Study On Land Use/Cover Change And Its Driving Forces In Upper Reaches Of Minjiang River

The upper reaches of Minjiang River is the main water source of Chengdu Plain and an important ecological barrier in the upper reaches of the Yangtze River,which plays an important role in the ecosystem balance of the whole upper reaches of the Yangtze River.Because of its special location conditions and fragile ecological environment,it has become a hot spot for scholars.This study is based on this typical region,taking Landsat 7 ETM in2000,2005,2010 and Landsat 8 remote sensing images in 2015 as data sources,and using human-computer interactive interpretation method to extract the land use classification layers of the upper reaches of Minjiang River.The spatial and temporal patterns of land use in the upper reaches of Minjiang River were quantitatively analyzed.Based on the MODISNDVI data from 2000 to 2015,the vegetation coverage in the upper reaches of Minjiang River was estimated by using the pixel dichotomy model,and the spatial and temporal characteristics of vegetation coverage were studied.The changes of land use/cover landscape pattern in the upper reaches of Minjiang River were evaluated by calculating the landscape pattern indices.Based on the geographical detector model,the natural factors affecting the spatial distribution of land use/cover in the upper reaches of Minjiang River are analyzed,and the population pressure in the upper reaches of Minjiang River is statistically analyzed.The influence of policy factors on land use/cover change in upper reaches of Minjiang River is analyzed qualitatively.The main conclusions of the study are as follows:(1)Grassland and woodland are the main types of land use in the upper reaches of Minjiang River,and the total area of them accounts for more than 95%of the total land area on Minjiang River.The area of cultivated land is close to 3%and the area of construction land is less than 0.2%.The average area of water area is about 120km~2.The unused land is scattered all over the upper reaches of Minjiang River.The overall characteristics of land use area change are as follows:the area of cultivated land and construction land continues to increase,the area of forest land continues to decrease,and the change of grassland and water area is not obvious enough.The general characteristics of conversion of land use types are as follows:woodland,strong conversion between grassland and grassland,expansion of cultivated land to forestland and grassland,obvious phenomenon of compressed cultivated land for construction land,the unused land is mainly formed because of the destruction of forest and grassland.The waters changed little.The spatial characteristics of land use type change are as follows:the main stream of Minjiang River and the two sides of main tributaries,woodland and grassland transition zone and other areas have strong changes in land use type,among which the transition of grassland is scattered in the south-central area of Songpan County.The central part of Lixian County,the northeast of Wenchuan County,the central part of Maoxian County,the central part of Heishui County,etc.Forest land transfer mainly occurred in the south of Songpan County,on both sides of the Heishui River,in the east of Lixian County and in the central area of Wenchuan.The overall level of land use change in the upper reaches of Minjiang River was relatively slow.The highest dynamic degree of land use change was in 2005-2015(20.86%),and the lowest was in 2000-2005(0.96%).(2)The mountains in the upper reaches of Minjiang River are dense,grassland is widespread,and the overall condition of vegetation cover is good.The annual average vegetation coverage is more than 0.77,and the area with vegetation coverage greater than 0.7is more than 80%of the total area in the upper reaches of Minjiang River.Distributed in most areas of the upper reaches of Minjiang River;The area of no vegetation or less vegetation area with vegetation coverage less than 0.1 is less than 3%,mainly distributed in the upper reaches of Minjiang River,southwest and northeast of Minjiang River,where the altitude is more than 4000m.The vegetation coverage in the upper reaches of Minjiang River was relatively stable during the study period,the average coefficient of variation was 12.30%,and the area with no significant change in vegetation coverage accounted for 67%of the total area of the upper reaches of Minjiang River.In the later period of the study,the vegetation coverage in the upper reaches of Minjiang River shifted from high grade vegetation coverage to lower grade vegetation coverage,indicating that the quality of vegetation covedecreased.(3)The land use patches in the upper reaches of Minjiang River were mainly cultivated land,woodland and grassland patches,while the number of other land patches was relatively small.The overall change characteristics of landscape pattern of land use were as follows:the number of patches and the density of patches increased,the fractal dimension of patches fluctuated greatly,the patches tended to be broken,and the tendency of patch aggregation was gradually separated.The complexity and evenness of the landscape increased.Among the land use landscape types,construction land and cultivated land landscape patch fragmentation degree is higher,but with the promotion of urbanization in the space towards agglomeration;The area of grassland and woodland is larger,the spatial distribution is relatively concentrated,and the degree of fragmentation inside the patch is low.However,due to the influence of various factors such as nature and human activities,the aggregation degree of the patches of woodland and grassland landscape shows a downward trend.The degree of landscape fragmentation increased;The fragmentation degree of unused land and water landscape patches was enlarged stage by stage,and the change of aggregation was not obvious.(4)Land use/cover change in the upper reaches of Minjiang River is not only controlled by natural factors,but also influenced by social and economic factors.The results showed that geomorphologic type,soil type and elevation were the main factors affecting the spatial distribution of land use degree in the upper reaches of Minjiang River.Three natural factors,temperature,elevation and soil type,are the most important factors affecting the spatial distribution of vegetation coverage in the upper reaches of Minjiang River.The growth of consumption population and the development of social economy are the fundamental reasons for the increase of construction land and cultivated land area in the upper reaches of Minjiang River,and also the important reasons for the decrease of woodland and grassland area.In order to ensure ecological security in the upper reaches of Minjiang River,it is necessary to strengthen the implementation of policies such as returning farmland to forest and grassland,moving ecological immigrants,closing mountains and raising forests,planting trees and afforestation.