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• Hillslope erosion dynamic model integrating vegetation effect was established. • Unified baseline parameters were obtained across different land uses. • Effect of vegetation on soil detachment and sediment transport was revealed. • Contribution rates of vegetation to sediment reduction along slopes were quantified. Developing a process-based soil erosion model that comprehensively considers the effects of vegetation is complex but crucial for evaluating sediment reduction during vegetation restoration. Current understanding of the effect of vegetation on sediment reduction along hillslopes across different land uses is limited. In this paper, we developed a dynamic model of hillslope erosion that integrates the effects of vegetation on soil detachability and hydrodynamics (VED) based on the feedback mechanism between soil detachment and sediment transport. The VED was calibrated and validated with runoff plot data for woodland, grassland, and farmland in the Wuding, Yan, Jing and Wei Rivers of the Loess Plateau in China. Unified baseline parameters and decay coefficients of vegetation were obtained. The model validation results indicated that the coefficient of determination, Nash–Sutcliffe simulation efficiency and relative error ranged from 0.59 to 0.99, 0.56 to 0.90, and −45.99 % to 46.36 %, respectively. The decay coefficients for soil detachment capacity (Φ), sediment transport capacity (T c), and the sediment reduction effect of vegetation exhibited the following order: woodland > grassland > farmland. Compared with VED, existing process-based soil erosion models failed to effectively characterize the influence of vegetation on Φ and T c on the Loess Plateau, with differences of several orders of magnitude. The individual contributions of runoff, soil detachability, and hydrodynamics with vegetation impact on sediment yield were quantified by VED. The contribution rates of vegetation to sediment reduction decreased gradually with increasing slope length because the soil detachment capacity of bare slopes exceeded that of vegetated slopes, resulting in a faster increase in sediment yield. The contribution rates of vegetation to sediment reduction for woodland, grassland, and farmland with 20 %–60 % vegetation covers were 66.75 %–99.95 %, 57.43 %–99.63 %, and 27.91 %–88.63 % in the boundary conditions of this study, respectively. The results reveal the potential for integrating VED into other distributed watershed hydrological and sediment models to assess the effects of vegetation restoration on sediment reduction at a watershed scale. [ABSTRACT FROM AUTHOR]

• SY and Sr of the SL was the largest, but the sediment MWD was the smallest. • Change in the flow discharge did not affect the distribution pattern of sediment particles. • Dynamic coupling of runoff hydraulics and EG morphology drove soil loss. • SDR was the most direct influencing factor on the Sr in EGs. Ephemeral gully (EG) erosion represents the early stage of large-scale gully (locally named as "Benggang") development on Ultisols in the granite hilly area. However, the EG erosion process and the mechanism underlying sediment production have not been fully elucidated. In this study, a series of field scouring experiments were conducted under different flow discharge levels (10, 15, 20, 25, and 30 L·min−1), and the primary EGs in three typical soil layers with different weathering degrees (red soil layer (RL), transition layer (TL), and sandy layer (SL)) was dynamically monitored. The results indicated that the ratio of width to depth of the EGs rapidly decreased as the flow discharge increased. At 30 L·min−1, the average sediment yield and transport rate (Sr) of the SL were 4.85 and 5.39 times, greater than those of the RL, respectively. The sediment particles were concentrated within the 1–2 mm particle size range, accounting for over 35 %. In addition, the mean weight diameter of the sediment particles in the RL was the largest. The sediment distribution characteristics did not respond to flow changes but were affected by EG morphological changes. With increasing flow discharge, the flow shear stress and stream power increased. Further analysis revealed that the coupling effect of flow hydraulic characteristics and EG morphological evolution drove the EG erosion sediment production process, and the Sr of the SL was highly sensitive to flow discharge changes. The soil disintegration rate was an internal factor that directly affected soil loss in the EGs, with a path coefficient of 0.71. The bulk density, organic matter content, and sand content were all indirect influencing factors. The RL exhibited greater anti-erodibility than the other layers and could serve as an effective protective layer for granite hilly areas. These findings provide a theoretical basis for improving soil loss management systems. [ABSTRACT FROM AUTHOR]

