Global analysis reveals persistent land-cover thresholds for river water quality across climates and decades

Agricultural and urban land uses are widely recognized as major drivers of global surface water quality. However, it remains unclear whether the magnitude of the impacts scales universally and consistently across contrasting climatic and geographical settings based on the intensity of land-use change. Here we analyse more than 19 million river water quality observations from 117 countries over 28 years, linking ~ 101,000 monitoring stations to upstream basin land cover derived from ESA CCI. Cropland and urban areas reveal the strongest global positive correlations with nutrient and sediment concentrations, while forested landscapes are consistently associated with lower pollutant levels. A dose-response analysis (term borrowed from toxicology) based on 1.7 million basin–year records shows that land-cover impacts on water quality are not merely categorical but scale progressively across the full gradient of land-cover intensity. Nutrient concentrations increase by up to 10-fold for nitrate and 5.7-fold for total nitrogen with expanding agricultural and urban land use, whereas forest cover is associated with reductions of up to 12-fold in both nitrogen and phosphorus. Importantly, these relationships are strongly non-linear: the conversion of the first 20–40% of a basin to cropland drives disproportionately large increases in nutrient loads, revealing critical and actionable land-use thresholds for watershed management.