Mapping of land degradation in Vietnam using NDVI MODIS time-series

In Vietnam, land degradation is a serious problem for food security and development of society, particularly in tropical countries where livelihoods are often agriculture-based. The agricultural sector is a significant contributor to the country's economy, and land degradation can have severe impacts on crop yields and food security. The Normalized Difference Vegetation Index (NDVI) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) of the NASA's Earth Observing System has been used in several studies to assess vegetation productivity and land degradation. The MODIS NDVI dataset has finer resolution than the Global Inventory Monitoring and Modeling System (GIMMS) NDVI data, which has been used in previous studies to assess long-term biomass productivity decline in Vietnam. This study aimed to use the Terra MODIS NDVI at 1 km resolution to identify the decline of the long-term trend of vegetation productivity in Vietnam and to identify the areas of human-induced productivity decline by isolating the land degradation area from climate-driven impacts. The study used a pixel-based analysis of the temporal trend in biomass productivity based on the MODIS NDVI time-series for the period of 2001-2016. The trend was measured from the significant slope coefficient (A) in the linear equation NDVI = A × year + B, where A and B are constants. The trend was considered significant at p < 0.1. The significantly negative long-term trend of NDVI (slope coefficient A) can be attributed to either climate change or human activities. To differentiate areas of human-induced biomass productivity decline from those in which the decline is driven by climate dynamics, the study applied two methods: the trend-correlation stepwise analysis (Trend-Correlation) and residue trend analysis (RESTREND). The results showed that the area of significant decline of biomass productivity during 16 years in Vietnam were 15% of total national area. The degraded areas were found in the Northwest, the Central Coast, the Central Highland, the Southeast, and Mekong River Delta. The findings agree with the fact that the intensification of rice-based agriculture in these areas has increased considerably since the beginning of the Era of Renovation (Doi Moi) in Vietnam in 1986. The study's results may help researchers and policy makers identify the specific locations in Vietnam where more detailed actions may be required to mitigate vegetation productivity decline, thereby combating land degradation. The study's methodology involved the use of MODIS NDVI time-series data for the period of 2001-2016, which was analyzed based on linear regression to identify the improvement or decline of NDVI over long time. The value of slope (A) of annual NDVI over 16 years was ranged from -0.04 to 0.03. The significant test at p < 0.1 was carried out for slope A. The trend in long-term vegetation productivity in Vietnam over 16 years (p<0.1) is shown in Fig. 2, which indicates that the areas of significant improvements in biomass productivity comprise about 33% (109,063 km2) of total national land, and they were mainly found in the Northeast-Red River Delta, North Central Coast, and Southest-Mekong River Delta.