Water supply systems are critical components of urban infrastructure and significant contributors to global carbon emissions. These systems face an emerging challenge in balancing the increasing demands of water security with international climate mitigation goals. To combat water scarcity, many regions have transitioned toward energy-intensive water sources such as inter-basin water transfer and desalination, which significantly increase electricity-dependent indirect emissions. Concurrently, the global shift toward clean energy in electricity generation has provided a crucial mechanism for mitigating these emissions. However, the complex interactions among shifting water-source mixes, energy transitions, and socioeconomic drivers remain poorly understood, often obscuring the effectiveness of decarbonization strategies. Existing quantification frameworks frequently overlook the spatial spillover effects of economic development and the risk that new water security strategies will offset decarbonization gains. Here we show that China's carbon emissions from water-supply processes rose to 228 Mt CO₂ yr⁻¹ by 2022, despite initial declines driven by clean energy expansion. Using a three-stage quantification–decomposition–attribution framework, we find that while economic development generally suppresses emission in neighboring regions via technology diffusion, it exhibits a national U-shaped relationship with carbon output. Crucially, central China displays an inverted U-shaped pattern, suggesting a localized risk of high-carbon lock-in as industries and water demands shift. These findings reveal a critical paradox in the water–energy–carbon nexus where water security measures may inadvertently undermine climate targets. Our results advocate for integrated regional governance and differentiated policy interventions to safeguard both water and climate stability in rapidly developing regions.

In order to quantitatively reveal the influence of the implementation of the contour interception project on water system structure and function in the river network area in Donghai County, taking the Longliang River-Shian River network area in Donghai County as the research object, a comprehensive analysis method of water system structure and connectivity at regional and nodal levels was proposed based on the analysis method of water system morphological structure, graph theory and landscape ecology method, and complex network method. The morphological structure and connectivity changes of the river network area water system before and after the implementation of the contour water interception project were calculated. The results showed that, in terms of water system structure, the number and length of rivers increased by 76.39% and 77.95%, respectively, after the implementation of the contour interception project; in terms of regional connectivity, the contour interception project connected isolated river networks laterally, increasing the water system circularity, node connection rate, and hydrological connectivity indexes of the regional water network by 101.29%, 37.27%, and 35.23%, respectively, and significantly improving the overall structural connectivity of the water system. The nodal function connectivity was significantly improved, and the spatial distributions of node propagation ability and node importance were optimized due to the bidirectional connection ability of the contour interception project.

Taking the Rushan River Basin in Shandong Peninsula as an example, a conjunctive regulation model of surface and underground reservoirs with multiple time scales of hours, days, and months was constructed. A floodwater resources utilization scheme through conjunctive regulation of surface and underground reservoirs was proposed based on the optimization of regulation rules of surface reservoir discharge, river gate and dam water level controlling, and surface and underground reservoir water supply. The results show that, through conjunctive regulation of surface and underground reservoirs to raise the river control level, increase the proportion of underground reservoir water supply, and increase the infiltration of rivers before floods, the excess water from surface reservoirs can be transferred to underground reservoirs timely, which can empty the storage capacity of the surface reservoir and effectively increase the amount of floodwater resources utilization in the watershed. Under the conjunctive regulation scheme, the floodwater resources utilization in the Rushan River Basin increased by 8.4696 million, 7.2877 million, and 1.0635 million m³ , respectively, in 2007 (wet year), 1994 (normal year), and 1967 (dry year), verifying the practical application effect of the conjunctive regulation model of surface and underground reservoirs.

In response to the lack of joint regulation methodsof the complex engineering system of surface-underground reservoirs in coastal small and medium-sized river basins, which leading to difficulties in flood resources utilization, the structure and function of surface-groundwater reservoir system was analyzed, the surface reservoir regulation and dynamic processes of rivers and groundwater under gate dam regulation were quantitatively simulated, and the intelligent replacement models to improve the calculation speed of hydrodynamic processes were used.A multi-time scale model coupling nesting method of hour/day/month was developed to construct a joint regulation model of surface-underground reservoirs that takes into account calculation accuracy and optimization efficiency. This model realized the unified allocation of floods, surface groundwater interactions, and water supply, which can provide new ideas and technical support for the utilization of floodwater resources in coastal small and medium-sized river basins.

For the reservoirs used to regulate local surface water resources, new dispatching rules need to be formulated due to the influence of multiple water sources to improve the water resource utilization efficiency. Considering the low regulation and storage capacity of unconventional water, water transfer and supply rules are set for the reservoirs in the water receiving area on the basis of priority allocation of unconventional water, and the reservoir optimal operation model considering the efficiency of water transfer and supply is established. The ideal point method and genetic algorithm are used to optimize the operation rules. Taking the Mishan Reservoir in Weihai City as an example, the results show that compared with the standard operation strategy, the optimized reservoir operation rules reduce the average annual water transfer by 26.924 million m³ and the annual average waste water by 4.175 million m³. The utilization ratio of natural incoming water and external transferred water is improved. The dependence of external water transfer in dry years is higher than that in wet years and normal years, and the average water transfer volume is only reduced by 3.98 million m³. Under this regulation, the assurance rate of agricultural water supply can be increased from 75% to 88.4% by reducing the amount of external water transfer and increasing the allocation of unconventional water.

In order to reveal the impact of underground reservoir construction and extraction methods on the saltwater and freshwater transport law in coastal aquifers, a 300 m × 900 m × 30 m conceptual numerical model was established. The variable density current simulation method of unconfined aquifer was used to analyze the saltwater and freshwater transport law in coastal aquifer under different extraction scenarios before and after the construction of underground dam. The results show that the construction of underground dam can significantly reduce the scope and degree of seawater intrusion. Groundwater extraction can accelerate the intrusion and diffusion of residual saltwater in underground reservoirs within a certain time range. The coefficient of saltwater intrusion is exponentially related to extraction scale and the distance between extraction wells and underground dams. Reasonable layout of the location and scale of extraction wells can effectively reduce the negative impact of groundwater extraction on water salinity in the reservoir area.