| Xiaolangdi Dam is a comprehensive water conservancy project focusing on flood control,silt reduction,and anti-cold,while taking into account power generation,irrigation,and water supply.The operation of the Xiaolangdi Dam changed the natural runoff pattern of the lower Yellow River,shaped a brand-new relationship between water and sand,and also had a new impact on the agricultural landscape pattern and agricultural production practices in the middle and lower reaches of the Yellow River.Therefore,the study on the characteristics of changes in water and sediment regimes in the lower reaches of the Xiaolangdi Dam before and after the operation,clarifies the response of the agricultural landscape pattern and agricultural production practices in the lower reaches of the Yellow River to the changes in the water and sediment regulation of the Xiaolangdi Dam,and it is useful for realizing the regional agricultural production potential and building sustainable agricultural landscape pattern.This study takes the typical area downstream of the Xiaolangdi Dam as the research object,adopts the Mann-Kendall trend test method,mutation test method,and single factor variance method to analyze the change characteristics of the water and sediment regime in the downstream of the dam before and after the operation.The land utilization dynamic attitude,land use transfer matrix and landscape pattern index analyze the change characteristics of agricultural landscape pattern in the middle and lower reaches of the Yellow River,and use Pearson correlation analysis to establish the correlation between hydrological elements and landscape pattern changes.The single factor variance method and questionnaire survey method are used to analyze the difference of agricultural productivity level and agricultural production environment before and after the dam operation,and the relationship between agricultural production factors and various influencing factors is established through redundant analysis method.Based on the current operating mechanism of the dam,the FLUS model is used to predict the development of the agricultural landscape in the study area under different scenarios in the future.The main findings are as follows:(1)After the operation of the Xiaolangdi Dam,its downstream water and sediment regime changes have obvious characteristics.After the operation of the dam,the amount of water and sediment in the lower reaches of the Yellow River decreased,and the inter-annual and intra-year fluctuations in water and sediment were gentle;the average monthly runoff increased after the operation of the Xiaolangdi Dam in May,June,and July.Decrease,with the most significant changes in June and August;Before the operation of the dam,the high monthly average sediment concentration of the lower Yellow River appeared in July,August,and September,and the high monthly average sediment concentration appeared in May,July,and August after the operation of the dam;The operation of the dam slowed down the downward trend of annual runoff and runoff during the flood season in the lower Yellow River,and the runoff during the spring irrigation period increased significantly.(2)Before and after the operation of the Xiaolangdi Dam,the cross-sectional shape of the downstream river channel and the water environment outside the river channel have undergone significant changes.Before the operation of the Xiaolangdi Dam,the main channel section of the lower Yellow River was wide and shallow,and sedimentation caused the river bed to continue to rise.After the operation of the Xiaolangdi Dam,the lower reaches of the Yellow River have been continuously scoured,and the channel has developed in a narrow and deep direction,resulting in a significant drop in the downstream water level.The lower reaches of the Yellow River saw a drop in the water level at the same flow rate;a drop in the groundwater level along the Yellow River;a misalignment of the supply and demand time for irrigation from the Yellow River;and a decline in the water diversion capacity of culverts along the Yellow River.(3)Before and after the operation of the Xiaolangdi Dam,the agricultural landscape pattern in the downstream area changed significantly,and it was closely related to the hydrological changes of the Yellow River.From 1990 to 2018,the overall dynamics of land use in the study area showed a downward trend and then an upward trend.The size relationship is: the dam has the largest dynamic change during the stable period of water and sediment regulation(2009-2018),the dynamic change in the early stage of dam operation(1990-2000)is medium,and the initial stage of dam water and sediment regulation(2000-2009)Minimal dynamic changes.Before the operation of the dam,the conversion of beaches,canals,and dry land in the study area was frequent;the dry land landscape in the study area has obvious advantages,with good landscape connectivity and high landscape heterogeneity.After the operation of the dam,the beaches,canals,and paddy fields in the study area were mainly transformed into dry land,and the area of reservoirs and ponds increased;the heterogeneity of the landscape in the study area decreased,and the shape of various landscape patches tended to become irregular and complicated.Before the operation of the dam,there was a significant positive correlation between the heterogeneity of the agricultural landscape in the study area and the annual runoff of the lower Yellow River;the shape change of the agricultural landscape was affected by the fluctuation of the runoff during the spring irrigation period(March to May).After the operation of the dam,affected by the annual runoff of the lower Yellow River and the drop in water level,the degree of fragmentation of the agricultural landscape has intensified,and the shape of the various elements of the landscape tends to become irregular and complicated.(4)There are significant differences in agricultural production and water use in the study area before and after the operation of the Xiaolangdi Dam.After the operation of the dam,the sown area of crops,grain sown area and output in the study area have increased significantly,and the changes before and after the dam operation are significantly different.After the operation of the dam,the actual area of the lower Yellow River for irrigation from the Yellow River continued to decline,and people began to change irrigation methods and raise water-saving awareness.After the dam operation,the correlation between hydrological factors,groundwater level,number of pumped wells and various factors of agricultural production was increased;the correlation between precipitation and annual average temperature and agricultural production was weakened.(5)Based on the current operating mechanism of the Xiaolangdi Dam,the FLUS model is used to simulate the agricultural landscape in the study area in 2027,including natural development,ecological protection,and cultivated land protection.In the context of natural development,the area of construction land has increased significantly,and the area of cultivated land and other non-agricultural landscapes has decreased;In the context of ecological protection,the area of woodland,grassland and reservoir ponds increased,while the area of arable land decreased slightly;Under the arable land protection scenario,the area of arable land is stable,and the area of river channels and ponds increases.Under the three scenarios,the area of reservoir ponds maintained an upward trend.The combination of ecological protection and cultivated land protection can be used as the future direction of agricultural landscape development in the study area. |
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