Effects of alfalfa coverage on runoff, erosion and hydraulic characteristics of overland flow on loess slope plots

Abstract

An evaluation of the interactions between vegetation, overland and soil erosion can provide valuable insight for the conservation of soil and water. An experiment was conducted to study water infiltration, runoff generation process, rate of sediment erosion, and hydrodynamic characteristics of overland flow from a sloping hillside with different draw-off discharges from alfalfa and control plots with 20°slope. The effect of alfalfa on runoff and sediment transport reduction was quantitatively analyzed. Alfalfa was discussed for its ability to reduce the overland flow scouring force or change the runoff movement. Compared to the bare-soil plots, alfalfa plots generated a 1.77 times increase in infiltration rate. Furthermore, the down-slope water infiltration rate for the bare soil plots was higher than in the up-slope, while the opposite was found in the alfalfa plots. In addition, alfalfa had a significant effect on runoff and sediment yield. In comparison to the control, the runoff coefficient and sediment transportation rate decreased by 28.3% and 78.4% in the grass slope, respectively. The runoff generated from the alfalfa and bare-soil plots had similar trends with an initial increase and subsequent leveling to a steady-state rate. The transport of sediment reduced with time as a consequence of the depletion of loose surface materials. The maximum sediment concentration was recorded within the first few minutes of each event. The alfalfa plots had subcritical flow while the baresoil plots had supercritical flow, which indicate that the capability of the alfalfa slope for resisting soil erosion and sediment movement was greater than for bare soil plots. Moreover, the flow resistance coefficient and roughness coefficient for the alfalfa plots were both higher than for the bare-soil plots, which indicate that overland flow in alfalfa plots had retarded and was blocked, and the flow energy along the runoff path had gradually dissipated. Finally, the ability to erode and transport sediment had decreased.

Effects of Vegetation Stems on Hydraulics of Overland Flow Under Varying Water Discharges

“…This figure is available in colour online at wileyonlinelibrary.com/journal/ldr This figure is available in colour online at wileyonlinelibrary.com/journal/ldr the unit stream power and the ability of runoff to carry sediment, and the larger the vegetation stems diameters were, the greater the contribution on Froude number reduction was. This behavior has also been found under different vegetation types and covers Wu et al, 2011). However, Froude number in our study was much greater than those of .…”

“…This is also the reasons why Pan & Shangguan () and Wu et al . () found Reynolds number decreased with vegetation cover increased on mobile beds. Therefore, further study is needed to isolate each part of the vegetation to discover the possible influence on soil erosion.…”

“…The decrease of Froude number on vegetated slopes indicated that vegetation could decrease the unit stream power and the ability of runoff to carry sediment, and the larger the vegetation stems diameters were, the greater the contribution on Froude number reduction was. This behavior has also been found under different vegetation types and covers (Pan & Shangguan, ; Wu et al ., ). However, Froude number in our study was much greater than those of Pan & Shangguan ().…”

“…On natural slope, the vegetation stems could increase the infiltration and reduce the runoff and then weaken the soil erosion. This is also the reasons why and Wu et al (2011) found Reynolds number decreased with vegetation cover increased on mobile beds. Therefore, further study is needed to isolate each part of the vegetation to discover the possible influence on soil erosion.…”

Sediment deposition and overland flow hydraulics in simulated vegetative filter strips under varying vegetation covers

“…Flow rates had a negative effect on f ‐values, while the sediment concentration had a positive effect (Table ). The negative relationship between hydraulic resistance and flow rate is generally accepted (Abrahams and Li, ; Wu et al ., ). A positive correlation between hydraulic resistance and sediment concentration was also found by Zhang et al .…”