The Relationship Between The Different Time Scales Vegetation Coverage Change And Climatic Factors In The Yellow River Basin

Vegetation is a bond that links soil, climatic, hydrologic and other elements in the whole ecosystem, but also an indicator of global climate change. The impacts of climate change on terrestrial ecosystems and its feedback is one of the hotspots in global change research. In this study, the Yellow River Basin was selected to study area. The geographic information system(GIS), remote sensing(RS) technologies and Matlab, IDL/ENVI software were employed, as well as the time series NDVI datasets, the daily temperature and precipitation observation data and auxiliary data were used in this study. The detailed contents of this study is three-fold: the spatio-temporal change of vegetation cover in study area during the period of 1999-2013 and its driving factors, the seasonal variation of vegetation cover in the Yellow River Basin and its relationship between climatic factors, the spatio-temporal change of the different types vegetation. The results show that:(1) From the different time scales in spatial distribution of vegetation coverage, the yearly vegetation in the Yellow River Basin was high in the east, west and south, while relatively low in the north in recent 15 years. The vegetation coverage in downstream plains is the highest, much higher than high altitude cold area in upstream and the basin area of west Lvliang mountains. The vegetation coverage of Longdong and Longzhong Loess plateau of Gansu Province, Inner Mongolia plateau, is the lowest due to which locate in inland of China and lack of rainfall. The vegetation coverage in different seasons ranks as follows: summer > autumn > spring > winter. In spatial, the vegetation coverage of middle reaches in the Yellow River Basin area and downstream plains higher than the A’nyemaqen Mountains and Huangshui valley of upstream, while in Longzhong Loess plateau, Inner Mongolia Plateau, perennial less rainfall, the vegetation coverage is pretty low. The growth season(from May to September) vegetation coverage distribution of the Yellow River Basin show the peak value in the curve changes, centering on August. In July and August, the vegetation grow best and the highest vegetation coverage area extended to the agricultural region of middle reaches in the Yellow River Basin.(2) The different time scales of annual vegetation coverage temporal variation in the Yellow River Basin show fluctuated upward trend in recent 15 years, rate is 0.053/10 a.The four seasons vegetation coverage are increased, respective rate is 0.044/10 a, 0.085/10 a, 0.064/10 a, 0.025/10 a, inter-annual time variation have the characteristic of: summer>autumn>spring>winter. The growth season vegetation coverage increased on the whole, rate is 0.081/10 a, the inter annual variation of growth season vegetation coverage vegetation from May to September is 0.071/10 a, 0.076/10 a, 0.091/10 a, 0.089/10 a, 0.081/10 a, the vegetation coverage were increased in recent 15 years. The growth season vegetation NDVI is between 0.38 and 0.52, higher than the other months, due to enough water and good temperature over the same period stage, the vegetation coverage reach the peak in this year.(3)The spatial distribution variation of vegetation coverage in the Yellow River Basin has been improved in recent 15 years, accounting for 83.73% of the total area. In upstream、middle reaches、downstream of the Yellow River Basin all have distribution, mainly in the central and southern of Gansu, southern of Ningxia, central and northern of Ordos Plateau, north-central of Shanxi, western of Shanxi, Huangshui valley etal. Each season vegetation spatial variation is given priority to with mild improvement and basically remain unchanged. In summer and winter the vegetation coverage mainly unchanged, proportion of 55.680%, 71.365% respectively. In spring and autumn, mild improvement is given priority to the proportion of 76.108% and 56.254%. The spatial variation of growth season vegetation coverage in the Yellow River Basin is given priority to with basic unchanged or mild improvement. On May to September, May, June, July, the veget-ation coverage is basically unchanged, proportion is 56.106%, 54.783%, 60.058%, 56.416%. The vegetation coverage of August and September are given priority to improvement(significantly, moderate, mild), the proportion is 83.768%, 60.91%; Moreover, the growth season vegetation coverage varies with different temperature, precipitation, topography and other factors in recent 15 years.(4) The major vegetation types of Yellow River are evergreen coniferous forest, deciduous coniferous forest, deciduous broad-leaved forest, mixed forest, shrub, grassland, crops, and different time scales of every types of inter-annual vegetation coverage change is different. On the yearly time scale, various types of vegetation show the rise trend in recent 15 years, rate is 0.041/10 a, 0.051/10 a, 0.059/10, 0041/10 a, 0.053/10 a, 0.059/10 a. On the seasonal time scale, the vegetation coverage increases in Spring while cease to change in Winter. On the growth season, the rate is: 0.059/10 a, 0.067/10 a, 0.067/10 a, 0.073/10 a, 0.063/10 a, 0.085/10 and 0.085/10 a.(5) During in recent 15 years, the vegetation coverage of the Yellow River Basin response to average temperature and precipitation varies in different time scales. But in spatial distribution, in the low temperature districts such as high altitude mountains area of upstream, temperature is the main factor affecting vegetation growth; In these areas, the water supply enough and mainly from ice and snow, therefore, the response to precipitation is low. While in the dry districts such as Longdong and Longzhong Loess plateau of Gansu province, Hedong sandy region of Ningxia, Inner Mongolia plateau, the temperature is high all the year round but the precipitation is low, therefore, the precipitation is the main factor. The west of Lvliang Mountains which located in middle reaches of the Yellow River Basin that spreading ridge of crops planting area, the vegetation coverage response to the same period mean temperature and precipitation in spring and summer have the high characteristics, but low in winter and autumn. Compared with middle reaches and upstream of the Yellow River Basin, downstream plains region have high temperature and enough precipitation, vegetation coverage is high. In the year of vegetation growth season, response for the same period mean temperature is high and low to precipitation; because the vegetation required large quantities of heat and water in summer, so response for temperature and precipitation is high, the other time for its response is low.