On the need to account for soil erosion rates in water quality indices for tropical Rivers: Insights from Cesar River, Colombia

Abstract

A large body of research suggests that soil erosion in developing countries is reaching critical rates and that its offsite effects (e.g., increased sediment concentration in water bodies) could be amplified by global warming. In most cases, Water Quality Indices (WQI) do not differentiate between natural and man-made loads, potentially biasing assessments and management decisions. However, this discrimination is particularly important in tropical rivers where intense rainfall, high erosion rates, and sediment connectivity strongly influence water quality. This contribution introduces the Integrated Water Quality Index with Sediment Load (WQIssy), a sediment-sensitive metric which incorporates the quantitative input of natural erosion into water quality metrics. In operative terms, WQIssy integrates RUSLE-GGS (Rosas & Gutierrez, 2020) soil erosion estimates with PCA-based sub-indices. WQIssy is applied to the Cesar River (∼20,000 km2), one of the main tributaries of the Lower Magdalena River (Colombia). To this end, 11 water quality parameters derived from 15 monitoring stations (year 2023) and 9 virtual stations derived from the DynQual model were analyzed. Our results show that natural erosion increases turbidity (>100–500 NTU) in the downstream section of the Cesar River, modifying the WQI classification in the middle and lower reaches of the river, where several stations went from "acceptable" to "fair" or "poor" category in the WQIssy scale. These findings underscore the fact that erosion-driven sediment dynamics can significantly alter water quality interpretation in tropical rivers. By distinguishing natural and anthropogenic inputs, WQIssy reduces overdiagnosis and supports cost-effective river restoration and basin management in erosion-prone tropical basins.