On Vegetation Cover Change And Its Influencing Factors In Typical Inland River Basins Of Northwest Ecological Environment Fragile Regions

Northwest region of China is one of the most sensitive areas to global change.It is influenced by the fragile ecological environment,dry climate,sparse vegetation and frequent sandstorm,and these are infavourable to the regional economic and social sustainable development.In recent years,climate warming and other relevant extreme eventshas obviously caused temporal and spatial changes of vegetation coverage in the northwest regions of China.Thus,studies on vegetation changes and their driving factors are helpful to understand the temporal and spatial heterogeneity of regional vegetation changes,to reveal the influence of driving factors,and to strengthen the construction of ecological areas.At present,some scholars have explored the dynamic change of vegetation coverage at different period and scale by using remote sensing technology,and they have achieved fruitful results.However,their studies are mostly based on administrative divisions or large spatial scales,and few of them use multi-watershed systems to analyze vegetation changes in fragile ecological environments.The understanding of regional vegetation change and its influencing factors is still far from enough,and it is still lack of effective systematic demonstration for different time and space scale,especially in some typical areas.In this paper,three complete inland river basins which are the Shiyang River Basin,Heihe River Basin and Shule River Basin are studied.Regional MODIS NDVI data of 2000-2015,and GIMMS NDVI data of 1980-2006 in typical area,as well as hydrological data,climatic data,DEM data,vegetation type data have been adopted to study the changes of vegetation cover and climatic factors,and the relationship between them.In this paper,the correlations between vegetation coverage change and runoff have been studied.The main research contents and conclusions are as follows:(1)In this paper,the linear regression,M-K mutation test,wavelet analysis,coefficient of variation and correlation analysis have been adopted to study the changes of vegetation coverage and climatic factors in different basins.The results indicate thatthe vegetation coverage is overall lower in Shiyang River Basin,Heihe River Basin and Shule River Basin.The NDVImax distribution of the longitude is low in the west side and high in the east side,the distribution of the latitude is high in the south,while it is low in the north.Altitude and slope have a significant effect on vegetation spatial distribution.From 2000-2015,the improvement of vegetation coverage in the inland river basin of China northwest was obvious.The average annual growing rate of NDVImax was Shule River Basin higher than Shiyang River Basin,and it was the lowest in theHeihe River Basin.The seasonal differences of vegetation cover were significant.The general trend of climate changes in inland river basins of northwest area was getting warmer and more humid,but the growing rates of temperature and precipitation in the past 16 years were slower than before.The coefficient of variation NDVImax in the study area was on the rise,indicating that the overall stability of vegetation in three inland river basins was weak,and there are still some risks of vegetation coverage degradation in the inland river basins of China northwest region in the future.(2)Based on the residual calculation,simple correlation analysis,partial correlation analysis and complex correlation analysis methods,the effects of temperature and precipitation and human activities on vegetation coverage in the inland River Basins have been studied.The results show that,the vegetation cover of the inland river basin in northwest China was closely related to temperature and precipitation conditions.In Shiyang River,Heihe River and Shule River Basin,vegetation cover was highly influenced by both temperature and precipitation.Precipitation was the main factor affecting the vegetation of inland river basins in northwest China,especially in the Shiyang River Basin and Shule River Basin.The vegetation growth in Heihe River Basin was more manifested by the common effects of precipitation and temperature.The residual analysis shows thatthe influence of human activities was high in three inland basins.(3)Based on the Hurst index,the study shows that the area of continuing degradation will be slightly larger than that of continuing improvement in the future,and the degraded area gradually will increas from east to west.It indicates that there will be still a certain risk of vegetation cover degradation in inland river basins of northwest ecological environment fragile areas.(4)Based on the correlation analysis,Wavelet analysis,and comparative analysis,the relationship between vegetation coverage change and runoff has been studied.The results show that,the trend of vegetation NDVImax in the typical area of Heihe River Basin was consistent with the runoff in a year.The regional vegetation NDVImax and runoff all have a 4-year quasi-period and similar secular variation trend,especially during 2000-2010.The correlation analysis showed that the runoff had a positive effect on the NDVImax of vegetation,and the runoff accumulation from the end of the first year’s growing season to July and August in next year was significant with the NDVImax of the vegetation.Because of the combined effects of temperature,precipitation and non-climatic factors,runoff was not the dominant factor of regional vegetation growth.The main innovation of this paper is reflected as follows:(1)Aiming at the situation of single spatial scale in the study of regional vegetation cover change,three adjacent inland river basins of fragile ecological environment have been selected as the research object.The characteristics and evolution trend of vegetation coverage have been systematically studied.The dynamic changes of vegetation in the inland river basins of China northwest have been further revealed.(2)Based on the theory and method of linear regression,M-K mutation test,Morlet Wavelet analysis and correlation analysis,the characteristics of regional climate change and its impact on vegetation cover have been analyzed at different time and space scales,and the response mechanism of vegetation change to climate in inland river basins of China northwest have been revealed.(3)Besides the climatic factors,the effects of non-climatic factors such as terrain and human activities on the vegetation coverage in the study area have been discussed.The relationship between vegetation and surface runoff has been analyzed quantitatively.Thus the analysis of the factors affecting regional vegetation change was more comprehensive.