Spatio-Temporal Variation of NDVI Change and Driving Forces in the Cascade Hydropower Development Zone of the Jinsha River

Vegetation dynamics and their relationship with climatic factors are key studies of how terrestrial ecosystems respond to climate systems. Thus,we explored the characteristics of NDVI and the relationship between NDVI and precipitation and temperature based on the SPOT- VGT dataset and precipitation and temperature series datasets. We found that NDVI and temperature rose with fluctuations between 1999 and 2012,and the linear tendency rates were 0.044/10 a and0.3° C /10 a,respectively. However,precipitation decreased at a tendency rate of- 181.7mm/10 a.Inter- annual variation in NDVI showed a trend of unimodal change with the maximum at August and minimum at March. Inter- annual variation of NDVI is increasing as a whole,but in the last four years the NDVI has decreased. Changing trends in pixel NDVI were apparent,and about92.6% of the whole pixels were increasing,of which 56.1% were significant. The trend coefficients around hydropower stations were relatively low. At a yearly scale,excessive precipitation had an inhibition on NDVI while temperature was the main positive factor driving change in NDVI. At a monthly scale,correlations between NDVI of the current month and precipitation of the month before the current month were better,showing a hysteresis response of NDVI to precipitation.Overall,the vegetation ecosystem was improving in the past 14 years although the recent four years NDVI decreased due to drought. Relationships between monthly NDVI and climatic factors represented temporal rhythm controls of precipitation and temperature on NDVI. Human activities(e.g. ecological restoration projects)were very important reasons for increasing NDVI.