Hydrological Connectivity And Restoration Model Of Coastal Wetlands Based On Water Salinity Inversion

The coastal wetland is located in the land-sea interlaced zone,and it is one of the wetland ecosystems with the highest productivity and the most fragile ecosystem.Yancheng National Rare Birds Nature Reserve is located in Yancheng City,Jiangsu Province.It was included in the"World Natural Heritage List"in July 2019.It is the largest existing coastal wetland in China.With the development of society,in the coastal wetlands along the eastern coast of Jiangsu,a series of activities such as tidal flat reclamation,port construction and land reclamation have intensified the destruction and degradation of coastal wetlands.The report of the 19th National Congress of the Communist Party of China stated that it is necessary to accelerate the reform of the ecological civilization system and strengthen the protection and restoration of wetlands.Water is a key factor in the formation and development of wetlands.Hydrological connectivity is a key part of wetland restoration.In this study,the northern buffer zone of Yancheng National Rare Bird Nature Reserve was selected as the study area.Combine field survey data with GF-2 remote sensing data,and extract and invert water bodies and salinity in the area through BP neural network method.Based on the water body salinity quantification hydrological process connectivity,on this basis,combined with the hydrological structure connectivity indicators,the regional hydrological connectivity at different scales is evaluated and graded through the coefficient of variation method and the principal component analysis method.At the same time,it conducts hot spot analysis on hydrological connectivity evaluation at different scales to determine the restoration target,restoration priority and restoration mode of the wetland in the region.The research conclusions are as follows:(1)The area of water bodies in the region is 114.37km~2,accounting for 60.66%of the total area,and the main types are breeding ponds and ditches.The area of the aquaculture pond is94.45km~2,and the total number is 1686.It is mainly distributed in the northwest and east coast.The area of the ditch is 19.92km~2,and the total number is 1659.Among them,there are 837 inlet canals and 822 outlet canals.The supporting distribution of breeding ponds and farmland are all over the research area.(2)The inversion range of water salinity is between 0.05S/m-3.5S/m.The salinity of freshwater ranges from 0.05S/m-1S/m,and they are mainly distributed in the farmland in the northwest,farmland in the southwest and farmland in the northeast.The salinity of saline water is higher than 1S/m,and it is mainly distributed in the eastern coastal aquaculture ponds and surrounding ditches.The reason for the huge difference in salinity of water bodies in the region is the different types of land use.The inflow direction of the freshwater distribution area is the Subei main irrigation canal,and the land use type is mainly freshwater aquaculture ponds.In areas where saltwater bodies are distributed,the direction of the inflow of the ditches and channels is the ocean,and the land use type is mainly saltwater aquaculture ponds.(3)This study evaluates the hydrological connectivity of coastal wetlands from two perspectives:the internal scale of the region and the land-sea scale.From the evaluation results,the hydrological connectivity of the internal scale of the region is relatively good,and it is divided into four grades:excellent,good,medium,and poor.Excellent grade and good grade accounted for about 55%of the total area of the study area,medium grade accounted for 9.14%of the total area,and poor grade accounted for 35.36%of the total area.In terms of the distribution of the four levels and the types of land use,the connectivity of water bodies has a greater impact on them.The types of land that can store water have better hydrological connectivity at scales than those that cannot.The sea-to-land hydrological connectivity decreases from sea to land,but the rate of change is different.This is due to the huge difference in salinity of water bodies in the region.It is divided into first,second,third,and fourth levels from good to bad.The area where the first level is located accounts for 16.70%of the total area,the second level accounts for 27.01%of the total area,the third level accounts for 31.91%of the total area,and the fourth level accounts for 24.30%of the total area.(4)In this study,the coastal wetlands in the northern buffer zone were divided into the eastern coastal saltwater dominant recovery area and the inland freshwater dominant recovery area based on the salinity inversion results of the water body.According to the hotpot analysis results of hydrological connectivity of different scales,the priority of wetland restoration in the region is divided.The priority of restoration is from I to VII.The higher the priority,the lower the level.Among them,the I-level priority area overlaps with the salt water dominant restoration area.The total area of this level of the restoration priority area is 30.02km~2,accounting for 16.06%of the total area of the study area;the second-level priority area is the freshwater restoration area with the second priority.The total area is 16.59km~2,accounting for 8.87%of the total area;Level III priority area,with a total area of 26.50km~2,accounting for 14.18%of the total area;Level IV priority area,accounting for 26.71%of the total area;Level V priority area accounting for the total area The total area of the VI priority area is 11.52%of the total area,and the total area of the VII priority area is 12.88 km~2,accounting for 6.89%of the total area of the study area.On this basis,through correlation analysis,the smallest unit of wetland restoration in the area is determined to be a 150m×150m grid.On the basis of determining the minimum recovery unit,this study combined with the previous research conclusions,extracted the grids with water surface ratio higher than95%in the recovery unit as the easy recovery unit,and after excluding the scattered easy recovery units,the easy recovery unit in the study area was finally determined.The restoration areas are mainly distributed in Ⅰ-Ⅲ priority restoration areas.