Spatial distribution of overland flow velocity along straw-mulched slope under extreme rainfall

Abstract: The velocity of overland flow is one of the important parameters to simulate the hydrological processes and soil erosion on sloping farmlands. In this study, a solute centroid model was selected to determine the velocity distribution, according to the salt transport of overland flow, in order to clarify the erosion mechanism on the straw mulching slope under extreme rainfall. A systematic experiment was conducted under an artificial rainfall intensity of 160 mm/h at the State Key Laboratory of Soil Erosion and Dryland Agriculture on the Loess Plateau. The velocity measurements were performed at the slope lengths of 1.15, 3.15, 4.15, and 5.15 m under the slope gradients of 5°,10°, and 20°, and the mulching rate of 2, 4 and 8 t/hm2. The experimental soil flume was in the size of 0.25 m wide, 8 m long, and 0.3 m deep, which was filled with the clay loam that composed of 31.8% clay, 43.3% silt, and 24.8% sand, and sampled from Yangling, China (34°16′50″ N, 108°3′13″E). The wheat straw was air-dried and chopped to 5-7 cm before being laid over the surface soil. A downspout rain simulator was also selected, where the rainfall uniformity was 85%, and the rainfall height was 18 m. A flow velocity was determined using an electrolyte centroid. After that, some hydraulic parameters were calculated, including the unit width discharge, water depth, Reynolds coefficient, Froude number, and Darcy-Weisbach resistance coefficient. The measurements showed that the mean velocity was ranged in 0.071-0.352 and 0.017-0.117 m/s at the mulching rate of 0 and 8 t/hm2, respectively. There was a smooth curve of electrolyte transport under the condition of low straw coverage rate (2 t/hm2), whereas, no smooth curve without the straw mulching treatment. The smoothness of electrolyte transport under the 100% straw coverage was between those of no and low straw coverage. The straw mulching greatly weakened the disturbance of raindrops on the overland flow, but increased the roughness of the eroded underlying surface, leading to a lower average flow velocity. Once the mulching rate reached a critical point, there was an intensive disturbance of mulching straw on the overland flow. The distance to the maximum velocity of overland flow on the slopes with a high mulching rate was much shorter than that with a low mulching rate. In addition, the mean flow velocity was ranged in 0.017-0.352, 0.027-0.323, and 0.033-0.272 m/s under the slope gradients of 5°, 10°, and 20°, respectively. The flow velocity increased significantly, with the increase of slope gradient and length. But the change rate of flow velocity decreased, with the increase in the slope length, until the velocity gradually tended to be stable. There was a strong linear relationship of flow velocities between the straw mulching and bare slope, where the proportional coefficient was 0.285. The flow regime of overland flow depended mainly on the mulching straw and the critical value of Reynolds number, which was less than 500 in the laminar flow. Once the overland flow was stable, the flow rate and velocity of overland flow were stable without changes with time. The water depth of the bare soil slope varied from 0.8 to 1.4 mm, whereas, the water depth of the covered slope varied from 2.0 to 5.1 mm. Consequently, the straw mulch can be widely expected to obstruct the water flow, further reducing the flow velocity for the low water level, compared with the bare soil slope. The disturbance effect of straw mulch on the water flow was also strengthened in the downstream section of the slope, with the increase of catchment areas. The finding can greatly contribute to clarifying the effects of soil erosion on the straw-mulched slope under the conditions of extreme rainfall.