Hydrological and erosive response of soil surfaces to rainfall intensity as affected by gravel fragment coverage

Abstract

Gravel fragment cover is an important factor in determining soil erosion rate, particularly in arid and semiarid lands where vegetation coverage is very low. For example, in North China, soil is a shallow gravelly loam with 30% coarse fraction, and gravels affect local agriculture and forestry dramatically. This study was conducted to quantify the effects of gravel fragment coverage on slope erosion. Three rainfall intensities (30, 60, and 90 mm h⁻¹) were conducted on five gravel fragment coverages (0%, 10%, 20%, 30%, and 40%) by simulated rainfall. The results showed that both runoff and sediment yield rates decreased significantly with the increase of gravel fragment coverage, and they were negatively correlated with rainfall intensity. Gravel fragment cover reduced cinnamon soil (Haplustalfs) loss from slopes. Equations were established to quantify the effects of rainfall intensity and gravel fragment coverage on runoff and sediment yield. Mean runoff velocity, Reynolds number, and Froude number of gravel fragment-covered slopes varied from 0.88 to 3.17 cm s⁻¹, 8.35 to 46.21, and 0.21 to 0.59, respectively. They generally increased with increasing rainfall intensity and decreased with increasing gravel fragment coverage. Sediment yield rate was in a logarithmic function with resistance coefficient. Mean shear stress (τ), runoff power (ω) and unit runoff power (P) of gravel fragment-covered slopes ranged from 0.836 to 1.548 Pa, 0.013 to 0.072 N m⁻¹ s⁻¹, and 0.003 to 0.009 m s⁻¹, respectively. Relationships between runoff shear stress and sediment yield rate, and runoff power and sediment yield rate were both linear. The critical shear stress of the bare slope was 0.48 Pa, which was increased by 0.30 to 0.82 Pa in fragment-covered slopes. The critical runoff power of the bare slope was 0.0003 N m⁻¹ s⁻¹, and gravel fragment cover increased runoff power by 0.0112 to 0.0187 N m⁻¹ s⁻¹. The results of Pearson analysis indicated that Reynolds number was the most sensitive parameter to estimate runoff hydraulic characteristics. In conclusion, gravel cover resulted an effective conservation measure to prevent erosion and conserve rainwater.