Hydrological response in a highly urbanized watershed in China

Urbanization has a strong signal on the hydrologic cycle, leading to reduced infiltration and faster and larger runoff. However, the detailed analysis of the contribution of urbanization to different quantiles of discharge is still lacking; particularly, less is known in watersheds that have been experiencing such large and rapid urbanization as those in China. Here, we focus on the Wenyu watershed, a rapidly urbanizing basin located in the Beijing metropolitan area. Using a statistical attribution framework, we examine the hydrological response to the increasing urbanization across a wide range of discharge quantiles, from low to high flows; moreover, we perform analyses at the seasonal scale to capture differences in the physical processes at play during the year. Results indicate that the selected GAMLSS (generalized additive models for location, scale, and shape) models can capture well the variability in streamflow in this highly urbanized basin, with the average Spearman correlation coefficients between observations and the median of the fitted models of 0.84, 0.79, and 0.81 in spring, summer, and winter, respectively. Overall, urbanization played a different role in the different seasons and discharge quantiles. More specifically, we find its strongest impact to be in winter and spring, and for low and median quantiles. The role of precipitation is the strongest in summer, and it increases as we move toward the upper tail of the discharge distribution, especially above the 55th percentile, for which precipitation is selected as the only important predictor. Recycled water, on the other hand, tends to play a more dominant role in winter and spring.