Evolution Of Landscape Ecological Risk And Its Relationship With Terrain Gradient In Heihe River Basin In Recent 40 Years

The rapid development of urbanization and population increase have led to serious ecological problems.It is necessary to clarify the temporal and spatial changes of land use in the watershed and to explore the characteristics of landscape ecological risk.Landscape ecological risk research based on terrain gradient can not only study the dynamic changes of risk from the horizontal direction,but also characterize the landscape ecological risk in the vertical zone with large terrain differences.Therefore,taking the Heihe River basin with obvious vertical distribution characteristics of "mountain oasis desert" as the research area,taking the research of "land use landscape ecological risk relationship between landscape ecological risk and terrain gradient" as the main line,this paper selects multi temporal remote sensing land use data and DEM data to study the spatial and temporal characteristics of land use change in the upper,middle and lower reaches of Heihe River Basin from 1980 to 2018,On this basis,the landscape ecological risk index model is constructed to study the risk change characteristics and laws of the study area in recent 40 years.Finally,the terrain factors such as DEM,slope,undulation and TNI index were selected to explore the differences of landscape ecological risk grades on the terrain gradient.The specific conclusions are as follows:(1)The overall land use has changed significantly in the past 40 years.In the upper reaches of the basin,grassland and unused land dominate,followed by woodland.The area of arable land,water area and construction land increased 11.142km~2,181.08km~2 and 4.6km~2 respectively,and the area of woodland,grassland and unused land decreased by 2.81km~2,84.5km~2 and 150km~2 respectively.Except for grassland and unused land in the middle reaches of the basin,arable land The area accounts for the largest proportion and is increasing year by year.From 1980 to 2018,the area of arable land,forest land,and construction land increased by 1035.43km~2,3.78km~2 and 282.897km~2,respectively,and the other types of land use decreased to varying degrees;the percentage of the lower reaches of the basin Eighty of them are unused land,and the area is increasing,and the area of cultivated land and construction land have increased by 289km~2 and 152km~2 respectively.The area of grassland has decreased the most in the past 40 years,which is 1589km~2.It can be seen that the downstream grassland is seriously degraded.Facing serious ecological problems.(2)The characteristics of landscape ecological risk changes in the past 40 years are obvious.Among them,the area with higher risk and the area with high risk increased more to3047km~2 and 2197km~2,respectively,the low-risk area also increased by 162km~2,the medium-risk area decreased the most at 4720km~2,and the lower-risk area decreased by685km~2.From a spatial perspective,the upper reaches of the basin are mainly low-risk and low-risk,the middle reaches of the middle and higher risks,and the downstream is medium-risk.The risk level in the western part of the basin is higher than that in the eastern part.On the whole,the risk in the Heihe River Basin is increasing.(3)The risk level changes significantly on the terrain gradient.The advantages of low-risk areas in terms of slope,elevation,undulation and TNI are all medium-to-high terrain positions,low-risk areas are distributed over medium-to-high-risk areas with topographic factors,medium-risk areas and higher-risk areas are mainly Distributed on low topography,high-risk areas except for low topography,are distributed in the other topography.In general,the changes of landscape ecological risk levels in the gradient of geomorphic factors are basically the same.