Dynamic simulation-based assessment of water supply risks and emergency water demand in coastal cities

Frequent saltwater intrusion induced by extreme climate events poses significant challenges to water supply security in coastal cities. This study developed a supply–demand balance model for urban water supply systems based on the system dynamics (SD) method, employing the supply–demand gap and water stress index (WSI) as risk indicators. Dynamic simulations were conducted in Zhongshan City in China across five development and 17 saltwater boundary scenarios, and corresponding emergency water reserve requirements were proposed for emergency durations of 10–60 d. The results showed no supply–demand gaps from 2016 to 2023, although water supply was notably affected by saltwater intrusion. The highest risk occurred in 2021 when saltwater fronts reached the Renyi and Dafeng intakes, resulting in a peak WSI of 0.45. Water demand would peak in 2035 across all development scenarios. The economic development scenario exhibited the highest demand, the conservation development scenario the lowest, and the comprehensive development scenario ranked the second lowest, with the latter balancing economic and social development with resource conservation, enhancing its policy relevance. Across the 17 saltwater boundary scenarios, the conservation development scenario demonstrated the lowest WSI values (0–6.74) and water supply risk level, followed by the comprehensive development scenario (with WSI values of 0–7.11), while the economic development scenario demonstrated the highest WSI values (0–7.84) and water supply risk level. Under worst-case saltwater conditions with 60-d emergency reserves, supply–demand gaps in 2030 would reach 7.938 × 10 7 m 3 and 8.928 × 10 7 m 3 in the conservation and economic development scenarios, respectively, and increase to 10.164 × 10 7 m 3 and 12.354 ×10 7 m 3 by 2035. This methodology offers actionable insights for coastal cities to optimize development strategies and emergency water reserve planning.