The evolution of the overland flow velocity along the distance downslope on smooth and granular beds in different cases is investigated by means of the electrolyte tracer via flume experiments.The results demonstrate that a non-uniform flow regime and a uniform flow regime exist in the development process of the overland flow.Owing to the different attributes of beds’roughness,the position of those zones with different flow regimes varies correspondingly:(1)the overland flow on granular beds enters into the uniform regime much sooner,additionally,the roll waves tend to appear because of the presence of the proper flow resistance imparted by the roughness(coarse sands),and large slopes(20o and 25o)which makes the flow velocities and depths to undulate spatially.Furthermore,the flow resistance of the overland flows with different roughness elements,that is the non-sands,the fine sands and the coarse sands,is calculated.A quadratic interpolation method of the third order accuracy is employed in the calculation of the longitudinal flow resistance.The results show that it is rational to use the bed slope to approximate the hydraulic energy slope over a relative small roughness(the present roughness),however on the other hand,if the mean flow velocities and depths rather than the local parameters are used to calculate the flow resistance,a considerable error will be induced within the non-uniform regime of the overland flows,including the acceleration zone and the roll-wave zone. The evolution of the overland flow velocity along the distance downslope on smooth and granular beds in different cases is investigated by means of the electrolyte tracer via flume experiments. The results demonstrate that a non-uniform flow regime and a uniform flow regime exist in the development process of the overland flow. Orwing to the different attributes of beds’roughness, the position of those zones with different flow regimes vary correspondingly: (1) the overland flow on granular beds enters into uniform regime much sooner, additionally, the roll waves tend to appear because of the presence of the proper flow resistance imparted by the roughness (coarse sands), and large slopes (20o and 25o) which makes the flow velocities and depths to undulate spatially. Morerther, the flow resistance of the overland flows with different roughness elements, that is the non-sands, the fine sands and the coarse sands, is calculated. A quadratic interpolation method of the third order accuracy is emp loyed in the calculation of the longitudinal flow resistance. the results show that it is rational to use the bed slope to approximate the hydraulic energy slope over a relative small roughness (the present roughness), but on the other hand, if the mean flow velocities and depths rather than the local parameters are used to calculate the flow resistance, a considerable error will be induced within the non-uniform regime of the overland flows, including the acceleration zone and the roll-wave zone.