Future climate change and urban growth together affect surface runoff in a large-scale urban agglomeration.

• Climate change and urban growth combined caused surface runoff increases in an urban agglomeration. • At regional and city scales, climate change was the main driver of the runoff change. • At sub-city scale, urban growth might have the greatest effect on the runoff change. • Solutions at multiple scales are needed to surface runoff in mega-cities. Climate change and urban growth have been shown to alter hydrological processes, particularly surface runoff. While there is widespread evidence of the combined effects of climate change and urban expansion on surface runoff at the watershed scale, little is known about their combined effects at different scales in large-scale urban agglomerations. Using integrated spatial models, we simulated the combined effects of climate change and urban growth on surface runoff in the Beijing-Tianjin-Hebei urban agglomeration, China, in 2030. We found that the surface runoff increased under both climate change and urban growth scenarios but the relative contributions of these two factors to surface runoff change varied from the regional to the sub-city scale. At regional and city scales, climate change was the main control on surface runoff change, while urban growth had the most effect on surface runoff change in the main urban areas of some cities, i.e., the sub-city scale. The results indicate that one single factor does not give reliable information for managing surface runoff in megacities, as the effects of other factors are ignored. These findings highlight that integrated solutions must be tailored to the appropriate spatial scale to effectively regulate surface runoff change across large-scale urban agglomerations. [ABSTRACT FROM AUTHOR]