Land Use Change And Landscape Pattern Evolution Effects On Runoff In The Upper Reaches Of Sanchahe River Basin

The upper Sanchahe River Basin in Guizhou Province is one of the areas with severe soil erosion in the southwest of China.The rocky desertification in this basin is severe,water resources are scarce,and the ecological environment is fragile.In recent years,the implementation of ecological projects such as"Grain for Green"and“Protection forest construction”has effectively controlled soil erosion and improved the ecological environment of the river basin.At the same time,the land use patterns and landscape patterns of the river basin have also undergone major changes,which have a major impact on the water cycle process and water environment security of the river basin.Therefore,the upper Sanchahe River Basin,the water source area of the Qianzhong Hydraulic Project is selected as the research area.These data of the meteorological,hydrological and land use were collected.The Arc GIS 10.2,Fragstats 4.2,mathematical statistics and SPASS software were used to analyze the Land use/cover and landscape pattern changes and their impact on runoff.First,the Mann-Kendall rank correlation method and anomaly percentage curve were used to analyze the change characteristics of precipitation and runoff in the upper reaches of the Sancha River from 1900 to 2015.The cumulative anomaly method was used to determine the abrupt years in precipitation and runoff changes,and the evolution process was divided.The contribution rate of precipitation and human activities to runoff changes are separated according to the Slope Changing Ratio of Cumulative Quantity.Secondly,the Arc GIS 10.2,Fragstats 4.2 and mathematical statistical software were used to analyze the characteristics of land use/cover and landscape pattern changes in the upper Sancha River from 2000 to 2015.Finally,the qualitative and quantitative methods were used to study the impact of land use/cover change on runoff.At the same time,the Person correlation analysis was used to analyze the impact mechanism of landscape pattern on runoff changes,which can provide a scientific basis for the sustainable use of water and land resources in the river basin.The main conclusions are as follows:(1)From 1990 to 2015,precipitation showed a decreasing trend,but the trend was not significant,with an average annual decrease of 5.71mm,while runoff depth showed a significant downward trend,with an average annual decrease of 12.44mm.The cumulative anomaly method is used to determine that there is a sudden change in precipitation and runoff in 2000.The evolution of precipitation and runoff can be divided into A1(1990-2000)and A2(2000-2015)two stages.Taking 1990-2000 as the reference period,the cumulative slope change rate comparison method(SCRCQ)was used to calculate that the contribution of precipitation to runoff reduction from 2000 to 2015 was 55.57%,while the contribution rate of human activities to runoff reduction was 44.43%.It shows that the reduction of precipitation from 2000 to 2015 was the main factor causing the reduction of runoff.However,human activities have played a role in reducing runoff.(2)From 2000 to 2015,the land use types in the upper reaches of the Sancha River were mainly forest land,grassland and arable land,which alternately distributed in space.Among them,woodland and grassland accounted for about 60%of the total area of the river basin,and cultivated land accounted for about 30%.The total area of water and unused land accounts for about 10%.From 2000 to 2015,the land type transfer changes were mainly cultivated land,forest land,grassland and construction land.Especially after the"Grain for Green"and urbanization construction,the transfer changes between land types gradually accelerated.From 2000 to 2015,the cultivated land decreased by 259.01km~2,mainly changed into forest land,grassland and construction land.Forest land,grassland,construction land and water area increased by 66.52km~2,77.28km~2,94.24km~2 and 25.17km~2.The increase of forest land mainly comes from cultivated land and grassland.The increase of grassland mainly comes from cultivated land and woodland.The increase of construction land mainly comes from cultivated land,woodland and grassland.The increase of water area comes from cultivated land and woodland.However,the unused land area change not significantly.(3)From 2000 to 2015,the overall landscape pattern of the watershed developed towards the direction of deepening landscape fragmentation,weakening landscape connectivity,decreasing landscape dominance,increasing landscape heterogeneity,and evenly spaced landscape.In patch type,forest land,grassland and cultivated land were the dominant landscape types in the watershed.After"Grain for Green",the degree of fragmentation of forest land,grassland and cultivated land were deepening,the degree of dominance were gradually decreasing,and the shape of landscape were gradually simple and regular.Affected by urbanization and population growth,construction land tends to be centralized,landscape dominance increases,and landscape shapes tend to diversify.(4)The qualitative and quantitative methods were used to analyze the impact of land use/cover change on runoff in the upper Sancha River,the results show that:From 2000 to 2015,the increase in the area of forest land and grassland increased the retention and consumption of precipitation,which had a significant impact on the reduction of runoff in the watershed,while the decrease in cultivated land area had a weaker effect on runoff.(5)The Person correlation analysis was used to analyze the relationship between landscape pattern and runoff changes,the results show that:At the landscape level,the maximum plaque index(LPI),area-weighted average plaque shape index(AWMSI),and spread index(CONTAG)were positively correlated with runoff depth,indicating that the landscape fragmentation was deepened,the landscape heterogeneity was increased and the landscape space was evenly distributed,which increases the retention and consumption of precipitation and reduces the formation of surface runoff.However,Landscape dominance index(LPI),shape index(AWMSI)and aggregation index(CONTAG)were positively correlated with runoff,but the reduction of the landscape dominance and aggregation degree and the simplification of patch shape were not conducive to the increase of runoff.The main reason for this influence mechanism was that forest land and grassland were the dominant landscapes in the watershed.The increase in the area of forest land and grassland,the deepening of fragmentation and the increase in the degree of adjacency hinder the formation of surface runoff,resulting in a reduction in watershed runoff.