Hydrological-ecological Responses Of Riparian Zone Of Grassland Meandering River Based On Connectivity

Connectivity refers to the flow and distribution of water,sediment,energy and organisms in the region.Connectivity is a key indicator to evaluate the ecological health of rivers and regions.In view of the current lack of understanding of the hydrological connectivity of inland rivers in semi-arid areas,especially grassland meandering rivers,and the unclear relationship between hydrological connectivity and ecological response in the riparian zone,this paper selects the typical grassland meandering river-the upstream and downstream reaches of Xilin River as the study object.In the summer flood season of 2021 and 2022,topographic survey,hydrological dynamic monitoring and grassland vegetation community survey were carried out in the riparian zone.Path analysis and redundancy analysis were used to explore the hydrological connectivity and ecological response relationship of the meandering river sections.The results show that :(1)In terms of time dimension,the wet year(2021)is mainly characterized by river water recharge to groundwater,and the normal year(2022)is characterized by groundwater weakly recharge to river water.In wet years,environmental factors(EC,p H,DO,etc.)have a high degree of response to hydrological connectivity,while in normal years,the local distribution of environmental factors is significantly different and is not sensitive to hydrological connectivity.The rainfall and flood events during the summer flood season in the wet year and the normal year both increased the connectivity between the river and the riparian zone,which was a key period to drive the redistribution of nutrients in the riparian zone.However,in the wet year,the difference in hydrological connectivity between the longitudinal upstream and downstream increased,and the difference in hydrological connectivity between the transverse concave bank and the convex bank decreased.The degree of connectivity between the near and far riverbanks was enhanced,and the consistency of hydrological elements was high.In contrast,the difference of vertical hydrological connectivity in normal years decreases,and the difference of hydrological connectivity between horizontal concave and convex banks,far and near regions increases.(2)Along the longitudinal direction of the river,the hydrological connectivity of the upstream meandering river section is significantly stronger than that of the downstream.The upstream water level fluctuates frequently,and the lateral exchange between river water and groundwater is also more active.The exchange rate q is-0.05～ 0.28 m / d,while the exchange rate q of the downstream meandering river section is-0.15 ～ 0.14 m / d.The electrical conductivity(EC)and p H increased from upstream to downstream,the dissolved oxygen(DO)concentration decreased,and the soil total nitrogen(TN)and total phosphorus(TP)decreased slightly.(3)Along the transverse direction of the vertical river channel,the hydrological connectivity of the convex bank(inside)of the meandering river channel is stronger than that of the concave bank(outside).The hydrological connection between convex bank groundwater and river water is closer,while the concave bank is mainly affected by regional groundwater.The fluctuation of groundwater in the convex bank and the concave bank is more active in the near bank(5m)than in the far bank(10m).The environmental factor data in the area with high connectivity are closer to the normal distribution,and the outliers are less than those with weak connectivity.(4)The connectivity degree and ecological response of different hydrological years are obviously different.The area with high vertical hydrological connectivity has large biomass and rich vegetation diversity.The upstream hydrological connectivity is strong.The riparian vegetation has many species,large coverage and low herbaceous vegetation.The average biomass in wet and normal years is 101 g and 37 g,respectively.The downstream hydrological connectivity is weak,the vegetation type is single,the coverage is small,the growth is high and sparse,and the average biomass in wet and normal years is 54.5 g and 14.5 g,respectively.In the transverse direction,due to the long flooding time of the convex bank and the low growth of vegetation,the average biomass of the convex bank in wet and normal years is smaller than that of the concave bank.The sensitivity of hydrological connectivity factors from the greatest to the smallest is EC>p H>q>h(groundwater depth)>DO>TP>TN.The results provide a scientific reference for the restoration of grassland river corridor ecosystem by using the connectivity principle.