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 m(3) s(-1), which occurs for ca. 50% of dry seasons, represents the threshold for salinity 0.45 psu (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 10 years until 2040. We estimate that water diversions of 56.2 x 10(9) m(3) (1800 m(3) s(-1)), 59.2 x 109 m(3) (1900 m(3) s(-1)) and 61.3 x 109 m(3) (2000 m(3) s(-1)) will occur in 2020, 2030 and 2040, and a rise of sea level of 0.12 m by 2040 (from 2010), equivalent 506 m(3) 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 120-128 days (in 2020, 2030, and 2040, respectively), for extreme low-flow conditions. These periods exceed the present 68-day maximum freshwater storage in Qing-caosha 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. (C) 2014 Elsevier Ltd. All rights reserved.