Assessment of urban runoff and initial pollutant wash-off responses based on future scenario precipitation

In the context of climate change, the future precipitation patterns have brought great uncertainty to the drainage systems of mountainous cities, which further exacerbates the risks of urban flooding and non-point source pollution. This study aims to access the impact of projected rainfall on drainage loads and pollutant wash-off dynamics based on three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, and SSP5-8.5) in Qijiang District, Chongqing. The CMCC-ESM2 climate model was combined with the Storm Water Management Model (SWMM) to perform the simulations. Results showed that as the severity of the scenarios set in the scenario simulation increased, both the rainfall intensity and the pollutant load showed a gradual upward trend. Under the SSP5-8.5, for a recurrence period of 100a, the 24h design precipitation increased by 38.65% compared to the historical baseline. In contrast, the increase was more moderate in the SSP1-2.6 and SSP2-4.5 scenarios. Surface runoff exhibited a disproportionate amplification across all scenarios due to infiltration saturation. The 10a runoff magnitude under SSP5-8.5 approximated the historical 50a level. Total pollutant loading showed a stepwise increase across the three pathways, where COD and TSS exhibited greater sensitivity to climate fluctuations than TN and TP. The first flush effect (FF30) strengthened alongside emission intensities, with indices under SSP5-8.5 persistently exceeding the 0.6 strong-effect threshold. This puts a massive strain on existing infrastructure, both hydraulically and environmentally. By elucidating the systematic response of urban catchments to climate change, this research provides a vital scientific basis for informing resilient urban water management in mountainous regions.