Assessing seasonal trends and variability of vegetation growth from NDVI3g, MODIS NDVI and EVI over South Asia

The characterization of vegetation dynamics over South Asia (SA) has been primarily conducted using satellite time series of Advanced Very High Resolution Radiometer (AVHRR) Normalized Difference Vegetation Index (NDVI). However, various vegetation indices may show diverse trend patterns over the same area. This study analysed the consistency of the vegetation spatiotemporal trends from AVHRR version 3 NDVI (NDVI3g) with the Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI and Enhanced Vegetation Index (EVI) over SA during various seasons, assuming that MODIS products are of higher quality. Results showed that the spatiotemporal vegetation trends derived from the NDVI3g were analogous to both MODIS NDVI and EVI indicating greening over semi-arid regions where croplands dominate and browning over tropical/subtropical forest areas. Correlations among them were better during winter monsoon. Discrepancies occurred in tropical/subtropical densely vegetated (humid) and complex topographic areas specifically during summer monsoon (SM). This study improved the understanding of the heterogeneous vegetation trends over the vast complicated terrain of SA. It was revealed that NDVI3g is reliable to quantify vegetation trends over SA, however, calibration errors still could introduce biases during SM season.