Evaluating the Applicability of wflow_sbm Model with Seamless Parameter Maps for Streamflow Simulation in Small and Medium-Sized River Basins

Flood simulation in small and medium-sized catchments across China is constrained by limited hydrometeorological data and pronounced hydroclimatic heterogeneity. The wflow_sbm model offers promising potential, as seamless parameter fields can be estimated from global datasets via pedotransfer functions (PTFs), enabling explicit representation of the spatial and temporal variability of catchment characteristics. However, the extent to which parameter sensitivity differs between humid and semi-humid regions with distinct runoff-generation mechanisms remains insufficiently understood. Moreover, it is unclear whether parameters derived by PTFs are directly applicable to small and medium-sized catchments or require regional adjustment.

In this study, the wflow_sbm model is applied to two representative Chinese catchments: the humid Tunxi basin and the semi-humid Chenhe basin. Distributed parameters are derived from global datasets using HydroMT model setup and preprocessing framework. We (1) systematically analyze the sensitivity of three key parameters about soil water dynamics (KsatHorFrac, InfiltCapSoil and SoilThickness) in humid and semi-humid basins, (2) assess the applicability of seamless parameter maps derived by PTFs and evaluate the necessity of regional adjustment, and (3) benchmark the performance of wflow_sbm against the well-established Xin’anjiang (XAJ) model in China, including multi-site validation to assess spatial robustness.

Results reveal clear regional differences in parameter sensitivity: KsatHorFrac and InfiltCapSoil dominate runoff responses in the humid Tunxi basin, whereas KsatHorFrac and SoilThickness exert the strongest control in the semi-humid Chenhe basin. The PTF-derived SoilThickness (~2 m) in Chenhe leads to systematic underestimation of flood volume and peaks. Reducing it to ~0.2 m substantially improves model performance and is consistent with vadose-zone depth estimates from the XAJ model, highlighting SoilThickness as a key control in semi-humid basins. The results also show that the wflow_sbm model achieves performance comparable to XAJ in both catchments, with an average NSE of 0.85 in Tunxi and generally NSE >0.7 in Chenhe. The good performance at the internal stations in Tunxi (average NSE > 0.70) further demonstrates that the parameter maps derived by PTFs are applicable and reliable for small and medium-sized basins.

Overall, wflow_sbm is applicable for flood simulation in small and medium-sized catchments in humid and semi-humid regions and is particularly advantageous in data-scarce basins. However, if its application in semi-humid regions requires appropriate adjustment of SoilThickness, which can be guided by parameter ranges inferred from the XAJ model.