Research On Land Use Change And Its Ecological Effect In The Dengkou Oasis Irrigated From The Yellow River

Land use change which is the most prominent landscape symbol of global change in the earth's surface system exerts a huge influence on the process,structure and function of ecosystems with different scales so that it has become the core content and focus field of global change research.It is of great significance to investigate the spatial and temporal change of land use and its ecological effects for the rational development and utilization of regional water and soil resources as well as the protection of ecological environment.The Yellow River irrigated oasis is not only an important carrier for the impoverished people in the northwest arid areas to realize ecological migration and get rid of poverty,but also the first ecological barrier to prevent the sandstorm from entering the Yellow River.In recent decades,with the rapid growth of the population and the intensification of the development of soil and water resources,ecological and environmental problems such as land desertification,shortage of irrigation water and decrease of biodiversity not only threaten the survival of desert-oasis but also limit their ecological protection,which have aroused great attention from all walks of life.In this research,the Dengkou county,Inner Mongolia which is a typical Yellow River irrigated oasis was selected as the study area.The most suitable classification methods for Dengkou oasis was compared by the performance among K-Nearest Neighour(KNN),Support vector machine(SVM)and Randomforest(RF),and the land cover classification results of seven periods during 1988-2018 was gained based on the comparative result.The spatial and temporal variation rules of the quantitative characteristics,dynamic attitude,comprehensive degree and transfer characteristics of land use are studied on the basis of statistical analysis and grid analysis,and then the spatial autoregressive analysis is used to measure the contributions of 8 influencing factors,such as natural environment and social economy,so as to identify the driving factors of land use change.Take the Largest Patch Index(LPI),Effective Mesh Size(MESH),Contagion Index(CONTAG)and Shannon's Diversity Index(SHDI)as an indicator of landscape fragmentation,the spatial gradient variation characteristics of landscape fragmentation from the Yellow river to oasis internal were investigated using Buffer analysis.The characteristic scale was determined by method that combines semi-variograms with Moving windows,and the spatial evolution process of landscape fragmentation space is analyzed on the basis of this scale.The ecological risk distribution map of Dengkou oasis in 7periods was obtained with the landscape ecological risk index model,spatial autocorrelation analysis,semi-variance analysis and ordinary kriging interpolation.On the basis of the revised ecosystem service value coefficient per unit area of Dengkou oasis,the spatial-temporal evolution characteristics of ecological service value are explored by combining with grid analysis,cold hot spot analysis,center of gravity transfer analysis and ecosystem services value flow analysis,and the impact of land use change on it is quantitatively studied.Finally,based on the above research content,the spatial and temporal change of land use and its ecological effect of Dengkou oasis in the past 30 years are obtained,so as to provide theoretical basis and scientific reference for the construction of ecological civilization and the sustainable development of economy of Dengkou oasis.The conclusions are as follows:(1)Among the four machine learning algorithms,RF performs better than the other three algorithms in terms of stability,ease of use and processing efficiency during parameter tuning.The classification accuracy of RF is slightly lower than that of ANN and there was no significant difference.Therefore,RF can be used as the first method to extract land use classification information in Dengkou oasis.Elevation,Short-wave infrared band and some spectral indices are of great importance to the overall accuracy of individual land use types.(2)The number of each land use types in Dengkou oasis changed significantly from 1988 to 2018,which was manifested in the decrease of large area of bare sand land,the dramatically increase of cropland and built-up land,as well as the slightly increase of ecological land areas such as woodland,rangeland and open water.Both the land use degree and the land use dynamic attitude show an increasing trend of fluctuation,but they are generally at a low level;spatially,the distribution of these two indices can be characterized by high in the east-central region and low in the west as well as high in the west and low in the east.Land use transfer was characterized by stages and 2010 was an obvious turning point.Previously,the major types of land use transfer were the mutual transformation amog the bare sand land,rangeland and cropland,and the transformation area was relatively small,which was mainly distributed in the central and eastern parts of the oasis.The major type of transfer was the conversion of large area of bare sand land to cropland,and the region shifted to the western,southwestern and southern oasis with the drastic transformation of woodland,open water and built-up land.Human activities,precipitation and elevation are the driving factors of land use change in Dengkou oasis,among which human activities play a dominant role.(3)The landscape pattern of Dengkou oasis changed significantly from 1988 to 2018,with the continuous decrease of LPI,MESH and CONTAG as well as continuous increase of SHDI,and the degree of landscape fragmentation gradually intensified.The landscape fragmentation of all buffer zones showed an increasing trend except for the shore of the Yellow River,and the difference between different regions is obvious,among which the buffer zones within 5-20 km from the Yellow River had the most significant fragmentation intensification.Based on the moving window,1800 m is the characteristic scale of landscape fragmentation for space evolution in Dengkou oasis.Within this characteristic scale,the spatial distribution characteristics of landscape fragmentation in Dengkou oasis was generally manifested by the intensified radiation in the urban center and along the Yellow River.(4)The ecological risk of Dengkou oasis showed a downward trend of fluctuation which was manifested in the sharp decrease of the area of high ecological risk area and the large increase of the low,lower and medium ecological risk area from 1988 to 2018.The spatial distribution of ecological risks is characterized by stages and 2015 was an obvious turning point.The spatial pattern of ecological risks always shows that the low ecological risks are distributed in the northeast,central and the coastal areas of the Yellow River in the oasis,while the high ecological risks are distributed in the northwest of the oasis and the whole southern region before 2015,while the area of the high-ecological risk area in the whole southern region decreased sharply and the spatial distribution characteristics of ecological risks changed significantly after 2015.There was an obvious spatial positive autocorrelation of ecological risks in these seven periods,and the spatial clustering pattern of ecological risks in different periods was more obvious in local areas from 1988 to 2015 and gradually turned to uniform distribution after 2015.The conversion of bare sand land and waste land into cropland and built-up land is the leading factor causing the change of ecological risk.(5)The total value of ecosystem services in Dengkou oasis fluctuated and increased during 1988-2018,which was mainly contributed by the increase of the ecological service value of cropland.The ecosystem service value of cropland,open water and rangeland exceeded 76% in different periods,which is the main body of the ecosystem service value of the whole research area.The function of ecosystem regulation and support services was much larger than that of production and cultural services,and soil conservation,food production,climate regulation and gas regulation adding the most value.The ecological service value of the seven periods showed obvious spatial distribution characteristics of high in the southeast and low in the northwest,among which the difference of spatial distribution was not obvious from 1988 to 2010,and significant changes occurred in local areas in 2015 and 2018.The value-added hot focus of the ecological service value change from east-central region towards the southwest,and loss of cold spots center was transfered from the Yellow River to the oasis internal and then returning to southeast region,the main cause of the change of the ecological service value can be attributed to the mutual transformation among cropland,open water,and the rest of different land use types.