• The sediment transport capacity was developed for loessal slopes. • Two-dimensional erosion numerical model was developed for loessal slopes. • Erosion numerical model could accurately simulate slope runoff and sediment process. Simulated slope erosion sediment yield and its development process have great significance for quantitative evaluation of erosion and its sources. In this study, a loess slope erosion experiment was implemented indoors to establish a sediment transport capacity formula suitable for loess slope erosion. A two-dimensional slope erosion numerical model was developed with corrected sediment transport capacity formula, and the model was verified by a simulated indoor slope rainfall erosion experiment. The results showed that the corrected slope sediment transport capacity formula is suitable for loess slopes. The proposed two-dimensional slope erosion numerical model could simulate runoff and sediment processes in a relatively more accurate. The most severely eroded areas were the middle and lower parts on erosion spatial contribution based on the simulation erosion results. Good agreements between the simulated and observed results were found for erosion and its spatial distribution. The research results can further improve loessal slope erosion process simulation and prediction. [ABSTRACT FROM AUTHOR]

• Effective sediment sizes were determined along a gradient of soil structural stability. • Sediment average size exhibited a unimodal variation with soil structural stability. • Suspension-saltation dominated sediment transport for low and high stable soils. • Suspension-saltation and rolling jointly dominated erosion for medium stable soils. • Soil structural stability determined rainfall intensity effect on sediment transport. Sediment transport patterns (suspension, saltation and rolling) not only depend on flow hydrodynamic characteristics, but also on sediment properties, both of which are largely influenced by soil structural stability. However, how sediment sizes and transport patterns response to original soils over a wide range of structural stability remains unclear. Herein, field plot rainfall simulation experiments on five soils along an increased gradient of structural stability were conducted at two rainfall intensities of 45 and 90 mm h−1 to investigate effective sediment size characteristics and associated transport patterns in sheet erosion. Differing from runoff and sediment yield, sediment sizes were most influenced by soil structural stability (F = 386.9) rather than rainfall intensity (F = 246.7, p < 0.001). Mean weight diameter of sediment (0.16–0.84 mm) and the proportion of coarse particles (>0.25 mm) exhibited a unimodal variation with increased soil structural stability. The effective sediment size distribution shifted from unimodal with the peak at < 0.10 mm for low and high stable soils to bimodal with two peaks at < 0.10 mm and 0.5–1 mm for medium stable soils. Correspondingly, suspension-saltation (>74 %) dominated sediment transport for low and high stable soils, which was determined by fine sized fragments from aggregate breakdown and small flow transport capacity, respectively; while both suspension-saltation (<0.10 mm) and rolling (>0.25 mm) jointly contributed to sediment transport for medium stable soils, which was controlled by the interaction between fragment size and flow transport capacity. Moreover, the positive effect of rainfall intensity on sediment sizes was more remarkable for the medium than for the low and high stable soils. A conceptual model of sediment transport mechanisms in response to soil structural stability is firstly proposed, which merits consideration in the future development of erosion modeling and soil conservation techniques. [ABSTRACT FROM AUTHOR]

• Demonstration of rise and fall of carbon discharge with time. • Demonstration of enrichment of sand in loose layer above soil. • Demonstration of no carbon enrichment is sediment at the steady state. Soil carbon losses from sheet and interrill erosion areas have an impact on the global carbon cycle. Sediment discharged in many experiments where artificial rainfall has been applied to sloping soil surfaces show greater proportions of carbon in the sediment than found in the cohesive soil source. In many cases, conclusions drawn from these experiments fail to recognise that the enrichment of carbon in the sediment discharge is a result of unsteady conditions that occur before the steady state develops. It is known that particles of sand of different sizes move at different rates when raindrop induced salation occurs with the result that initially the particle size distribution is finer than the source but tends towards that of the source as time progresses. The presence of carbon in aggregates influences the density of aggregates. Here a qualitative model of sediment transport of sand and coal particles by raindrop induced saltation is used to demonstrate time varying enrichment of coal resulting from the effect of carbon on particle settling velocity. The model also demonstrates how the layer of loose particles that develops on top of the cohesive soil surface influences the relative amounts of carbon and sand in the sediment discharges. The steady state is not achieved until the slowest moving particle detached at the farthest distance from the downslope boundary is discharged. That may not be achieved in many of the experiments reported in the literature. [ABSTRACT FROM AUTHOR]

