Evaluation of Water Supply Risks and Emergency Water Volume in Coastal Cities Based on Dynamic Simulation

Abstract Water supply security is the basis for high-quality urban development. Amid escalating extreme climate events, the upstream intrusion of saltwater in the Pearl River Estuary poses significant challenges to the water supply security of cities. This study establishes a city water supply system model based on the system dynamics method, then evaluates the urban water supply risks in Zhongshan City from 2016 to 2023 using the supply-demand gap and water stress index as risk assessment indicators. Additionally, it simulates the dynamic water supply risks under five development scenarios (conventional, economic, population, conservation, and comprehensive) and 17 saltwater intrusion scenarios from 2024 to 2035. Based on the simulation results, the study then proposes the emergency water reserve requirements for 2030 and 2035 under emergency response levels of 10, 20, 30, 45, and 60 days. The results indicate that Zhongshan's water demand has increased from 2016 to 2023, with water supply capacity meeting demand but significantly impacted by saltwater intrusion. The highest water supply risk and water stress index (peaking at 0.45 in 2021) occur when the saltwater front reaches the Quanlu, Renyi, and Dafeng intake points simultaneously or individually. Among the five development scenarios, water demand peaks in 2035, with the highest demand under the economic development scenario and the lowest under the water-saving scenario. The comprehensive development scenario, which balances economic growth and resource conservation, achieves a 2035 water demand of 266.90×104 m³/d, a 5.19% reduction compared to the conventional scenario, thus making it more acceptable to policymakers. Under the 17 saltwater intrusion scenarios, the water-saving scenario exhibits the lowest water stress index (0-6.74) and thus the lowest water supply risk, followed by the comprehensive scenario (0-7.11) and the economic scenario with the highest water stress index (0-7.84) and risk. The current water reserves can usually meet demand under scenarios where the saltwater front only reaches below Quanlu or Dafeng, but additional emergency reserves will be necessary for higher intrusion levels to meet various emergency duration requirements. When the saltwater front reaches Yinggezui and a 60-day emergency level is considered, the water-saving scenario has the smallest water deficit, while the economic scenario has the largest, with deficits of 7,938×104 m³ and 8,928×104 m³ in 2030 and 10,164×104 m³ and 12,354×104 m³ in 2035, respectively. These findings offer valuable insights for coastal city policymakers in selecting urban development paths and enhancing emergency water reserve capabilities.