Water diversion and sea-level rise: Potential threats to freshwater supplies in the Changjiang River estuary

The densely-populated mega-city of Shanghai relies increasingly on freshwater from the Changjiang estuary (70% now). However, this strategy is facing potential threats due to extensive water diversion in the lower Changjiang basin and future sea-level rise. Given this, the present study evaluates the ability of Shanghai to source its water from the estuary, especially in the dry season. Flow <15,000 m³ s^-1, which occurs for ca. 50% of dry seasons, represents the threshold for salinity 0.45psu (chloride 250 mg/L) above which the estuary is unusable for freshwater. Correlating discharge and salinity, maximum salinity and related time duration, and taking the future water diversions and sea-level rise into consideration, we extrapolated salinity events into the future at intervals of 10years until 2040. We estimate that water diversions of 56.2 × 10⁹ m³ (1800 m³ s^-1), 59.2 × 10⁹ m³ (1900 m³ s^-1) and 61.3 × 10⁹ m³ (2000 m³ s^-1) will occur in 2020, 2030 and 2040, and a rise of sea level of 0.12m by 2040 (from 2010), equivalent 506 m³ s^-1, ca. 19.4% of the total reducing discharge of 2040 into the estuary (ca. 28% projected to the worst case of February of 2040). Based on scenario building, the pattern of salinity distribution would remain >0.45 for 20-65, 75-90 and 120-128days (in 2020, 2030, and 2040, respectively), for extreme low-flow conditions. These periods exceed the present 68-day maximum freshwater storage in Qingcaosha reservoir, which is meant to secure freshwater for Shanghai in the future. Urgently countermeasures are needed to secure the Shanghai's water in the future.