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In the Nilgiris region, accelerated soil erosion is a major land degradation problem, resulting in loss of fertile topsoil, reduced agricultural productivity, reservoir sedimentation, and environmental degradation. This study developed a simplified Erosion Susceptibility Index (ESI) methodology for identifying critical erosion-prone areas within a watershed by selecting a representative, KG-4-1 watershed as study area belonging to Kundah catchment of the Nilgiris district, Tamil Nadu, India. The approach integrates land use-specific factors including slope, land management conditions and vegetation characteristics, and a weighted scoring system derived from univariate regression-based relationships between individual factors and Universal Soil Loss Equation (USLE) estimates. Study findings clearly demonstrated that only 11.46% of the watershed area exhibits critical erosion susceptibility (ESI >0.62) with potential soil loss exceeding 20 t ha −1 yr −1 , while 55.06% area falls under non-critical (ESI < 0.33) indicating potential soil loss within the limit of 5 t ha −1 yr −1 . Analysis of treatment projections with suitable conservation interventions in different land units of the watershed indicated substantial potential for reducing watershed-scale soil loss through targeted treatment of erosion-prone areas, with 60% area of the watershed identified as treatable. Scenario analysis showed that treating the identified critical and very critical erosion-prone areas could achieve ~53% of the total attainable soil loss reduction. Further, inclusion of moderately critical areas (ESI >0.48, 25.77% area) enables capture of a major share of the total attainable soil loss reduction potential at the watershed scale. Validation of ESI in another representative watershed (Thambatti, 500.9 ha) situated in the Nilgiris, confirmed methodology transferability, showing 10.70% area exhibiting critical to very critical susceptibility. Thus, this methodology provides a robust and practical tool for field practitioners to prioritize conservation interventions and optimize resource allocation in watershed development and safeguarding land resource.