The global trend of urbanization is accelerating, leading to the formation of megalopolises – highly concentrated urban areas with integrated cities. Megalopolises offer significant economic advantages, driving countries to pursue their development. However, sustainable urban expansion requires careful consideration of resource limitations, particularly water. The United Nations highlights the growing global water demand, projected to increase by 20-30% by 2050, largely driven by industrial and domestic sectors. Megalopolises, with their high population and production activities, are particularly vulnerable to water crises, which can hinder their development. A strong correlation exists between the economic growth of existing world-class megalopolises and abundant water resources, underscoring the critical role of water in economic development. Existing world-class megalopolises are generally not water-scarce, thus the effects of water scarcity on megalopolis development have been less studied. China's ambitious goal to develop a world-class megalopolis in the Beijing-Tianjin-Hebei (BTH) region presents a unique case study. The BTH region suffers from severe water scarcity, with annual per capita water resources significantly below national and global averages. Compared to established world-class megalopolises, the BTH region also lags in economic size, tertiary industry dominance, and inter-city connectivity. To achieve its goal, the BTH region needs to address these shortcomings while simultaneously mitigating its water shortage. This necessitates city-level analysis due to significant heterogeneity in economic development and water resources across the region. The study uses the BTH region to investigate the effect of water scarcity on urban development by examining the water gap and the effectiveness of water conservation measures.

Existing research on the impact of water scarcity on urban development primarily focuses on water-carrying capacity, often defined as the maximum water resources sustainable for a healthy socio-economic system. However, the definition of water-carrying capacity remains ambiguous. Previous studies also often overlook the implications of intra-regional economic connections due to data limitations. Inter-city economic connections are crucial because they influence water use patterns in different cities. Inter-city input-output models are recognized as effective tools for assessing these connections and capturing city-level heterogeneities, but their application has been limited by data availability. This study addresses these gaps by utilizing city-level input-output tables for the BTH region to analyze the interaction between economic interconnections and water constraints. The study is unique in comprehensively examining the restrictive effect of water resources on world-class megalopolises, examining urban development from the perspective of the water gap, and using an inter-city input-output optimization model to analyze urban development under water constraints while considering intra-regional economic connections.

This study employs an inter-city input-output optimization model to simulate the BTH region's water requirements for achieving world-class megalopolis status. The model incorporates city-level differences and inter-city connections, using data from city-level input-output tables of Beijing, Tianjin, and 11 cities in Hebei. The model's stability and reliability have been verified in previous studies. The study uses two benchmarks: the Yangtze River Delta megalopolis (smaller economic size) and the Great Lakes megalopolis (larger economic size), to set constraints for economic size, industrial structure, and inter-city connections in the optimization model. The model conducts multiple simulations with different objectives and constraints: 1. **Minimum water requirement:** Minimizes water use while meeting the benchmark constraints (economic size, industrial structure, inter-city connections). 2. **Maximum GDP without conservation measures:** Maximizes GDP subject to the available water resources constraint. 3. **Effects of water conservation measures:** Minimizes water use under various scenarios incorporating water conservation measures (improving water use efficiency and reducing agricultural water use). These conservation measures' effectiveness is evaluated based on government-set targets. The simulations use the inter-city input-output table of the BTH region (2012) as the base data. The model's equations represent the inter-sectorial and inter-city dependencies in terms of intermediate and final demands. Different objective functions (minimizing water use, maximizing GDP) and constraints (economic size, industrial structure, inter-city connections, water use, water use efficiency, agricultural water use) are implemented in different simulations to analyze the water requirements under different scenarios.

The simulation results show that to reach the benchmarks of the Yangtze River Delta and Great Lakes megalopolises, the BTH region would require 37.58 billion m³ and 53.26 billion m³ of water annually, respectively. Considering the region's average annual water resources of 20.40 billion m³, significant water gaps exist (17.18 billion m³ and 32.86 billion m³, respectively). Even for the more achievable Yangtze River Delta benchmark, the water gap is nearly equal to the current water resources. Without additional water conservation measures, the BTH region's maximum achievable GDP is estimated to be 1.34 trillion USD, only a 12% increase over the current level, and significantly below that of established world-class megalopolises. Improving water use efficiency modestly reduces the water requirement, but substantial gaps remain. Reducing agricultural water use proves more effective, leading to more significant reductions. However, only when both measures are implemented can the BTH region reach the Yangtze River Delta benchmark. The study reveals that the water transfer from the South-to-North Water Transfer Project (SNWTP), while potentially helpful, faces uncertainties due to high costs and limited utilization in industrial production. The study emphasizes the importance of reducing agricultural water use in Hebei, which is a major wheat and maize producer. Winter wheat, a water-intensive crop, is a major contributor to groundwater depletion. Replacing winter wheat with more water-efficient crops could substantially reduce water use. However, this may impact farmers' livelihoods, requiring compensatory measures. The study also explores adjusting the BTH megalopolis boundary by excluding four cities in southern and central Hebei. This reduces the water gap by 35% (Yangtze River Delta benchmark) and 40% (Great Lakes benchmark), making the world-class megalopolis goal potentially more achievable. The success of this strategy depends on factors like maintaining geographical continuity, targeting agricultural regions with weak economic connections to other BTH cities.

The findings demonstrate that water scarcity significantly constrains the BTH region's goal of becoming a world-class megalopolis. Achieving the benchmarks requires either drastic water conservation efforts or a redefinition of the megalopolis's boundaries. The study highlights the trade-offs between economic development goals and environmental sustainability. The effectiveness of water conservation measures depends on their joint implementation. Adjusting the BTH megalopolis boundary is a more feasible option that balances economic development and water resources. This offers valuable lessons for other water-scarce regions aiming for similar urban development goals. The study acknowledges potential underestimation of the water challenge due to using annual average water resources and ideal simulation conditions.

This study shows that water scarcity significantly hinders the BTH region's development as a world-class megalopolis. Reaching the goal necessitates either intensive water conservation measures or a strategic boundary adjustment. The findings provide vital references for policymakers in water-stressed regions. Future research should consider the trade-offs between water conservation and other societal impacts, including climate change and consumption patterns.

The study may underestimate the water challenge because it uses annual average water resources, potentially overestimating water availability. The model's simulation provides an 'ideal circumstance' result, which may not reflect real-world achievability. Further, the study uses 2012 data, which may not fully capture recent changes in water use or conservation efforts.