Relationship Between Land Use And Stream Water Quality In The Bahe River Basin,China

River water is an important part of surface water resources.The condition of its environment is directly related to human economic development and the healthy operation of ecosystem.With the development of economy and society and the growth of population,many rivers have been affected by human activities in recent years.And there have been some ecological and environmental problems,such as river drying up and water quality deterioration and so on.Land use patterns directly reflect the different categories of human activities,which are closely related to the river water environment quality.As the largest tributary on the south bank of Weihe River,Bahe River is an important economic and natural resource in Xi’an City,which has irreplaceable ecological and environmental functions.With the rapid development of Xi’an’s economy and society,the land use patterns of Bahe River Basin have also changed significantly,which is definitely affect the water quality of Bahe River.In view of this,the Bahe River Basin was taken as the study area,and analyzed the change characteristics of land use in the Bahe River Basin from 2000to 2019.Then three different methods were used to evaluate the water quality situation of Bahe River,which include single factor evaluation method,traditional fuzzy comprehensive evaluation method and improved fuzzy comprehensive evaluation method.Finally,correlation analysis,redundancy analysis(RDA),geographically weighted regression model(GWR)and other methods was used to study the impact of land use structure and landscape pattern changes on the water quality of the Bahe River at the buffer scale.And make recommendations to improve the water quality based on the analysis results.The main conclusions are as follows:(1)In the past 20 years,the land use structure of Bahe River Basin has changed significantly,and the areas of cultivated land and construction land have changed the most.The calculation results of land use variation degree and dynamic degree shows that the change degree of construction land is the largest,with the variation degree and dynamic degree reaching 65.66%and 3.46%respectively.The comprehensive index of land use degree has increased for the past 20 years,which indicates that the development and utilization of land resources have been strengthened in Bahe River Basin.The transferred out area of cultivated land is the largest in Bahe River Basin from 2000 to 2019,and its main transfer direction is forest land and construction land.The transferred in area of forest land is the largest,followed by construction land,and their main transfer source is cultivated land.The characteristics of landscape pattern change in Bahe River Basin show that landscape fragmentation and landscape diversity has increased,landscape connectivity has reduced,and spatial adjacency probability of different patch types has increased.(2)The total nitrogen(TN)pollution in Bahe River is severe.The improved fuzzy comprehensive evaluation method is used to evaluate the water quality of Bahe River.The results show that the B_T value of fuzzy comprehensive evaluation is between 2.958 and 4.013,and the water quality is classed gradeⅢ-Ⅴ.The proportions of gradeⅢ,ⅣandⅤare11.77%,82.35%and 5.88%respectively.And the water quality at the junction of Chanhe River and Bahe River is the worst.The comparison of different water quality evaluation methods shows that the improved fuzzy comprehensive evaluation method is obviously better than the single factor evaluation method and the traditional fuzzy comprehensive evaluation method,which can fully and reasonably mirror the river water quality.(3)Construction land,grassland and unused land have relatively large impact on the river water quality,while forest land has relatively less impact on the river water quality.Construction land accounts for a large proportion on both sides of the river,which is greatly affected by human activities and is more conducive to the formation of surface runoff and the collection of various pollutants.Construction land has a significant impact on the river water quality,so it is the key factor of leading river water pollution.The accumulation of domestic waste and construction rubbish in the unused land makes pollutants easily enter the river with surface runoff.And the concentration of pollutants in the river will be increased.Grassland and forest land play the role of intercepting pollutants,preventing water and soil erosion and conserving water resources.So the widespread distribution of grassland and forest land can improve the water quality of the river.Forest land may be affected by human activities,resulting in the effect of forest land on river water quality is weaker than grassland.Affected by the distribution characteristics of cultivated land on both sides of the river,the real impact of cultivated land on river water quality is covered by other land use types in a certain extent,so cultivated land does not show the role of deterioration of river water quality.Within the400 m buffer zone,the correlation between river water quality index and land use type area proportion is the most significant,and the river water quality has the greatest response to land use structure.Therefore,the land use structure within the 400 m buffer zone can better reflect the spatial change of the river water quality.The impact of construction land on river water quality changes with the change of spatial location.The construction land have a greatest impact on the river water quality before and after the confluence of Chanhe River and Bahe River.(4)Landscape pattern on both sides of the Bahe River has a certain impact on the river water quality.The more fragmented the landscape is,the higher the landscape diversity is,and the more complex the spatial adjacency distribution among different patch types is,the easier pollutants are intercepted in the process of migration,which is more conducive to the improvement of river water quality.The stronger the aggregation degree and connectivity of landscape patches,the more favorable for the migration of pollutants to water and the greater the threat to the river water quality.Further analysis shows that the distribution characteristics of construction land,as the dominant landscape type in the buffer zone,have a great impact on the river water quality.Construction land with small area distribution can improve the river water quality,while which with high connectivity and large area distribution may lead to the deterioration of the river water quality.