Global river water quality under climate change and hydroclimatic extremes

Climate change and extreme weather events (such as droughts, heatwaves, rainstorms and floods) pose serious challenges for water management, in terms of both water resources availability and water quality. However, the responses and mechanisms of river water quality under more frequent and intense hydroclimatic extremes are not well understood. In this Review, we assess the impacts of hydroclimatic extremes and multidecadal climate change on a wide range of water quality constituents to identify the key responses and driving mechanisms. Comparison of 965 case studies indicates that river water quality generally deteriorates under droughts and heatwaves (68% of compiled cases), rainstorms and floods (51%) and under long-term climate change (56%). Also improvements or mixed responses are reported owing to counteracting mechanisms, for example, increased pollutant mobilization versus dilution during flood events. River water quality responses under multidecadal climate change are driven by hydrological alterations, rises in water and soil temperatures and interactions among hydroclimatic, land use and human drivers. These complex interactions synergistically influence the sources, transport and transformation of all water quality constituents. Future research must target tools, techniques and models that support the design of robust water quality management strategies, in a world that is facing more frequent and severe hydroclimatic extremes. Maintaining good water quality is vital for healthy ecosystems and human water use, yet water quality responses to hydroclimatic extremes and changing climate are poorly understood. This Review explores river water quality under climate change and extremes. River water quality is generally deteriorating under droughts and heatwaves (68% of case studies), rainstorms and floods (51%) and multidecadal historical and future climate change (56%), although improvements and mixed responses are also reported.Droughts and heatwaves result in lower dissolved oxygen and increased river temperature, algae, salinity and concentrations of pollutants (such as pharmaceuticals) from point sources owing to lower dilution. By contrast, low flow during these events leads to reduced pollutant transport from agricultural and urban surface runoff, contributing to lower concentrations.Rainstorms and floods generally increase the mobilization of plastics, suspended solids, absorbed metals, nutrients and other pollutants from agricultural and urban runoff, although high flow can dilute concentrations for salinity and other dissolved pollutants. The sequence of extreme events (such as droughts followed by floods) also impacts the magnitude and drivers of river water quality responses.Multidecadal climate change is causing water temperatures and algae to generally increase, partly causing a general decrease in dissolved oxygen concentrations. Nutrient and pharmaceutical concentrations are mostly increasing under climate change, whereas biochemical oxygen demand, salinity, suspended sediment, metals and microorganisms show a mixture of increasing and decreasing trends.The main driving mechanisms for multidecadal water quality changes in response to climate change include hydrological alterations, rises in water and soil temperatures and interactions of hydroclimatic drivers with land use. These impacts are compounded with other human-induced drivers.Our findings stress the need to improve understanding of the complex hydroclimatic-geographic-human driver feedbacks; water quality constituent fate, transport, interactions and thresholds; and to develop technologies and water quality frameworks that support the design of robust water quality management strategies under increasing hydroclimatic extremes.