南水北调西线工程是我国“四横三纵”国家水网主骨架尚待建设的最后一环，而水源区可调水量是西线工程规划论证面临的焦点和难点问题。本文从水资源本底条件与演变趋势，生态环境与经济社会发展需水，调蓄水库与输水工程规模，调水对水源区及其下游水资源开发利用、水生态环境、水力发电、航运等方面影响的角度，提出了与水源区可调水量相关的10个问题；以长江上游分布式水文模型为基础，建立了南水北调西线工程水源区可调水量模拟分析模型，力图系统、定量地回答所提问题。本文的主要研究结论有：在仅考虑满足水源区河道内外生态环境和经济社会需水的前提下，“上线+下线”组合方案多年平均可调水量为1.59×10¹⁰ m³，“完全下线”方案在建设岗托水库进行联调时，多年平均可调水量将达到1.74×10¹⁰ m³；在综合考虑调水对水源区水平衡、水力发电和航运影响及其可接受程度后，南水北调西线水源区可调水量为1.22×10¹⁰~1.26×10¹⁰ m³。长远来看，应谋划西南片区水网与国家水网主骨架、“大动脉”的连接与融合，在减小南水北调西线工程调水影响的同时，提高南水北调西线工程的整体供水能力以及对气候变化等外部条件的适应性。

The Western Route of the South-to-North Water Diversion Project is the final link to be constructed for the four-horizontal three-vertical framework of China’s national water network. The adjustable water volume in the water source areas is a focal and challenging issue in the planning and demonstration of the Western Route. This study raised ten questions related to the adjustable water volume in the water source areas of the Western Route from the aspects of water resource background conditions and evolution trends, ecological and socio-economic water demands, scales of reservoirs and water conveyance projects, as well as impacts of water diversion on water resource development and utilization, aquatic ecological environment, hydropower, and navigation in the water source areas and their lower reaches. Based on a distributed hydrological model of the upper reaches of the Yangtze River, a simulation and analysis model for calculating the adjustable water volume in water source areas of the Western Route is established to quantitatively answer the ten questions raised. The main conclusions are as follows. First, under the premise of solely satisfying ecological and socio-economic water demands within and outside the river channels of the water source areas, a plan that combines upper and lower routes presents an average adjustable water volume of 1.59×10¹⁰ m³ over multiple years, while a plan for solely constructing a lower route can reach an average adjustable water volume of 1.74×10¹⁰ m³ by constructing the Gangtuo Reservoir for coordinated operation. Second, comprehensively considering the impacts of water diversion on water balance, hydropower, and navigation, as well as their acceptability in the water source areas, the adjustable water volume is between 1.22×10¹⁰ and 1.26×10¹⁰ m³. Third, in the long term, the plan should focus on connecting and integrating the water network in the southwestern region with the main framework and major streams of the national water network of China. This will reduce the impacts of water diversion by the Western Route Project, increase the overall water supply capacity of the project, and promote its adaptability to external conditions such as climate change.