Based on Clark unit hydrograph model, a variable distributed unit hydrograph model for urban drainage system is presented with different rainfall intensity. Firstly, urban watershed is decomposed into several individual sub-catchment units; then, different unit hydrograph for each sub-catchment unit is modeled as a single linear reservoir; finally different unit hydrograph is delayed and summed to outlet according the time of concentration along underground pipe network. The values of two model parameters including response parameter and delayed time of concentration can be directly estimated without the need of calibration considering the effect of rainfall intensity on flow velocity. The north region of Future Science Park in Beijing was selected as the study area, application results showed that: (1) the unit hydrograph for whole catchment area was different, in which the peak values increased with the increase of rainfall intensity, and the increasing rate showed gradual decrease tendency; (2) the simulated runoff processes are in reasonable agreement with the monitored runoff processes, in which the average value of R2 was 0.73, which varied from 0.4 to 0.89, the average value of NSE was 0.26, which varied from -0.77 to 0.87, the average value of RE was -0.18, which varied from -0.77 to 0.36; (3) the unit hydrograph for each sub-catchment unit was spatially varying, and varied with the rainfall intensity, in which with the increase of rainfall intensity, the response parameter increased by the 0.4 power function and delayed time of concentration decreased by the -0.6 power function, respectively.

The actual rainfall–runoff control effects of a low-impact development (LID) system comprising multiple facilities within a large-scale area need to be monitored and examined, including not only the rainfall volume capture effect but also the rainfall control mode, which is influenced by the confluence relationship. The confluence relationship between an impervious surface and LID facilities can be manifest as a series or parallel system. For individual rainfall events smaller than the designed rainfall, rainfall control operates in event-capture mode for a series system and in partial-capture mode for a parallel system. Although the rainfall volume capture ratios of series and parallel systems might be equivalent, a series system is more suitable for water quality improvement and peak flow reduction. This study considered the Future Science Park of Beijing as a case study site. Monitoring indicated that the rainfall volume capture effect is acceptable because the average runoff coefficient is approximately 0.10; however, rainfall control operates in partial-capture mode. Investigation and analysis showed that runoff is generated by three mechanisms: impermeable road surfaces, infiltration excess, and pipe storage-water emission. When rainfall volume is less than the designed rainfall, the little runoff that occurs is generated mainly by the impermeable road network. The rainwater infiltration capacity of permeable facilities might reach 44 mm. In future, to improve rainfall–runoff control effects, impermeable roads and green belt areas alongside the roads should realize series confluence.