Attribution of climate change imprint on riverine nutrient export from diffuse pollution sources to African coastal waters

Nutrient pollution derived from anthropogenic activities impacts both inland and coastal waters, altering the aquatic ecosystem and resulting in serious environmental issues. As climate change is affecting most of the hydroclimatic variables across the world, a fundamental concern in river ecology is therefore to understand the degree to which the spatial patterns and variations of nutrient concentration and loading in rivers during the last decades can be associated with climate change. This study detects and attributes the impact of historical climate change on long-term changes in the flux of nutrients from diffuse pollution sources into the coastal waters of Africa. An impact attribution approach is employed by forcing a continental process-based water quality model (Soil and Water Assessment Tool – SWAT+) for Africa with a set of observational and counterfactual climate data from the impact attribution setup of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP3a). The nonparametric Mann–Kendall test is used to identify trends while long-term mean annual river nutrient simulation differences between a model setup with observational and counterfactual climate data are calculated to allow for quantification of the climate change attribution. Results show spatial differences with climate change reasonably contributing to both an increase and decrease of both riverine Total Nitrogen and Total Phosphorus to African coastal waters. However, the climate change imprint on the riverine nutrient export is starting to emerge within the 21st century years for most rivers. These findings show spatial differences in the sensitivity of impacts of climate on riverine TP and TN export to coastal waters while highlighting the most impacted rivers in Africa.