Earth Surface Processes
LIU He-ping, WANG Xiu-ying, LIU Bao-yuan
Flow velocity is not only an important hydraulic property of overland flow, but also an indispensable index to calculate other hydraulic parameters. The purpose of this paper is to investigate the spatial distribution of rill and interrill flow velocity along the hillslope. Simulated rainfall and 7 plots (5 degrees steepness) with the lengths of 1, 2.5, 4, 5.5, 7, 8.5 and 10 m, repectively, were employed. The tested soil was a silty loam, sampled from Yanqing County, Beijing. For each slope length, the experiment consisted of two identical rainfall events (dry run and wet run), timed apart about 24 hours. Each rainfall lasted 2 hours with varied rainfall intensity (27-142 mm/h) and a total rainfall of 121 mm. Flow velocities in rill and interrill area were measured with dying tracing at different slope positions between 60 and 70 min of each rainfall. The results showed that the interrill flow velocity, on plots of all slope lengths and for both dry and wet runs, varied from 0.03 to 0.1 m/s, whereas the rill flow velocity ranged between 0.15 and 0.45 m/s. The latter was 2 to 4 times of the former. Both rill and interrill flow velocity were power functions of L, the distance downward from the upper end of the plot, and the exponents of L for rill were larger than those for interrill, being 0.409-0.420 and 0.175-0.218, respectively, which was due to the concentration of flow into rills along the slope. Additionally, the rill velocity in wet run was higher and accelerated faster than that in dry run, whereas it was opposite for interrill flow velocity, which may be attributed to the fact that more water was delivered into rills and the rills were stabler and smoother in wet run. The results are helpful in understanding the processes and mechanism of rill and interrill erosion.