Comment on “Development and testing of a new storm runoff routing approach based on time variant spatially distributed travel time method” by Jinkang Du, Hua Xie, Yujun Hu, Youpeng Xu, Chong-Yu Xu

presented a time variant spatially distributed travel time method in order to simulate storm runoff response for a watershed in China. There are a number of issues raised in their paper that require further thoughts. One of the important issues is the assumption of travel time being dependent on the flow velocity rather than the wave celerity. Another problem deals with the introduction of a parameter (K) that accounts for the estimation error for roughness coefficient and bed slope. While the default K value must be unity, a value of 7.5 was calibrated. Other issues deals with ignoring the effect of flow accumulation in overland cells, calibration of a sensitive channel threshold parameter, trying to keep the mass conserved through the routing procedure while the isochrone locations vary in time, and the application of SCS infiltration method during the no-rain period. This commentary elaborates on the above issues.