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Understanding rill erosion processes is important in the prediction of soil erosion and the prevention of soil loss. However, limited information is available concerning the impacts of rainfall on rill erosion on steep slopes. Colluvial deposits with steep slopes make up the packed material underlying the collapsing walls in benggang, which collapse due to hydraulic pressure and gravity. They contain loose materials and large amounts of coarse particles. The objectives of this study were to investigate the impacts of rainfall intensity and slope gradient on the rill erosion process, rill development and rill flow dynamic mechanisms on the steep slopes of colluvial deposits. The colluvial soils were subjected to simulated rainfall in a 5-m2 (5-m by 1-m) flume at heavy rainfall intensities (100, 120, and 140 mm h−1) and on five steep slopes (20, 25, 30, 35, and 40°). Rill erosion contributed significantly to colluvial slope erosion; on average, rills accounted for 61% of the soil loss, and the effects of slope gradient were greater than those of the rainfall intensity. After rill development, rill density, rill length, width, and depth all significantly increased. Correspondingly, the soil loss rate sharply raised and irregularly fluctuated. Moreover, the collapse of rill heads or sidewalls tended to increase the relative contribution to rill erosion and rill development. The rill flow was characterized by transitional and subcritical flow regimes. The rill flow velocity was the most sensitive hydraulic parameter, and the unit stream power provided the optimal hydrodynamic parameter to characterize the dynamic mechanisms of rill erosion on colluvial deposits. The collapse of rill heads or sidewalls could result in negative values for critical shear stress, critical stream power, and critical unit stream power of rill erosion, which were −19 Pa, −5.3 N m−1 s−1, and −0.09 m s−1, respectively. These results provide a better understanding of the mechanism of rill erosion on steep slopes.

Mulching is an effective soil conservation practice for permanent gullies in southern China. Knowledge of the sediment characteristics that occur in mulched soils of colluvial deposits could improve the utility of mulching for soil conservation. A rainfall simulation experiment was designed to evaluate the effects of mulch on the runoff, erosion, and particle size distribution of eroded sediments. Straw mulch coverage of 0, 25, 50, 75, and 95% was tested with simulated rainfall. The effective particle size distribution of the sediment was compared with the ultimate particle size distribution to investigate the detachment and transport mechanisms involved in sediment mobilization. Mulching delayed the runoff initiation time and reduced the average runoff rate. Compared with bare soil, the increased mulch coverage decreased the soil loss rate by 13.0 to 90.3%. Moreover, the peak sediment concentration decreased from 80 to 200 g L− 1 under the different mulch coverage conditions. The optimal straw application rate was 1.5 to 3.0 Mg ha− 1 in the permanent gully's deposits. The relationship between instantaneous kinetic energy of rainfall and the proportion of effective clay- and sand-sized particles was well represented using an exponential equation. The effective clay-sized sediments under the different mulch coverage conditions were 2 to 4 times more common than those of the original soil, although there were only 13.9% sand-sized particles in the sediment when the mulch coverage was 95%. The silt-sized sediment was transported as primary particles under the different mulch coverage conditions. The effective to ultimate ratio of silt-sized particles fluctuated around 1. There were depletions of clay and silt in the colluvial deposit soil with mulch cover, and the enrichment ratios of clay and silt were larger than 1 while most of the enrichment ratios for sand were

Colluvial deposits with loose, coarse material are easily erodible in permanent gullies, but the mechanisms of erosion and sedimentation during overland flow remain obscure. Hence, the processes and mechanisms of the transportation of soil particles by overland flow were investigated in this study. Experiments were carried out in a 5.0 m long by 1.0 m wide flume using colluvial deposits. The slope gradient varied from 36 to 84%, and the flow rate ranged from 0.72 L m − 2 min − 1 to 2.88 L m − 2 min − 1 . The runoff rate and sediment yield rapidly increased with increasing overland duration. Runoff and sediment were highly variable when the flume was treated with a high flow rate compared with a low flow rate, with the fluctuation of sediment concentration under the high flow rate usually reaching 500 g L − 1 . The slope gradient and overland flow rate have strong impacts on sediment transport capacity. The mean flow velocity and the unit stream power can be an optimal composite force predictor for estimating sediment transport capacity. Experimental results also revealed that the percentage of gravel-sized particles increased with increasing flow rate and slope gradient, but silt and clay fractions observed opposite trend. The average enrichment ratio (ER) of gravel was usually 2.16 L m − 2 min − 1 , the bed load transport became an important mechanism; however, the simulation model overestimated these values.

