| The Yangtze River is an important geographical unit of China,especially in its lower reaches and its estuary.Facing the increasing pressure of economy and population,a series of environmental problems have arisen.Environmental management and protection are extremely urgent.It is of great significance to carry out the research on the lower reaches of the Yangtze River to the estuary.Many scholars have used hydrological data,satellite data and numerical models to carry out research on river runoff,islands and saltwater intrusion.However,the research on long-term Yangtze River runoff variation,quantitative analysis of river island evolution,and high-resolution model simulation of long-term saltwater intrusion are still rare.The explanation of their change mechanism is not very clear.In this paper,the long-term variation of the runoff measured by Datong Station in the lower reaches of the Yangtze River is analyzed in detail.The moving average method is used to remove the noise.It is found that the runoff fluctuations from 1937 to 2016 are severe and the annual average runoff in 80 years is 908.561 billion m~3.It shows a decreasing trend overall reaching an annual average reduction of 1.0877 billion m~3.The annual runoff reduction before and after the Three Gorges Reservoir built is 1.1639 billion m~3 and 6.4412billion m~3 respectively.The impoundment of the Three Gorges Reservoir has accelerated the rate of runoff reduction.It is found that there are three runoff change cycles of 11.9,7.3 and2.5 months by power spectrum analysis method.It is found that the change of runoff in the lower reaches of the Yangtze River was mainly caused by the change of precipitation.After El Ni?o years,precipitation will increase abnormally resulting in an increase in runoff,and factors such as temperature and PDO have less impact on runoff.Based on the observed runoff,sediment discharge and precipitation data,river islands in two typical regions of the lower reaches of the Yangtze River from 1985 to 2015 are extracted from Landsat4-5 TM(Thematic Mapper)and Landsat-7 ETM+(Enhanced Thematic Mapper)remote sensing images.They are the 7 islands in the Nanjing section and the 4 islands in the Yangzhong section.The results show that the island area in Nanjing section decreased from1985 to 2015 with an average annual decrease of 19.97 hectares However,its rate of decline has gradually slowed down,while the overall area of islands in Yangzhong section has increased showing the characteristics of first growth and then shrinking.The biggest influencing factor on islands in these two regions is sediment discharge.The islands in Yangzhong section are more affected by ocean tide than the islands in Nanjing section.Human influence has shown an upward trend in recent years.The high-resolution FVCOM(An Unstructured Grid,Finite-Volume Coastal Ocean Model)model of the study area is constructed to simulate the full-depth flow field and salinity distribution.Salinity shows the characteristics of"high-low-high"in terms of time.The saltwater intrusion distance in the Yangtze River estuary from 1993 to 2012 shows an intensifying trend.The intrusion distance in 1998 reached the lowest value.From 2001 to2008,the distance increased 9 kilometers.From the vertical section structure of the intrusion,in summer,due to the heating of the water in the sea surface,the stratification is enhanced and the mixing is weakened.The entire intrusion process exhibits a certain periodicity.Power spectrum analysis of the saltwater intrusion distance reveals that the period under 99%confidence is 341-410 days,27 days,23.95 days,13.65-16.13 days,and 4.51-9.63 days,which caused by runoff,tide,and both of pressure fluctuations and tide.It is found that runoff is the biggest influencing factor,followed by meridional wind,and the northerly wind prevailing in winter caused obvious Ekman transport to the land direction,which aggravated saltwater intrusion,while the impact of zonal wind and tide on the intrusion distance is small. |
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