Study On The Change Of Water-sediment Characteristics And Its Impact On Agricultural Landscape Under The Water-sediment Regulation Scheme Of Xiaolangdi Dam In The Lower Yellow River

The operation of dams and the implementation of the water-sediment regulation scheme(WSRS)drove the hydrological and geomorphological processes of downstream surface water and groundwater,and affected the development of agricultural landscapes outside the channel.However,few researches on dam operation affected the changes in agricultural landscape patterns outside the channel through the evolution of surface and groundwater hydrological processes.The Xiaolangdi Dam(XLD),located at the last gorge in the middle reaches of the Yellow River,has changed the hydrology and geomorphological processes of the lower Yellow River,affecting the relationship between surface water and groundwater,while satisfying the functions of sediment mitigation,flood control,agricultural irrigation and hydroelectric power generation in the downstream.Thus indirectly driving the evolution of agricultural landscape outside the river.Therefore,in this study,the agricultural landscape in the lower Yellow River from Huayuankou(HYK)to Jiahetan(JHT)and beyond the reach were taken as the research object.Based on water and sediment,meteorological,topographic and agricultural data,using correlation analysis,machine learning,IHA-RVA,distributed lag non-linear model(DLNM),variance partitioning analysis(VPA)and landscape pattern index,this study analyzed the relationship between surface water and channel morphology,between surface water and groundwater,and the influence of surface water and groundwater on agricultural landscape in typical suspended reaches of the lower Yellow River influenced by the WSRS of XLD.In order to clarify the impact of XLD operation on the hydrological and ecological processes of typical suspended reaches of the lower Yellow River,which has important theoretical and practical significance for realizing the management and governance in watershed,the development and utilization of water resources and the sustainable development of ecosystem.The specific results are as follows:(1)XLD operation has changed the water and sediment process and channel geomorphology in the lower Yellow River.Under the influence of XLD operation,the runoff(HYK: 42.62%;JHT: 42.19%)and water level(HYK: 90.40%;JHT: 44.04%)in the lower Yellow River have changed greatly.In the interannual variation,water levels at the same flow rate,suspended sediment concentrations(SSC),sediment budget and incoming sediment coefficient(ISC)decreased,and the channel morphology of the main channel became narrower and deeper.In the seasonal variation,the SSC in the pre-dam period has a significant seasonal variation,but SSC weakens or disappears in the post-dam period.The seasonal variation of water level of the same flow rate was not great after dam construction.Within a given hydrological year,the cross-sections varied in width and depth.In the post-dam period,the cross-sectional geomorphic coefficients decreased and the river channel deepened in the post-WSRS season.In the relationship between water and sediment characteristics and river geomorphic characteristics,the results of random forest model show that water level(WL)and SSC were the main factors influencing channel geomorphic evolution.However,the main factors vary according to season and reach.ISC and SSC were the main factors influencing channel geomorphic evolution in the pre-WSRS season at Huayuankou and Jiahetan,respectively.The SSC was the most important factor at both stations in the post-WSRS season.(2)The XLD operation changed groundwater in the lower Yellow River.The main performance is that groundwater level near the Yellow River decreased significantly,especially,the groundwater level in June decreased significantly at the 0.05 level.The highest groundwater levels at the four monitoring wells were all observed in August during the pre-dam period,which are consistent with the flow rate(FR),SSC,WL,and ISC.By contrast,in the post-dam period,the highest groundwater levels were observed in October,while the highest FR and WL values were observed in June,and highest SSC and ISC values were observed in July.Based on the extreme random tree model,this study found that,between the post-dam to pre-dam periods,changes in WL and ISC contributed most significantly to changes in groundwater level in annual and in the mid-and post-WSRS seasons,while WL and SSC contributed most significantly in the pre-WSRS season.Between the mid-WSRS and post-WSRS seasons,changes in SSC and the combined effect of water-sediment contributed the most to changes in groundwater level along the lower Yellow River in pre-dam and post-dam periods,respectively.In general,the groundwater in the pre-dam period was sensitive to changes in water and sediment characteristics,the response of monthly groundwater level to changes in water and sediment characteristics lagged by 2-5 months.(3)Since 2000,the ecological environment of the lower Yellow River has changed.The FR,temperature and grain output gradually increased,while the SSC,WL,groundwater level,rainfall,evaporation,regional effective irrigation area and the proportion of farmland gradually decreased.The agricultural landscape affected by the environment has also changed accordingly.This mainly manifested in the increase of dry land,river canals,reservoirs and ponds,and construction land,and the decrease of paddy field,forestland,grassland,and beach land.The patch density(PD)and edge density(ED)increased(0.3;9.33),while the perimeter-area fractal dimension(PAFRAC)and aggregation index(AI)decreased(-0.12;-0.47).In the relationship between environmental change and agricultural landscape,SSC was significantly correlated with PAFRAC,and river distance was significantly correlated with ED(p < 0.05).In general,the hydrological factors(44.88%)was the main factor affecting the change of agricultural landscape,followed by meteorological factors,topographic factors and agricultural factors.