Hydraulics of overland flow influenced by litter incorporation under extreme rainfall

Abstract

Water erosion on hillslopes is a worldwide environmental problem, which is a rainfall‐induced process, especially extreme rainfall. The great intensity of extreme rainfall strongly enhances the power of overland flow to detach soil and transport sediment. Plant litter is one of the most important constituents of ecosystems that often covers the soil surface and can be incorporated into topsoil. However, little attention has been paid to its effect on flow hydraulics owing to the veiled nature. This study aimed to examine the effects of incorporated litter on the hydraulic properties under extreme rainfall condition. To reach this goal, six litter rates of 0, 0.05, 0.10, 0.20, 0.35, and 0.50 kg m−2 and four litter types collected from deciduous trees, coniferous trees, shrubs, and herbs were incorporated into topsoil. Then, simulated rainfall experiments were performed on five slope gradients (5°, 10°, 15°, 20°, and 25°) with an extreme rainfall intensity of 80 mm h−1. The results showed that Froude number and flow velocity of the overland flow decreased, whereas flow resistance increased exponentially with litter incorporation rate. Litter type had an influence on flow hydraulics, which can mainly be attributed to the variations in surface coverage of the exposed litter and the litter morphology. Flow velocity and Darcy–Weisbach coefficient increased markedly with slope gradient. However, the variation of slope gradient did not modify the relationships between flow hydraulics and incorporated litter rate. The random roughness, resulting from heterogeneous erosion due to the uneven protection of surface exposed litter, increased linearly with litter incorporated rate. As rainfall proceeded, flow hydraulics varied with incorporated litter rate and slope gradient complicatedly due to the increases in flow rate and coverage of the exposed litter and the modification of soil surface roughness.

Comparison of the effects of litter covering and incorporation on infiltration and soil erosion under simulated rainfall

“…Consequently, greater runoff coefficients were found on litter incorporated slopes than those of litter covering slopes. The result is similar to the finding of Wang, Zhang, Wang, and Li () who reported that the increase in litter incorporated rate did not significantly influence infiltration. The different effects of litter covering and incorporated treatments on soil infiltration rate can be attributed to the variations in surface litter coverage and litter water repellency.…”

“…The uniformity of the simulated rainfall was greater than 85%. The detailed information can be found in a previous study (Wang, Zhang, Wang, & Li, ).…”

“…Some studies have been conducted to explore the influences of litter incorporation within top‐soil layer on hydrological and erosion processes at hillslope scale. Wang, Zhang, Wang, and Li () exhibited that the mean infiltration rate did not change markedly with litter incorporated rate. However, Cesarano, Incerti, and Bonanomi () found that the undecomposed litter induced soil water repellency, which may inhibit soil infiltration.…”

“…The resistance to flowing runoff increased accordingly, and thus flow energy decreased. The research of Wang, Zhang, Wang, and Li () found that flow velocity decreased exponentially with litter incorporated rate. Litter incorporation within top‐soil layer can decrease soil erodibility and enhance critical shear stress by physical binding and chemical bonding effects (Li et al, ; Sun, Zhang, Liu, & Luan, ; Sun, Zhang, Luan, & Liu, ).…”

Quantifying the effects of plant litter in the topsoil on the soil detachment process by overland flow in typical grasslands of the Loess Plateau, China

“…Plant litter also widely exists in the topsoil of natural lands, especially in areas with high erosion intensity (Wang, Zhang, Wang, & Li, ; Liu et al, ; Figure ). Plant litter can be incorporated into the topsoil during rain splash, sediment transport, and deposition in natural circumstances (Wang et al, ; Sun, Zhang, Liu, & Luan, ).…”