Could planting trees along urban rivers improve water quality?

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Nature-based solutions are increasingly advocated as a way to improve urban aquatic environments, among other benefits, and tree planting is widely adopted. But local complexities and spatial variation in urban hydrological responses make the extent of water-quality improvements difficult to predict.

To address this, researchers described QUESTOR-YARDSTICK (QUESTOR-YS), a standardised approach to process-based simulation of urban river quality. It removes the influence of point-source pollution and harmonises how river hydrodynamics and the size of the contributing catchment are represented. This focuses comparisons between cities on water-quality differences due solely to climate, river discharge and urban diffuse nutrient pollution. The approach could potentially quantify the relative sensitivity of urban water bodies in different cities worldwide to nature-based solutions, and can readily be extended to include wastewater effluents.

The approach's validity was demonstrated for a small river in Birmingham, UK. It was then demonstrated for 10 km of riparian tree planting in Birmingham, Oslo, Norway, and Aarhus, Denmark. Modelling suggests that planting could substantially improve three key water-quality indicators in each city under sensitive summer conditions: water temperature, chlorophyll-a and dissolved oxygen. The results indicate that the benefit from a fixed amount of planting depends on the existing extent of tree occupancy along the riverbank.