NDVI-based vegetation changes and their responses to climate change from 1982 to 2011: A case study in the Koshi River Basin in the middle Himalayas

Considerable researches during the past several decades have focused on monitoring changes in vegetation growth due to its important role in regulating the terrestrial carbon cycle and the climate system. In this study, we combined datasets of the satellite-derived Normalized Difference Vegetation Index (NDVI) and climatic factors to analyze spatio-temporal patterns of vegetation growth in 1982-2006 in the Koshi River Basin (KRB) in the middle Himalayas. In addition, the dataset from the global land surface satellite sensor from SPOT-4's Vegetation instrument in 1998-2011 was used to further verify the results of this study. At the regional scale, although a statistically significant increase in average growing season NDVI was observed (0.0008 yr− 1, P = 0.03) during the entire study period, there existed three distinct periods with opposing trends. Growing season NDVI significantly increased in 1982-1994 (0.0019 yr− 1, P = 0.03), and then decreased in 1994-2000 (− 0.0058 yr− 1, P < 0.001), while increased again in 2000-2006 (0.0049 yr− 1, P < 0.001) and in 2000-2011 (0.0034 yr− 1, P < 0.001). A sudden drop in NDVI in 1994-2000 largely contributed to these fluctuations in growing season NDVI over the KRB. Spring, summer and autumn NDVI significantly decreased in 1994-2000 (− 0.0077 yr− 1, P < 0.001; − 0.0067 yr− 1, P < 0.001; and − 0.0155 yr− 1, P < 0.001; respectively). Our further spatial analyses supported the existence of the sudden decrease in spring, summer and autumn NDVI in 1994-2000.