Soil infiltrability is a one of the key parameters in the fields of agriculture and near surface hydrology. The rapid and accurate measurement has a theoretical and practical significance. However, it is difficult to accurately measure the extremely high infiltration rate in the initial infiltration stage, due to the limitations of measurement principle. In addition, the presence of vegetation cover can also affect the acquisition of surface wetted area in the point source infiltration body, leading to an increase in the measurement error of infiltration rate. In this study, the system was proposed to achieve the automatic and accurate measurement of soil infiltrability with the presence of surface cover. The image correction and preprocessing were adopted to extract the surface wetted area. An infrared thermal imager was also used to record the progress of surface wetted area over time. The shape-based function of soil infiltrability curve was developed using the first-order derivative of the variation of soil wetted area over time. A complete soil infiltrability model was then established, according to the principle of water balance. A device was built to measure the soil infiltrability. The laboratory and field experiments were carried out to validate the measurement accuracy. The indoor experiments were conducted with the soils of sandy loam and silty loam, under surface slope gradients of 0˚, 5˚ and 8˚, and surface coverage of 20%, 40%, 50%, 60%, 65%, 70%, 75% and 80%, respectively. The field validation experiments were conducted on three sites with the natural surface covers, in order to measure the changes in surface wetted area and soil infiltration rate over time. The results show that the surface slope gradient and soil type shared no significant effects on the accuracy of surface wetted area monitoring, whereas, the surface coverage posed a significant impact on the recognition of surface wetted area. Especially, the recognition error of surface wetted area was about 5% with the surface coverage less than 60%, fully meeting the requirements for soil infiltrability measurement. The measurement error of surface wetted area significantly increased, when the contour line was covered by the vegetation from 70% to 80%. The determination coefficients of the fitted equations between the surface wetted area and time were not all less than 0.98, indicating the better performance of the model on the surface wetted area over time under the vegetation cover conditions. Both surface wetted area function and soil infiltrability function also performed the best in the laboratory and field experiments. The water balance errors of indoor and field infiltration experiments were both less than 2%, indicating a relatively high accuracy of soil infiltrability model without the disturbance of the natural soil surface. The finding can provide an alternative way easy to apply with a sufficient accuracy, particularly for the in-situ measurement of soil infiltrability in filed with the presence of natural surface cover.

The velocity of sheet flow on slopes is one of the key hydrodynamic parameters, and the velocity profile of sheet flow is important for understanding the soil erosion mechanism. The measurements of the slope sheet flow velocity were conducted under various underlying surfaces roughness through the tracing methods with the salt tracer and heat tracer, respectively, to investigate the influence of the underlying surface on the vertical velocity distribution of the sheet flow. The velocities of sheet flow were measured at three slope lengths (2, 3, and 4 m) under three slope gradients (5°, 10°, and 20°) and three flow rates (2, 5, and 8 L/min) on the slopes with various underlying surface roughness of polymethyl methacrylate, 80-mesh sandpaper (0.16 mm), and 24-mesh sandpaper (0.53 mm), respectively. The measurements were conducted in the water flumes with fixed bed, and the internal size of the flume was 5 m long, 0.2 m wide, and 0.2 m deep. The centroid velocity was adopted as the sheet flow velocity, and hydraulic parameters such as water depth, Reynolds number, Froude number, and Darcy-Weisbach resistance coefficient were also calculated to provide the background information of the experimental water flows. The results showed that the roughness of the underlying surface and the water depth both have significant effects on the velocity profile of the sheet flow (P<0.05). On the smooth underlying surface of polymethyl methacrylate, the boundary layer was fully developed to achieve a turbulent flow. The combined tracing method by using the salt and thermal tracers can be used to characterize the velocity profile of sheet flows when the water depth was 2-4 times of the bed roughness height and the flow was laminar; however, when the water depth were within or well beyond the roughness height, or when the flow was turbulent under the high velocity conditions, the combined tracing method failed to characterize the vertical velocity distribution of the sheet flow. A significant linear correlation was observed between the flow velocities measured by the two tracers, respectively, under the three experimental underlying surfaces (polymethyl methacrylate, 80-mesh sandpaper, and 24-mesh sandpaper), with the slopes of fitted lines of 1.015, 1.094, and 1.078, and the coefficients of determination R² of 0.892, 0.824, and 0.760, respectively. As the roughness of underlying surface increased, the velocity difference between the two tracers increased as the results of the degree of turbulence of the sheet flow increased, which in turn affected deference in the water flow velocity measured by salt and thermal tracers, respectively. The results can provide a reference for further understanding of the dynamic processes of sheet flow on the slopes.