Assessing dominant uncertainties in urban buildup/washoff processes under climate change: A case study in Western Switzerland

• We assess the impact of future rainfall on urban surface runoff pollution.
• We evaluate and compare climate predictions and b/w models as uncertainty sources.
• The highest sources of uncertainty come from the choice of the GCM/RCM model.
• Future computed concentrations are slightly higher in summer than those of today.
• Higher pollution implies higher risk for the environment and recreational activity.

Urban runoff is often contaminated. The interval, duration and intensity of rainfalls control the transport of pollutants. Future climate change will modify both frequency and intensity of rain events. Consequently, the concentration of pollutants in future runoff driven by buildup/washoff processes will be affected by such changes. We investigated to what extent modifications in future rainfall distributions will impact the pollutant concentration in urban surface runoff. Three emissions scenarios, multiple combinations of RCM/GCMs and modifications to rain event frequencies were used to simulate possible future rainfall distributions with various characteristics. Simulated rainfall timeseries were used as inputs for four pairs of buildup/washoff models, in order to compare future surface runoff pollution concentrations. We demonstrated that urban runoff pollution undergoes a perceptible increase in summer, and a decrease in winter, when runoff is more dilute due to rain water. The climate scenario chosen led to a higher source of uncertainty in the median runoff water quality estimates. The setting of b/w model parameters has minimal impact on predicted mean and median concentrations. However for the high concentration limit, the main sources of uncertainty differ, indicating that the choice of the water quality model dominates the source of uncertainties for extreme events.