Abstract. In this paper, we use the Hydrologic Modeling System (HEC-HMS) to simulate two flood events to investigate the effect of watershed subdivision in terms of performance, the calibrated parameter values, the description of hydrologic processes, and the subsequent interpretation of water balance components. We use Stage IV hourly NEXRAD precipitation as the meteorological input for ten model configurations with variable sub-basin sizes. Model parameters are automatically optimized to fit the observed data. The strategy is implemented in Clear Creek Watershed (CCW), which is located in the upper Mississippi River basin. Results show that most of the calibrated parameter values are sensitive to the basin partition scheme and that the relative relevance of physical processes, described by the model, change depending on watershed subdivision. In particular, our results show that parameters derived from different model implementations attribute losses in the system to completely different physical phenomena without a notable effect on the model's performance. Our work adds to the body of evidence demonstrating that automatically calibrated parameters in hydrological models can lead to an incorrect prescription of the internal dynamics of runoff production and transport. Furthermore, it demonstrates that model implementation adds a new dimension to the problem of non-uniqueness in hydrological models.

Sheet flow featured with shallow depth on vegetated slopes plays a key role on the dynamics of soil and water loss, yet the hydrodynamic characteristics of sheet flow pasting a vegetation stem simplified by an emergent cylinder have not been revealed. Laboratory flume experiments were conducted to investigate potential effects of a vegetation stem on velocity components, flow vortexes, and shear stress from time-averaged and time-resolved perspectives. Flow fields on the upstream flow of the cylinder at the symmetry plane were captured by using a high precision Particle Image Velocimetry (PIV, 63 pixel/mm) system. Four flow conditions with flow depths from 0.4 to 0.57 cm and cylinder Reynolds number from 2440 to 3806 were selected to fully evaluate the sheet flow condition. Time-averaged hydrodynamic features were analyzed in terms of streamlines, streamwise velocity, wall-normal velocity, and vorticity. Time-resolved features of two velocity components were then analyzed. Joint probability density functions of the two velocity components exhibited asymmetrical bimodal, indicating two preferred flow states occurred frequently, namely, backflow event and downflow event. Subsequently, analyses by linear stochastic estimation showed that the backflow event was induced by a reverse upstream flow starting from the leading edge of the cylinder and penetrating primary horseshoe vortex, which was motivated by the intermediate-flow mode proposed in previous open channel flow. Meanwhile, the downflow event was induced by a portion of fluid that was unable to penetrate the primary horseshoe vortex and then vertically impinged the flume bed, which was motivated by the downflow mode proposed in this study. As the critical hydrodynamic parameter for local scouring, shear stress was finally sketched. It was suggested that soil control measures should be implemented around the vegetation stem with a radius of 0.1D (D is the cylinder diameter), where the maximum shear stress mostly occurs. The newly defined flow mode could provide deeper insights into the mechanisms of sheet flow and promote the practice of soil erosion control on vegetated hillslopes.