The sediment transport capacity (Tc) is an important parameter for analysing and controlling soil erosion processes. However, few studies have investigated the Tc of soils containing gravel (grain sizes of >2 mm) on steep slopes. Colluvial deposits on steep slopes in benggang are composed of loose materials with large amounts of gravel. This study aimed to investigate the effects of gravel content on the Tc of overland flow using colluvial deposits with a range of slope gradients and flow discharges in a nonerodible flume (4 m long, 0.12 m wide and 0.1 m high). The experiments were carried out using six gravel mass contents (0, 10, 20, 30, 40, and 50%), four slope gradients (18, 36, 57, and 84%), and four flow discharges (0. 56, 1.11, 2.22, and 4.44 × 10-3 m2 s−1). The experimental results revealed that the measured Tc increased linearly as the gravel content increased. The effects of the slope gradient, flow discharge and gravel content on Tc were significant (p

Eroded sediment is an important feature of permanent gully's development; however, obtaining the specific characteristics of the sediment transport in the gully can be difficult. Thus, a composite fingerprinting technique, incorporating uncertainty analysis, has been employed to investigate the mean relative contribution of sediment sources in permanent gullies. In this study, 31 tracers were measured at 62 different sampling sites from three layers of a gully wall [surface layer (SL), sandy soil layer (SSL), and semi-weathering rock layer (SWL)] and 36 sediment samples from the slumping deposit and alluvial fan of the gullies. The sediment source tracing procedure was therefore used to assess the relative contribution of each source. The mean relative contribution from the SL to the sediment of the slumping deposit is higher in the active gully (G1), and is relatively lower in the potentially stable (G2) and stable gullies (G3); thus there was no obvious gully bank retreat in G2 and G3. In different places of the G1 alluvial fan, the mean relative contribution of the SL, SSL and SWL ranged from 9–11, 36–43 and 46–55%, respectively. However, the sediment contribution of the SL increased in G2 and G3 due to the changes in the erosion type. The mean relative contributions of the SL were 33 ± 5 (G2) and 30 ± 3% (G3). These findings have important implications for the establishment of a scientific basis for permanent gully sediment transport management and control policies.

The soil carbon reservoir is the largest carbon reservoir in terrestrial ecosystems and consists of soil organic and inorganic carbon stocks. Previous studies have mainly focused on the soil organic carbon (SOC) stock, and limited information is available about the soil inorganic carbon (SIC) stock. The Chinese Loess Plateau (CLP), which is located in the arid and semi-arid region of China, is an important inorganic carbon reservoir, with a thick soil layer that is rich in calcium carbonate. However, there are few reports on the SIC stock and its spatial distribution in this region. In the current study, the SIC densities and stocks for various soil types and land use patterns were evaluated based on 495 profiles with 2470 soil samples across the CLP, which were collected from the Chinese Second National Soil Survey. The results showed that in the top 1 m of soil across the CLP, the average SIC density is 17.04 kg/m(2), and the total SIC stock is approximately 10.20 Pg C (1 Pg = 10(15) g). The SIC stock of the CLP accounts for approximately 18.4% of the total SIC stock throughout China. The average values of the SIC stock in the 0-20, 20-50 and 50-100 cm depths of the CLP are 2.39, 2.92 and 4.89 Pg, respectively. Under different land use patterns, the order of the average SIC density is farmland approximate to grassland > forest in all soil layers. For the various soil types, the SIC density in the 0-100 cm layer is the highest in alkaline soil and lowest in subalpine meadow soil, whereas the SIC stock is highest in loessial soil, eolian sandy soil and sierozem, and the lowest in subalpine meadow soil. These differences are largely a result of the area occupied by each soil type and the climate conditions. The results of this study provide basic information about carbon reservoir in China and contribute to our understanding of the SIC stock on the CLP as it relates to the carbon balance of terrestrial ecosystems. (C) 2014 Elsevier B.V. All rights reserved.