中文摘要：

本文应用本系列论文Ⅱ中建立的长江河口水动力和盐水入侵三维数值模式,模拟长江河口20世纪50年代、70年代和2012年盐水入侵,定量分析不同年代河势下盐水入侵状况和变化程度及其原因。在北支,不同年代盐水入侵的变化是由分流比和潮差共同作用造成的。50年代北支盐水入侵较强,70年代大幅下降,中上段出现淡水,2012年盐水入侵极为严重,整个北支被高盐水占据,上段出现强烈的盐度锋面。50年代和2012年,北支盐水倒灌南支,大潮期间远大于小潮期间,2012年远强于50年代,70年代没有北支盐水倒灌南支现象。在南支,50年代、70年代南支大部分为盐度都小于0.45的淡水,在2012年大潮期间由于出现了强烈的北支盐水倒灌,南支上段出现盐度大于0.45的盐水。在南北港,在50年代盐水入侵最严重;大潮期间,北港净分流比南港大21.6%,北港盐度小于南港盐度,外海盐水主要通过南港入侵,出现南港盐水倒灌进入北港的现象。至70年代,南支主流转向南港,南港净分流比增大,比北港大10.4%,南港盐度明显小于北港盐度;南北港盐水入侵较弱。在2012年,南支主流再次转向北港,北港分流比比南港大10.4%,南港的盐水入侵再次强于北港。小潮期间,50年代由于南港分流比相比于大潮时更小,南港盐水上溯距离更远,上段盐度比更大;至70年代,北港分流比减少,盐水入侵减弱;至2012年,由于大潮时期北支倒灌的盐水在小潮期间到达北港,北港净盐通量比大潮时期大。由于潮动力减弱,小潮期间各年代垂向盐度分层更明显,盐水入侵变化与大潮期间一致。

英文摘要：

The 3D numerical model of the Changjiang Estuary built in the series paperⅡwas used to simulate the saltwater intrusion in 1950 s,1970sand 2012,and the changes of saltwater intrusion under different river regimes and their reasons were analyzed quantitatively in this paper.In the North Branch,the saltwater intrusion were controlled by the water diversion ratio and tidal range.In 1950 sand 2012,saltwater spilled over from the North Branch to the South Branch and the magnitude was greater during spring tide than during neap tide,and was greater in 2012 than in 1950 s.This phenomenon didn＇t appear in 1970 s.In 1950 s,saltwater intrusion was severe in the North Branch.In 1970 s,it decreased greatly,and a large amount of fresh water appeared in the upper and middle reaches.In 2012,the saltwater intrusion was very severe and the whole North Branch was occupied by saltwater,sharp salinity front appeared in the upper reaches.In the South Branch,fresh water occupied the most area in 1950 sand 1970s,and in 2012 saltwater with salinity greater than 0.45 appeared in the upper reaches because saltwater spilled over from the North Branch.In the North and South Channel,saltwater intrusion in the 1950 s was the most severe.During the spring tide,the net water diversion ratio in the North Channel was higher than that that in the South Channel by 21.6%,which caused the salinity in the North Channel was lower than that in the South Channel,resulting in the saltwater from the open sea intruded mainly through the South Channel,and returned over from the South Channel to the North Channel.In 1970 s,the mainstream of the South Branch turned to the South Channel,which caused the net water diversion ratio in the South Channel increased,and became higher than that in the North Channel by 10.4%,causing the salinity in the South Channel was lower than that in the North Channel;the saltwater intrusion was weakest in the South and North Channel.In 2012,the mainstream of the South Branch turned to the North Channel again and the net water