Impacts of major Yangtze River Basin projects on river discharge and estuarine salt intrusion

Basin-scale projects can significantly affect estuarine salt intrusion and freshwater resource utilisation. In the Yangtze River Basin, extensive reservoir construction has created a total effective storage capacity of 91.67 billion cubic metres. Together with water diversion by the South-to-North Water Diversion Project, these projects have induced a pronounced seasonal redistribution of river discharge. The river discharge at the Datong hydrological station shows a reduction of 4,200, 9,300, and 9,400 m3/s in August, September, and October, respectively – a change substantially influenced by these projects. The numerical simulation results reveal that under climate winds, the reduced river discharge from August to October exacerbates salt intrusion in the Yangtze Estuary but has not yet threatened the water intake safety of estuarine reservoirs. However, under adverse wind conditions (particularly typhoons in October), the river discharge reduction can significantly intensify salt intrusion and endanger freshwater supply. Conversely, reservoir releases from January to April effectively weaken salt intrusion. Under climate winds, the regulated discharge eliminates salt intrusion effects on estuarine reservoirs. Even during adverse winds, the increased river discharge substantially mitigates salt intrusion and reduces the unsuitable water intake time of the Qingcaosha Reservoir by over 40%. In summary, major basin projects secure freshwater availability in the estuary through seasonal river discharge redistribution, but caution is needed regarding the elevated salt intrusion risk caused by reduced river discharge at the end of the flood season.