• Mudstone deposits behave like noncohesive sediments in terms of erosion resistance. • High erosion resistance of laterites cannot be predicted merely by particle size. • Shields' parameter predicted by water content, clay content, and plasticity index. Soil erosion and sedimentation are major factors in landscape evolution and river morphology. The erosion resistance of sediments plays an important role in landscape evolution processes because it determines the threshold of an acting force at the initiative sediment motion, and the amount of sediment transports. This study examined the sediment properties and their correlations to the erosion resistance of mudstone deposits and laterites, which are two representative natural cohesive soils in hill and terrace topography in Taiwan. Soil physical properties and pore water chemical properties, and the erosion resistance were investigated and determined using laboratory tests and hydraulic flume experiments, respectively. Investigations of sediment properties showed that the mudstone deposits and laterites had significant differences in median particle size, soil texture, level of plasticity, internal structure, and pH value and electrical conductivity of their aqueous slurry. Results from the hydraulic flume experiments showed that the laterites had stronger erosion resistance, indicating by larger values of critical shear stress and the Shields' parameter, than the mudstone deposits. Through the stepwise multiple regression analysis, a quantitative equation was proposed to estimate the Shields' parameter using fine sediment properties including water content, clay content, and the plasticity index expressed in decimal fractions, of the two soils. Despite more experiments of the two natural soils with different ranges of the sediment properties are needed to validate the empirical equation for predicting the Shields' parameter, the proposed methodology provides an essential reference in which erosion resistance is an important parameter. [ABSTRACT FROM AUTHOR]

[Display omitted] • Total volume and peak discharge are the most important drivers of sediment transport. • Antecedent moisture condition is correlated only with the initial discharge of floods. • Hysteresis loops between SSC and Q are analyzed with the hysteresis index (HI). • Negative HI are predominant indicating delayed SS transport and distant SS sources. • In the catchment soil erosion from hillslopes is the dominant process. This study aimed to evaluate the drivers of suspended sediment (SS) transport by analyzing the flood events recorded from July 2010 to June 2011 in the Celone River basin (Apulia, S-E Italy). High-resolution data of rainfall, streamflow (Q), and measurements of suspended sediment concentrations (SSC) were used to carry out the multivariate analysis. Among the initial 18 variables, the Principal Component Analysis revealed that total volume (m3), peak discharge (Qmax, m3/s), and the flood duration were the most important factors influencing SS transport. Total rainfall and total kinetic energy recorded in the gauging station located in the lowland were also important factors, whereas the same rainfall variables recorded in the remote mountainous area of the basin had a minor influence on SS transport. Antecedent moisture conditions were significantly correlated only with the initial discharge of the flood. Hysteresis loops between SSC and Q were analyzed for each flood event by using the hysteresis index (HI). Results showed that this catchment was dominated by anticlockwise loops (negative values of HI) representative of delayed SS transport and of distant SS source areas in terms of the gauging station. These results also suggested that soil erosion from hillslopes was the dominant process compared with channel and bank erosion and that there was no exhaustion of sediment availability in the basin. [ABSTRACT FROM AUTHOR]

[Display omitted] • The SWAT model supports better management of erosion processes. • Computational models can be used with methods of visual recording erosion. • Simple statistical models aligned to SWAT is adequate to analyze erosive processes. • The spatially distributed erosion identifies the susceptible areas to water erosion. Water-driven erosion associated with the transport and deposition of sediments in watersheds may lead to contamination problems causing significant damage to land and infrastructure. The key factors that influence erosive processes are well known and include the precipitation rate, terrain topography, soil type, and vegetation cover. The soil and water assessment tool (SWAT) considers these factors and may be used to identify areas susceptible to soil erosion and to estimate soil loss and sediment transport and deposition. To corroborate this assumption, real data on rill erosion and sediment transport from the Mogi Guaçu River Basin in southeastern Brazil were used to validate SWAT estimates. The study shows that the model is able to identify soil-eroded areas and replicate the scale and variation patterns of sediment production, although not with great precision. Hence, it can successfully be used for prioritizing areas that require action and for evaluating alternative soil management and conservation practices to mitigate erosive processes. [ABSTRACT FROM AUTHOR]

