Riparian erosion hazard index (EHI) modeling: Deciphering the soil loss discrepancies in mountainous tropical rivers

Abstract

Riparian zones in upstream watersheds are important for hydrological stability but increasingly endangered by climate change and human activities. This study quantified the potential erosion hazard along the Way Ilahan and Way Merabung rivers in Lampung by using the Universal Soil Loss Equation (USLE) with 52 sampling points. The parameters considered were rainfall erosion (R), soil erodibility (K), topography (LS), and land management (CP). The mean annual rainfall erosion is 962.12 mm/yr with a high geomorphological asymmetry. On the left edge, the extreme LS value (267.90) indicates soil loss well beyond the tolerable threshold (TSL = 179.20 t/ha/yr). The most significant degradation was observed on the left bank of the Way Merabung River, with an erosion rate of 49,818.95 t/ha/yr and an Erosion Hazard Index (EHI) of 278.01, more than 270 times the natural soil regeneration capacity. Conservation modeling indicated that the combination of forest cover and contour planting on a 20% slope reduced the predicted soil loss to 51.39-427.46 t/ha/yr, changing the hazard classification from “very high” to “low-moderate”. These results highlight land management (CP factor) as a major driver of increased erosion. Urgent interventions such as agroforestry and mechanical contour planting on the vulnerable left bank are essential to reduce severe downstream sedimentation.