The connection of urban surface water bodies brings great challenges to the unified management of urban water resources. Despite this, many cities in the world are still actively promoting the connectivity of rivers and lakes, as the connectivity of water bodies is of significant practical significance to cities. The known benefits of this connection are to comprehensively improve water supply security, optimize the allocation of water and soil resources, as well as to improve water ecological restoration capabilities. The connection of surface water bodies is simply to reshape or build new water flow connection channels on the basis of natural and artificial rivers, lakes, and reservoirs, so as to create healthy hydrological cycles, material cycles, and energy cycles in the urban areas. Nevertheless, the structure and form of these connections are very diverse and complex, including river-river connections, river-lake connections, river-reservoir connections, lake-reservoir connections, and reservoir-reservoir connections, as well as the connection between rivers, lakes, and reservoirs and connection between water areas such as cities, wetlands, and irrigation areas.

A complex ring-shaped river network is usually formed after the surface water bodies are connected in some large international cities. By artificially changing its natural flow direction, the water in the river network can be "circulated". However, this synchronously results in the extension of the water flow path and the increase of the regulation dimension, and that makes the urban water system difficult to manage as a whole. Especially after the 1990s, the urban surface ring-shaped river networks and the complex underground drainage systems have become gradually integrated, and the urban water system has changed in form and structure, gradually forming a more complex system structure. In addition, major cities in the world are currently still expanding their capacity, which is accompanied by the continuous increase of water supply sources in cities, the continuous expansion of urban water circulation paths, and the increasingly complex water circulation structures, which further exacerbates the operation of complex water systems. Modeling, analysis, and simulation pave the cornerstone for designing complex urban water systems that is essential and crucial to the effective control of such systems. In the 1990s, the theory of complex adaptive systems was proposed, and the application of computer technology to the design of urban water systems promoted the construction of urban water systems with more complex structures and powerful functions. Since 2000, the rapid development of artificial intelligence technologies, such as computer vision, machine learning, and big data, has promoted the transformation of management methods for complex urban water systems.

This Special Issue aims to promote research on urban complex water systems, with the goal of bringing together the latest developments in global research in the modeling, analysis, simulation, and control of urban complex water systems. Suitable papers include innovative research in all aspects of urban water system planning, construction, waterfront space management, and water ecological environment governance and restoration, as well as ideas, technologies, and concepts in the planning, construction, operation, and management of urban complex water systems. We welcome original research and review articles.

Potential topics include but are not limited to the following:

• Research and practice on the integration of riverside urban agglomerations and ecological belts in major river basins
• Exploration and practice of people-water-city integrated urban water system planning
• Adaptive planning and simulation applications for urban water systems from the perspective of resilience
• Multi-objective urban waterfront landscape planning and designs
• Hydrodynamic analysis, holistic simulation methods, and water quality simulation models for complex water systems (ring-, tree-, fan-shaped, etc.)
• Refined configuration of complex urban water resource systems
• Multi-source joint dispatch models and applications for complex urban water supply systems
• Control of complex urban water systems, such as joint dispatch and automatic control of sluices, pumping stations, and reservoirs
• Urban flood control and drainage under complex meteorological conditions
• Monitoring, early warning, and fault diagnosis of complex urban water systems
• Health evaluation index and comprehensive evaluation methods for complex urban water systems