Comparative analysis of drought indicators for effective vegetation and soil moisture monitoring in the Oueme River Basin, Benin

This study analyzes and compares drought indices, specifically the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), with the Normalized Difference Vegetation Index (NDVI), the Vegetation Condition Index (VCI), the NDVI anomaly (An-NDVI), the VCI anomaly (An-VCI), and the Standardized Soil moisture Index (SSI) to determine the most appropriate index for vegetation and soil monitoring in the Oueme River Basin (ORB), Benin, West Africa. Using the Mann–Kendall (MK) trend test, drought characteristics assessment, and Pearson correlation coefficients, the study revealed an annual decreasing trend in the SPI and SPEI. Drought characteristics varied across different timescales, with increasing intensity and frequency observed over shorter periods. Both the SPI and SPEI exhibited a declining trend, with the SPEI showing a more pronounced decrease due to the influence of rising temperatures on potential evapotranspiration. Additionally, the SPI and SPEI demonstrated strong correlations with the SSI, while both indices exhibited positive correlations with NDVI, VCI, An-NDVI, and An-VCI. Notably, the SPEI showed higher correlation coefficients with these vegetation indices compared with the SPI, highlighting its greater sensitivity to environmental stressors. Thus, the study concludes that the SPEI-3 indices are more suitable for evaluating vegetation and soil conditions than the SPI-3 index.