Research On The Mixed-scale Effects Of The Yellow River Basin Landscape Structure Impact On The Water Quality

The Yellow River is the mother river of China.Under the influence of high-intensity human activities,the problem of water environment degradation in the river basin was quite serious.As the carrier of pollutants,the utilization mode and spatial pattern of land have a significant influence on water quality.How to explore the influence of landscape structure changes under the influence of human activities on the surface water quality in large-scale watershed,so as to more scientific planning of land and space management has become an important demand of the current national ecological civilization construction.However,the mechanism of water environment degradation in the large-scale watershed is very complex.Quantitative identification of the scale effect of land use change under human interference is still the difficulty and frontier in the current intersection of geography and environment.This research is oriented to the national strategic needs of ecological protection and restoration of the Yellow River in the new era,relying on the Yellow River Project of the National Central Research Institute Fund.In this study,using the Yellow River basin as the subject investigated,with the 113 monitoring stations（678 groups）water quality data in 2018.We developed a comprehensive research method that as mix scale division-driver factor identification-nonlinear analysis for main line.We established the response relationship between water quality and 45 landscape pattern indices at 28 different scales With the help of Redundancy analysis and Generalized additive model,We analyzed the influence of landscape structure on water quality under different scales.It quantitatively reveals the main drivers and scale effects of the landscape structure that cause the main water pollution changes in the Yellow River basin.The relevant results and conclusions of this study are as follows:（1）Identifying spatial differences in water quality of the Yellow River in dry and rainy seasons.The main pollution indicator of water quality in the Yellow River Basin is TN.In terms of space,the water quality of the Yellow River is in the order of middle reaches>downstream>upstream from poor to good.The change of water quality in dry and rainy seasons is not only affected by rainfall,but also closely related to landscape structure.Most of the sites have lower water quality concentrations in the rainy season.In places with developed agriculture and animal husbandry,the water quality sites in the upstream of some tributaries have worse water quality in the rainy season.Chemical oxygen demand(COD_Cr)is a sensitive indicator of water quality deterioration in the rainy season.（2）Reveal the characteristics of landscape structure at different scales in the Yellow River Basin.The division of 28 mixed buffer zones and the quantification of landscape index were carried out for each site.The mixed buffer is formed by the intersection of sub-basins,stream buffers（bs=200m,500m,1000m）,and circular buffers（bc=1km,5km,10km,15km,20km,25km）.The landscape composition of the Yellow River Basin is dominated by forest land,agricultural land and build-up land.In space,the proportion of forest land from upstream to downstream gradually decreases,and the proportion of cultivated land and construction area gradually increases.With the increase of the division range of the buffer zone,the discreteness of the landscape pattern at different scales increases（landscape segmentation degree and separation degree increase）,and the landscape becomes more fragmented（the largest patch area ratio and patch density decrease）.).Compared with the circular buffer scale and the sub-basin scale,the landscape composition diversity at the riparian scale is higher.（3）The best scale range of the influence of watershed landscape structure on water quality under the mixed buffer division mode is clarified.In general,compared with the dry season,the impact of landscape structure on YRB water quality tends to be more concentrated in the rainy season and to a greater extent.However,at different scales,the landscape indices with the greatest impact on water quality are different.From the 45 landscape pattern indices,the landscape variable groups with the greatest impact on the water quality in the rainy season and the corresponding optimal scales are:B-CA（sub-basin）,B-LPI（bc=1km,bs=200m）,A-CA（bc=20km,bs=1000m）,A-LPI（bc=25km,bs=1000m）,F-Split（bc=1km）,U-Split（bc=margin）,W-Split（bc=5km,bs=500m）,G-Division（bc=20km,bs=500m）.（4）Quantitatively revealed the nonlinear response relationship of water quality indicators of major pollutants in the Yellow River to landscape variable groups.The contribution of the same landscape index to different water quality indicators is different.According to the generalized additive model analysis,the explanation rates of key landscape variable groups for TN,TP and CODCr all reached more than 80%.Landscape index with significant impact on water quality changes are mainly related to the area of build-up land and agricultural land.Among them,the build-up land area（B-CA）at the sub-basin scale accounted for 43.7%of CODCr,and the pollutant output load increased rapidly when B-CA>50000ha.The agricultural land area（A-CA）has a significant contribution（50.2%）to the TN at the scale of bs=1000m,bc=20km.The largest patch index of agriculture（A-LPI）has a greater explanation rate for TP and CODCr changes at the scale of bs=1000m,bc=25km,and has the greatest impact when A-LPI=50%.The research shows that the impact of key landscape indices on different water quality indicators can be accurately identified by the mixed buffer zone division.Based on the relevant analysis of this large-scale watershed.It is found that the change of agricultural land in the Yellow River Basin is the main reason for the deterioration of water quality,and the main key factors and the control scope of the water environment degradation in the Yellow River Basin are initially revealed.This study can provide a reference for the practice of water resources protection and sustainable land use in the watershed.（5）The nonlinear response relationship of a single water quality indicator to the landscape variable group was established.Based on the research results of a large number of data and various analysis methods,It is found that human activities are the main reason for the change of water environment quality in the Yellow River Basin,focusing on the main constraints such as agricultural land.The suggestions for water environment governance and spatial development management and control for the Yellow River Basin are proposed.For the source landscape,intensive agriculture should be scientifically managed and controlled,the development scale of construction land in the sub-basin should be controlled,and the ecological red line should be implemented.Strictly control the discharge of grey water near the monitoring section.The pollution load of agricultural ground sources should be reduced and scientific agriculture should be developed.For the confluence landscape,the land use in the riparian zone should be reasonably developed and planned,so as to reduce the area of the unused land and enhance the connectivity of the forest land.