Spatial and Temporal Dynamics of Vegetation Greenness in the Sahel-Sudan Zone of North West Nigeria Using NDVI Time-Series Analysis

Vegetation greenness dynamics in the Sahel-Sudan transition zone of North West Nigeria reflect complex interactions between rainfall variability, land use change, and vegetation resilience, and understanding their spatial and temporal patterns using long-term NDVI data is essential for evidence-based dryland management. This study analysed spatial and temporal dynamics of vegetation greenness in Sokoto, Kebbi, and Zamfara States, covering the Sahel-Sudan transition zone, using a 22-year MODIS NDVI time-series dataset (2001 to 2022, 16-day composites, 250m resolution). Seasonal and interannual NDVI trends were decomposed using the Breaks for Additive Seasonal and Trend (BFAST) algorithm. Correlation analysis between NDVI anomalies and rainfall anomalies (from CHIRPS dataset) and temperature anomalies (from ERA5 reanalysis) was performed. Land use land cover change was mapped from Landsat imagery to contextualise NDVI trends. Results showed that 64% of the study area had significant positive NDVI trends (greening) over the 22-year period, consistent with the broad Sahel greening phenomenon reported in earlier studies. However, 23% of pixels showed significant browning trends, concentrated in heavily cultivated areas around major urban centres. Rainfall explained 72% of interannual NDVI variability, while temperature had a secondary but significant negative effect. BFAST detected 214 significant breakpoints in NDVI trends, corresponding predominantly to drought years and post-conflict agricultural abandonment events. Agricultural expansion accounted for the majority of greening in 40% of greening pixels, representing human-induced rather than natural vegetation recovery. The study provides fine-resolution evidence on vegetation dynamics and recommends land use-stratified NDVI analysis for future dryland monitoring programmes.

Keywords: NDVI, vegetation dynamics, Sahel-Sudan, North West Nigeria, BFAST