Soil degradation in tropical watersheds is a synergistic process where physical mass loss is intrinsically linked to chemical instability and elemental depletion. This study quantifies soil loss and evaluates the hydro-chemical drivers of erosion in the Wimbi Sub-watershed, Poso Basin, Indonesia, using an integrated Revised Universal Soil Loss Equation (RUSLE) and Geographic Information System (GIS) framework. Analytical focus was directed toward the interaction between rainfall erosivity (R=1,378.61), topographic factors, and the chemical properties of dominant soil types, specifically Dystropepts (59% coverage) and Hapludults (15%). Laboratory analysis reveals that high erodibility (K-factor up to 0.17) is primarily driven by the depletion of Soil Organic Carbon (SOC < 2.0%) and advanced mineral weathering under acidic conditions (pH 4.0–4.8). These chemical environments facilitate soil dispersion and reduce aggregate stability, leading to significant nutrient leaching, characterized by low Base Saturation (<20%) in Hapludult groups. Spatial modeling identified 1,417 ha (7.5%) as Critical Source Areas (CSAs), where steep topography (LS up to 9.5) and high erosivity converge with poor chemical buffering capacity. Findings indicate that these hotspots are not merely zones of physical detachment but represent primary sites for the export of essential elements into Lake Poso, threatening the lacustrine chemical balance and endemic biodiversity. The results underscore that effective watershed rehabilitation must move beyond physical stabilization to include chemical soil amendments that restore structural integrity and nutrient cycles. This study provides a vital empirical foundation for site-specific forest and land rehabilitation (FLR) strategies aimed at ensuring long-term hydrological and elemental stability in tropical basins.