| The Loess Plateau showed obvious regional differentiation regularity, which with serioussoil and water loss, and susceptible ecological environment. The water-wind erosioncrisscross region with a precipitation of400mm. Water erosion and wind erosion carried outalternately in this area, which made soil erosion was very violent. The coverage andprotection of vegetation is the key link of weakening the development of erosion duringerosion development, and its evolution direction and degree also become the guidecharacteristic factor of evaluating soil erosion. Utilizing8km/15d resolution GIMMS (GlobalInventory Modeling and Mapping Studies) NDVI (Normalized Difference Vegetation Index)date from1982to2006, and based on the methods of trend analysis, Center of gravityrevolution analysis and transition matrix model, the temporal and spatial evolution of NDVI/vegetation coverage and the relationship with precipitation and temperature in the water-winderosion crisscross region during recent25years was analyzed in this paper. The result showedthat:(1) In recent25years, the water-wind erosion crisscross region showed a significant(P<0.05) increasing trend in NDVI. NDVI also showed very significant (P<0.01) increasingtrend in autumn and significant (P<0.05) increasing trend in9-10months. Except the winter, asignificant correlation was observed between NDVI change of annual and spring, summer,autumn. On10years time scale, NDVI had great difference in spatial distribution. Continuousincreasing area was only22.3%, and centralized in northern Shaanxi of Wuqi/Zhidan inupstream of the Beiluohe river, Yanhe river area where Ansai/Yanan at, and southwest ofXining. Seasonal variation of NDVI was very significant in the eastern of Qinghai andnorthern of Shanxi.(2) With the method of the dimidiate pixel model, obtain the vegetation coverage ofstudy area and divided into5types. The medium-low vegetation coverage (20-40%) occupiedthe50%percentage of the whole area. Low vegetation coverage (0-20%) and moderatevegetation coverage (40-60%) occupied1/3area, and high vegetation coverage (>80%) onlyaccount for about0.13%. From1980s to the beginning of21stcentury, change rate washighest in high vegetation coverage and moderate vegetation coverage, and increased and decreased40.09%and18.88%respectively. In the periods of1980s,1990s and the beginningof21stcentury, more than80%area of vegetation coverage was not changed, and was mainlymedium-low vegetation coverage. From1980s to the beginning of21stcentury, improve areaof vegetation coverage occupied9.94%and degenerate area occupied4.99%. Area changewas mainly between low vegetation coverage and medium-low vegetation coverage, whichindicate that the two types of vegetation coverage were very instability.(3) Center of gravity of vegetation coverage from southwest to northeast during threeperiods was: medium-high vegetation coverage, high vegetation coverage, moderatevegetation coverage, low vegetation coverage and medium-low coverage, which wasconsistent with the actual vegetation coverage. In general, center of gravity of low vegetationcoverage moved to southwest and that of high vegetation coverage moved to northeast, whichindicated that in recent25years, with the effect of precipitation reduced, vegetation coverageof southwestern of Ningxia and middle of Gansu was decreased. While in the east area,conversion of cropland to forest and grassland of the ecological environment construction hasmade the vegetation coverage improvement.(4) Generally, the fragmentation and heterogeneity of landscape increased, the patchaggregation and advantage was decreased, and patches were well-distributed at spatial in thewhole region nearly25years. Changes of fragmentation, connectivity, spatial distribution andaggregation are significantly different on each vegetation coverage degree. Landscapeadvantage of the moderately low and low vegetation coverage was obvious, and has a majorcontribution to landscape pattern changes in the whole region. Moreover, landscape advantagewas decreased and the landscape fragmentation increased. Correspondently, landscapeadvantage of moderate, moderately high and high vegetation coverage was increased, andlandscape of high vegetation coverage showed a trend of fragmentation decreasing,aggregation was increasing and heterogeneity reducing. In this sense, it can be induced thatthe vegetation coverage was improved on the water-wind erosion crisscross region of theLoess Plateau during the25years. Regional difference was significant.（5）The region of NDVI continuous increasing was corresponding to the spatialdistribution of precipitation increasing trend excepting the region. On seasonal scale, therewas a significant lag correlation between precipitation and NDVI. And there was a obvioushysteresis effect on month scale. Annual temperature had little effect on annual NDVI, andthere was a similar relationship on season and mouth scale. On10years time scale, the regionof NDVI continuous increasing was corresponding to the spatial distribution of precipitationincreasing trend excepting the region nearby Dongsheng in Inner Mongolia where the NDVI increased while precipitation decreased. The phenomenon testifies that the ecologicalenvironment construction has brought better ecological environment benefit on the leadingeffect of precipitation at this region. |
PDF Full Text Request