• Soil erosion process on frozen-stony slope is experimentally investigated. • Sediment concentration on frozen-stony slope follows linear-exponent curves. • The impacts of thawed soil depth and rock fragment content on sediment transport process are quantified. Rill erosion on frozen-stony slope in high-altitude or high-latitude areas commonly undergo freeze–thaw cycle, which is significantly influenced by partially thawed soil with rock fragment. This research aims to investigate the influences of thawed soil depth (TSD) and rock fragment content (RFC) on rill erosion and quantifying the mathematical relationships among them. The laboratory experiment is conducted to evaluate the influences of TSD (1, 2, 5, and 10 cm) and RFC (0%, 10%, 20%, and 50%) on sediment delivery processes by concentrated meltwater flow under three slope gradients of 5°, 10°, and 15°, and three flow rates of 1, 2, and 4 L min−1 at nine rill lengths from 0.5 m to 8.0 m. Five runoff samples are obtained from each experiment run and used to calculate sediment concentration (SC). The SC increases linearly and then exponentially with rill length to approximate the extremum and fits well with piecewise functions. Restrictive effect of TSD and RFC on sediment supply is evident during linear stage induced by shallow TSD, which keeps a long distance under shallow TSD and low RFC to collect sufficient sediment for exponential period. However, TSD and RFC impacts become apparent during the exponential period and lead to higher sediment yield with RFC of 10% and 20% than that on totally thawed soil without rock fragment. In this research, the positive effect on sediment yield increases with RFC increases from 10% to 20%, which may need to be eliminated for efficiency in erosion control. In contrast to low RFC, the protection impact of rock fragment always exerts with RFC of 50%. The experimental data and the functional relationship can provide the basic knowledge for evaluating the model parameters such as soil detachment rate and erodibility of rill erosion on frozen-stony slope. [ABSTRACT FROM AUTHOR]

• Under the action of concentrated flow, the change of surface runoff of karst gently sloping farmland is mainly controlled by the magnitude of concentrated flow. • Under the action of concentrated flow, the underground runoff of karst gentle slope farmland is significantly negatively correlated with the slope. • Concentrated flow can lead to severe soil erosion in karst gently sloping farmland. Exposed bedrock in karst areas considerably alters runoff paths causing partial runoff to converge into a concentrated flow, which is a potential source of soil erosion risk. However, research is lacking on how concentrated flow affects regional soil erosion. In this study, the surface and underground dual structure in karst areas was simulated in a steel tank. Combined with runoff plot monitoring, the runoff conversion process and erosion characteristics of gently sloping farmland in karst areas under different water discharge rates were examined through indoor concentrated flow discharge scouring tests, and the influence of concentrated flow changes on the runoff spatial distribution and sediment yield in gently sloping farmland was clarified. The results demonstrated the following: (i) there existed a significant positive correlation between surface runoff and the water discharge rate (p < 0.01); the partial correlation coefficient value reached 0.87. The higher the water discharge rate, the more surface runoff formation was facilitated. (ii) When the slope remained the same, the underground runoff was relatively high at 3 L/min. Given the same discharge, a significant negative correlation between underground runoff and slope was found (p < 0.05); the partial correlation coefficient value reached −0.71. (iii) In response to a water discharge rate of 5 L/min, surface soil erosion was serious, and the maximum sediment transport modulus was 174.088 g/min∙m2. A significant cubic function relationship existed between the scouring time and surface sediment yield. Overall, a critical flow rate occurred between 3 and 5 L/min, which controlled changes in surface runoff, subsurface runoff and surface sediment yield. Under highly concentrated flow, drastic soil erosion could occur. These results can provide a scientific basis for the control and prevention of concentrated flow-induced erosion in karst areas. [ABSTRACT FROM AUTHOR]

• Rill erosions impacted by saturation & subsurface water flow are compared. • Method is suggested to quantify rill erosion affected by subsurface water flow. • A method is evaluated to determine rill erosion function with few data points. • Rill erosion models are applicable to slopes with subsurface water flow. Understanding rill erosion processes on purple soil slopes with subsurface water flow at different depths is of great significance to determine rill erosion model parameters. The purposes of this study were to quantify the influences of subsurface water flow at different depths on the rill erosion process. Laboratory experiments were carried out to measure sediment delivery under 5°, 10°, 15°, and 20° slope gradients, 2, 4, and 8 L min−1 flow rates and 0, 5, 10, and 15 cm subsurface water flow depths (SWFDs) below the soil surface at eight slope lengths of 1, 2, 3, 4, 5, 6, 7, and 8 m. The results showed that the rill erosion prediction model is applicable to rills on purple soil slopes with subsurface water flow at different depths. The effect of the slope gradient on the sediment concentrations is greater than that of the flow rate under various SWFDs. Slopes with shallower subsurface water flow depths from the soil surface produced higher soil erosion and sediment delivery, and the sediment concentrations of rill erosion under the impacts of SWFDs of 5, 10, and 15 cm from the soil surface were 61%, 50%, and 41% of that of the saturated soil slope, respectively. Moreover, the rill erosion process can be well simulated with sediment concentration data from 1 to 4 or 1–8 m rill lengths when the maximum sediment concentration at the sediment transport capacity level is determined. Our results provide a simplified and accurate method for predicting erosion processes with few data and reveal rill erosion processes under the effect of subsurface water flow. [ABSTRACT FROM AUTHOR]

