Comment on “Effects of different tillage practices on the hydraulic resistance of concentrated flow on the loess plateau in China” by J. Sun et al

For concentrated flows, which are characterized by small water depth and steep sloping beds, hydraulic conditions different from those typical of streams and rivers occur. In this study a new theoretically deduced flow resistance equation was tested using the experimental data by Sun et al. for three different tilled surfaces (Manual Dibbling, MD, Manual Hoeing, MH, and Contour Drilling, CD). At first, the profile parameter-relationship, which is the relationship between the velocity profile parameter Γ, the channel slope and the flow Froude number, was calibrated using rill flow data by Di Stefano et al. Then, the applicability of this relationship was tested by the measurements of Sun et al. The calculated Darcy–Weisbach friction factor values were generally lower than those measured by Sun et al. and this result was justified taking into account that the rill measurements used for calibrating the profile parameter-relationship simply correspond to grain resistance conditions. For taking into account both grain resistance and tillage induced surface roughness, the profile parameter-relationship was calibrated by the measurements by Sun et al. This calibration was carried out for each tillage practice (MD, MH and CD) and using all the measurements carried out during each 20 min – experimental run. This study was also developed using exclusively the data concerning the beginning of the experiment (0–4 min), in order to state the effect of the initial roughness, and those regarding the end of the experiment (16–20 min) to estimate the influence of the temporal evolution of roughness on the friction factor. The study proved that the Darcy–Weisbach friction factor due to grain resistance and tillage induced roughness can be accurately estimated using the proposed theoretical approach. A temporal variation of the Darcy–Weisbach friction factor was also recognized. Finally, the data were supportive of the slope independence hypothesis of flow velocity stated by Govers.