Optimizing Water Transfer Megaprojects under Climate Change: A Water–Energy–Land Nexus Assessment in China

Inter-basin water transfers (IBWT) are increasingly proposed as adaptation to climate-driven water scarcity, but their optimal scale depends on both future hydroclimate and the energy requirements. In China, several megaprojects are under planning to further mitigate the grand challenge of water scarcity in the north, notably the western route of the South-to-North Water Diversion (SNWD) project, yet their long-term optimization and Water–Energy–Land (WEL) implications remain uncertain. Here, we use the MESSAGEix-GLOBIOM integrated assessment model to optimize IBWT megaprojects in China during 2030–2060, and evaluate their WEL impacts under contrasting SSP3-70 and SSP1-26 Mitigation contexts. Results show that the designed capacities of planned IBWT megaprojects considerably exceed optimized requirements, even under extreme climate and high-demand scenarios, suggesting potential inefficiencies in the designed water allocation. Optimized IBWT deployment could reduce water scarcity by up to 76% in northern recipient basins with limited stress on donor basins, and buffering climate- and mitigation-induced fluctuations in water scarcity. IBWT also reshapes WEL interactions by reducing reliance on energy-intensive desalination and groundwater withdrawals, lowering national energy consumption for water, while increasing irrigation water availability and agricultural production. Specifically, the western SNWD route shows substantial potential, with an optimized annual capacity of 9.1 km³/yr under SSP3-70, but this decreases by 2.4 km³/yr under SSP1-26 Mitigation due to lower water stress and tighter carbon constraints, revealing a critical water–energy–carbon trade-off. These findings highlight the need to move beyond fixed design capacities toward integrated IBWT planning accounting for climate policy and cross-sectoral WEL interactions.