Dynamic Analysis Of Vegetation Cover And Relationships With Climate Faetors In Zhang-ye Region

In this paper, the temporal and spatial variation of NDVI of vegetation in Zhang-ye region during 2000~2008 is analyzed by using the gravity centre of vegetation, linear regression and R/S analysis based on the MODIS-NDVI data and it also analyzed the correlation and hysteretic nature between vegetation NDVI and climate factors of temperature, precipitation, sunshine hours and evaporation. The results showed that:(1).Annual variation of AMNDVI and ANNDVI in Zhang-ye region presented the trend of increasing. The vegetation cover of AMNDVI>0.1 increases unstable from 2000 to 2003, increases stable a little from 2003 to 2007 and decreases unstable from 2007 to 2008. The intra-annual variation exhibits stronger seasonal, that is, NDVI increased rapidly in April, NDVI reached the maximum value in June or July and decreased in September. Seasonal variation indicated that vegetation NDVI increased in summer and autumn and decreased in winter and spring.(2).The gravity centre of vegetation moving Eastward during 2000~2003indicated that vegetation increased in East or decreased in West; the gravity centre of vegetation moved Northward during 2006~2008, which revealed the vegetation increased in North or decreased in South. The intra-annual variations indicated that:the gravity centre of vegetation was unstable from January to May and from November to January, but stable from May to November. The gravity centre of vegetation moved Eastern from January to March and moved Northwestern from March to May, but it presented stable from May to November and the trend of clockwise changing, and it moved Southwestern from November to January. This is because the interaction of the temperature and precipitation(3).The spatial variation of AMNDVI in Zhang-ye region from 2000 to 2008 indicated: improve integrally, part-degenerate. The degradation region of vegetation, which took up 20% of the whole researching area, distributed in Qilian Mountain in South. The annual variation of growing season is mainly improvement. The increasing region of vegetation, distributes in Shan-dan, Min-le and Zhang-ye basin. The main land types were dry land and artificial grassland; artificial vegetation increased faster than natural vegetation. Degradation regions sporadically distributed over Zhang-ye basin and Qilian mountain due to much human activity. The region of almost invariant is gobi desert. The Slope of each month in growing season was different. The Slope showed significant degradation in Zhang-ye basin in May, middle-degraded in the South of Qilian Mountain in June and July and in in August and September the Slope of vegetation is better than the ones in May in Zhang-ye basin and in June or July in the South of Qilian Mountain. The order of Slope value of different land types in growing season was: grassland (dry land and artificial grassland) > cultivated land > Construction land> forest > unused land, and it indicated that artificial vegetation growing faster than that of natural vegetation.(4).The Hurst index of growing season of NDVI focused on 0.4~0.6, which dominated by weak sustainability and anti-sustainability, but the index of each month in growing season was different. The Hurst index showed the decreasing trend of persistence and increasing reverse- persistence from May to July, while the persistence was increased and reverse- persistence was decreased in August and September. The order of Hurst value of different land types was: Other unused land< Construction land<grass<cultivated land<unused land<forest.(5).The annual variation of climate factors in Zhang-ye region showed: temperature and precipitation presented the trend of increasing; the changes of sunshine hours manifested as double"V"curve; the change of evaporation decreased from 2000 to 2008 in Zhang-ye region. The intra-annual variation of climate factors showed that the changes of climate factors are most significant in July. It presented that the temperature, sunshine hours and evaporation decreased significantly, and precipitation increased most significantly.(6).The annual correlation coefficient between NDVI and climate factors showed that: the order of Pearson correlation coefficient is average temperature>evaporation> precipitation>sunshine hours. The order of partial correlation coefficient is evaporation> average temperature>precipitation. It showed that when rejecting the influence of average temperature and precipitation to NDVI, The response of NDVI to evaporation is most significant. (7).The correlation between NDVI and climate factors in different county or region of Zhang-ye showed that: the response of NDVI to temperature occurred in winter in most regions, but the one of Gan-zhou appeared in spring. The positive correlation between NDVI and precipitation appeared in summer which is most significant, particular maximum in Gan-zhou and Lin-ze. The correlation between sunshine hours and NDVI is weaker, not including Min-le. The correlation between NDVI and evaporation appeared in summer which is middle negative correlation and in winter which is middle positive correlation.(8).The hysteretic nature of NDVI to climate factors showed that: NDVI to temperature and precipitation have not lag phase, meaning the correlation of synchronization is maximum, not including Min-le. The lag phase of sunshine hours is one or two months. The lag phase of evaporation presents two months in eastern and one month in western.