Coupled hydrologic and hydraulic modeling for a lowland river basin in China

Spatially distributed hydrologic models are effective tools to simulate the terrestrial water cycle at a large scale, but limitations in flow routing would suffer modeling accuracy for their applications over river basins featuring hydraulically complex drainage systems. Passing outputs from the hydrologic model into the hydraulic model for flow routing as the so-called external coupling has been a common practice to improve modeling accuracy. However, such an approach could be cumbersome and less effective for real-time flood management. In this study, an internally coupled hydrologic-hydraulic model (ICM) was developed from the SWAT+ and SWMM models and was thoroughly evaluated for parameter sensitivity and modeling accuracy by comparing with the hydrologic model (HM) SWAT+ for their applications to a lowland watershed in Central China. The results indicate the most sensitive parameters for the vertical hydrological processes remain the same for ICM and HM although the rankings of the sensitive parameters changed and the ICM modeling accuracy has greatly improved over HM, with the NSE increased from 0.636 to 0.858 and from 0.595 to 0.831 for the calibration and validation periods, respectively. This study also showed a clear improvement of model performance by the synthetic coupled approach (SCM) or so-called external coupling. However, SCM is under performed than ICM and also had a much larger system bias than ICM. Overall, this study showed the fully coupled model is most suitable for improved simulations of hydrology and hydraulics over lowland watersheds, which could provide more reliable results for watershed management.