本文基于吴淞、高桥和宝钢测站不同年代盐度观测资料, 以及吴淞水厂、陈行水库和青草沙水库取水口实测盐水入侵天数, 结合大通径流量和长江口卫星遥感河势变化, 分析了20世纪70年代至今长江口盐水入侵演变过程. 吴淞、高桥和宝钢测站实测枯季盐度变化表明, 20世纪70年代长江口盐水入侵严重, 20世纪 80 年代盐水入侵趋弱, 1990—1996年盐水入侵减弱. 从盐度变化过程看, 盐度峰值宝钢测站先于吴淞测站, 吴淞测站先于高桥测站, 表明盐水入侵来源于上游北支盐水倒灌. 吴淞水厂、陈行水库和青草沙水库取水口各年盐水入侵次数表明, 1974—1981年长江口盐水入侵非常严重, 尤其是1974年、1979年和1980年, 吴淞水厂取水口盐水入侵天数均超过了70 d. 1982—1995年是长江河口盐水入侵较弱时期. 1996—2002年是长江河口盐水入侵增强时期, 1996年、1999年和2001年发生严重的盐水入侵. 2003—2020年是长江河口盐水入侵显著减弱时期. 2003年三峡水库建成及之后长江流域上游梯级水库的建设, 枯季径流量显著增加导致盐水入侵减弱. 从1974—2013年长江口卫星遥感图显示的河势变化上看, 20世纪70年代北支还是开阔的河道, 随着永隆沙、兴隆沙、新村沙的相继圈围并岸, 以及北支下段南侧滩涂的围垦, 北支明显变窄, 纳潮量减小, 导致在长时间尺度上北支盐水倒灌逐渐减弱. 这也从北支河势变化上说明了20世纪70年代盐水入侵严重, 之后尤其是21世纪以来盐水入侵趋弱的原因. 径流量和河势变化是长江口盐水入侵长期变化的主因, 长江口盐水入侵减弱有利于长江口水源地淡水资源保护.

In this study, we analyzed the evolution of saltwater intrusion in Changjiang Estuary since the 1970s based on: salinity data collected at the Wusong, Gaoqiao, and Baogang stations; days of saltwater intrusion at the water intakes of the Wusong water plant, Chenhang reservoir, and Qingcaosha reservoir; river discharges at Datong station; and satellite remote sensing data of estuarine topography changes. The measured salinity changes at Wusong, Gaoqiao, and Baogang stations in the dry seasons showed that the saltwater intrusion in the Changjiang Estuary was serious in the 1970s, became weak in the 1980s, and was weak from 1990 to 1996. The peak salinity at Baogang station occurred prior to Wusong station, and the peak salinity at Wusong station occurred prior to Gaoqiao station; these observations indicate that the saltwater intrusion originated from upstream saltwater spilling over from the North Branch. The annual days of saltwater intrusion at the water intakes of the Wusong water plant, Chenhang reservoir, and Qingcaosha reservoir indicate that the saltwater intrusion was serious from 1974 to 1981 and particularly acute in 1974, 1979 and 1980; in these cases, the days of saltwater intrusion at the water intake of Wusong water plant exceeded 70 days. The saltwater intrusion was relatively weak from 1982 to 1995. The saltwater intrusion intensified from 1996 to 2002, and serious saltwater intrusion occurred in 1996, 1999, and 2001. The saltwater intrusion from 2003 to 2020 decreased significantly. The construction of the Three Gorges reservoir in 2003 and the cascade reservoirs in the upper reaches of the Changjiang Basin after 2003 resulted in a significant increase in river discharge during the dry season; this phenomenon was the main driver for the weakening saltwater intrusion. The changes in estuarine topography from 1974 to 2013 were detected by satellite remote sensing images; in particular, the North Branch was a wide river in the 1970s. With the successive reclamations of Yonglongsha, Xinglongsha, and Xincunsha, as well as the reclamation of the south shoal in the lower reaches of the North Branch, the North Branch became narrow and the tidal capacity decreased; the sequence of events subsequently led to the gradual weakening of saltwater spillover from the North Branch into the South Branch in a long time scale. The topography changes of the North Branch also explain the drivers for the serious saltwater intrusion that occurred in the 1970s and the relative weakening of saltwater intrusion over time, particularly since the beginning of this century. River discharge and estuarine topography changes are the main drivers for the long-term changes in saltwater intrusion in the Changjiang Estuary. With the construction of more reservoirs in the upper reaches of the Changjiang River and further shrinkage of the North Branch, saltwater intrusion will continue to weaken. These changes are conducive to the safety of freshwater resources in the Changjiang Estuary.