundment in June, 2003. Existing studies document that individual lakes in this region have recently experienced dramatic changes under the context of enduring climatic drought, continuous population growth, and extensive human water regulation. However, spatial and temporal patterns of lake dynamics across the complete downstream Yangtze basin are poorly characterized; and the underlying changing mechanisms remain largely unclear. With an emphasis of the controversial TGD impacts, this dissertation presents a comprehensive investigation integrating remote sensing, spatial statistics, and hydrological modeling to (1) understand the recent lake dynamics across the downstream Yangtze Basin before and after the initial TGD operation and (2) diagnose the driving mechanism from inducing factors of both climatic variability and major anthropogenic activities, i.e., the TGD operation and human water consumption.
A widespread net decline in lake inundation area was revealed across the downstream Yangtze Basin during 2000-2011 from daily optical imagery acquired from the Moderate Resolution Imaging Spectroradiometer (MODIS). The decreasing trend was tested significant in all seasons, leading to an evident phase drop of the average annual lake cycle before and after the TGD operation (hereafter referred to as "post-TGD decline"). The most substantial decline appears in fall, which intriguingly coincides with the TGD water storage season. Regional lake dynamics exhibits contrasting spatial patterns, with decrease and increase of aggregated lake area within and beyond the Yangtze floodplain, respectively. Decreasing lakes in the floodplain, whose elevations are below the natural Yangtze level maxima, pose further puzzling of TGD's impacts on the lake system.
To investigate the underlying mechanism driving the post-TGD lake decline, a diagnostic approach was proposed based on the conceptual chain where the impact of an inducing factor (e.g., TGD's regulation) usually starts from the Yangtze flows, then propagates to the downstream Yangtze levels, and eventually the surrounding lake inundation areas. Using in situ measurements and hydrological modeling, daily Yangtze level changes were first quantified along the complete downstream range, as a combined result of i) TGD's flow regulation and ii) Yangtze channel erosion due to reduced sediment load. Derivative impacts on lake inundation areas were then assessed as empirical functions of lake-outlet level changes. Results uncover an altered inundation regime of the downstream lake system by TGD's water regulation, manifested as evident lake area decrease in fall and increase in spring and winter. As the most substantial influence, reduced lake area in fall explains ~18-83% of the observed post-TGD decline across the downstream Yangtze Basin. Concurrent Yangtze channel erosion slightly reinforced the area decrease in fall while counteracting ~34% of the area increase in winter. Human water consumption accumulated through the local river network led to constant Yangtze level decrease, which completely counteracted TGD-induced lake area increase in winter. However, human water consumption only adds minor contribution (< 6%) to the post-TGD lake decline due to slow increasing rates during 2000-2011. The major proportions of seasonal post-TGD lake declines were tested to be largely triggered by the decadal climatic drought across the downstream Yangtze Basin; however, the quantified anthropogenic impacts are evident and anticipated to increase in the coming decades due to chronic Yangtze channel erosion and continuous population growth.