Spatio-temporal Change of Vegetation Index NDVI in Northwest China and Its Influencing Factors

In this paper the spatio-temporal variation of vegetation cover in Northwest China and its influencing factors were analyzed by average method,trend line and correlation analysis during 1982 to 2006 using GIMMS/NDVI data.The annual average NDVI has been increased significantly with a rate of 0.5%/10a in Northwest China in recent 25 years.There were some significant spatial differences of the vegetation cover change.The trend line slope of NDVI were higher than 0.0005 in Xinjiang(Tianshan,Artai Mountains),Gansu(Qilian Mountains) and the eastern part of Qinghai,and it indicated the vegetation cover was significant increased in this areas.The trend line slope of NDVI were lower than-0.000 5 in the southern region of Qinghai,Shanxi and Ningxia border region,parts of Gansu and Xinjiang(Talimu Basin,Tulufan,Tahe,Tuoli),and it indicated the vegetation cover was degraded in this areas.The NDVI of woodland,grassland and cultivated land had an ascending tendency during the study period.Studies showed that: the vegetation cover change was caused both naturally and artificially in Northwest China.The temperature rose significantly in past 25 years in northwest China by an average rate of 0.67℃/10a,and precipitation increased by an average rate of 8.15mm/10a since 1986.There was a weak positive correlation between vegetation cover and temperature and precipitation inter-annual changes.However,there was a significant linear relationship between vegetation cover and the monthly average temperature and precipitation during the years.But when the average monthly temperature exceeded 20℃,the NDVI decreased and the correlation coefficient is 0.29;when the average monthly precipitation was 0~100mm,the NDVI increased and the correlation coefficient is 0.63.However,when the average monthly precipitation was more than 100mm,a clear growth trend no longer existed.The improvement in agricultural production and the ecological construction project are the very important reasons to increase the NDVI in Northwest China.