• Changes in spatial variation of rock fragment contents (SVRFC) due to tillage altered hydrological and erosional processes. • Yields of subsurface flow and deep percolation increased with decreasing SVRFC. • The decreasing SVRFC caused a decrease in surface erosion, but an increase in subsurface erosion. • Effects of rainfall intensity on hydrological and erosional processes were associated with SVRFC. Cultivation activities play an important role in shaping the spatial distribution of rock fragments in hilly agricultural landscapes. The spatial variation in rock fragment contents (SVRFC) can exert strong effects on the spatial heterogeneity of soil properties with great implications for hydrological fluxes and soil erosion. However, previous research always assumed that rock fragments were homogenously distributed, but little has been known concerning the SVRFC over the hillslope and its influence on hydrological and erosional processes. In this study, simulated rock fragment contents (RFCs) in soil layers at the upper (0.6 kg kg−1) and lower slope positions (0, 0.2, 0.4 and 0.6 kg kg−1) were established in combination with artificial rainfall experiments, to investigate the influence of RFC difference between the upper and lower slope positions on hydrological and erosional processes. Results show that the decreasing RFC difference reduced the yield of surface flow (5–100%), but increased yields of subsurface flow (58–792%) and deep percolation (19–130%). Additionally, the decreasing RFC difference reduced the runoff velocity of surface flow and surface erosion rate, but aggravated subsurface erosion. Especially under the 0 kg kg−1 RFC difference, the intense soil loss occurred at the subsurface which may be due to the highly developed preferential flow. With increasing rainfall intensity, great increments in yields of subsurface flow, deep percolation and subsurface sediments were found under the small RFC difference which had small increments in yields of surface flow and surface sediments. These results suggest that the changing SVRFC related to tillage erosion can exert strong influence on behaviors of hydrological and erosional processes. Our study provides a better understanding of the interaction between agricultural cultivation and soil–water loss in hilly agricultural landscapes. [ABSTRACT FROM AUTHOR]

• Upslope sediment-laden flow caused additional erosion in ephemeral gully zone. • Transport-dominated sediment regime was found under filed and laboratory conditions. • An increase in sediment concentration of inflow decreased downslope soil erosion rates. • Laboratory simulations verified the field monitoring results for extreme rainfall events. Ephemeral gully (EG) is an important linear erosion feature, often occurring at mid-slope position, that can be greatly influenced by upslope inflow and its sediment transport capacity. How sediment concentration in upslope inflow influences downslope EG erosion is still unclear. Thus, field monitoring on a natural hillslope and indoor simulations on a dual-box system were conducted to better understand their relationship. The results showed that both field monitoring and indoor simulation displayed the additional sediment delivery caused by upslope sediment-laden flow, verifying the transport-dominated sediment regime in EG systems. In field observations, the sediment increment coefficient (SIC , ratio of net sediment delivery caused by upslope sediment-laden flow to the total sediment delivery) on an event scale varied from 4.6% to 88.6%. In indoor simulations, the SIC changed from 24.9% to 87.5%. Indoor simulation experiments with a range of 0–378.2 g L−1 sediment concentration supplied by a feeder box, showed that SIC linearly decreased as sediment concentration of upslope inflow increased. Field monitoring showed complicated phenomenon because of natural random variations. For rainfall events with PI 30 (the product of precipitation and maximum 30-minute rainfall intensity) larger than 400 mm2 h−1 (similar to the designed extreme rainfall in laboratory simulations), SIC also generally decreased as sediment concentration of upslope inflow increased, especially when sediment concentration of upslope inflow was smaller than 400 g L−1. Thus, laboratory simulations verified the field monitoring results of extreme rainfall events with large rainfall amounts and intensities. This study illustrated the necessity of in-situ field monitoring and indicated that design of indoor simulation experiments needs more comprehensive considerations on complicated hydrological and sediment conditions. [ABSTRACT FROM AUTHOR]

• Upslope sediment-laden flow caused additional erosion in ephemeral gully zone. • Transport-dominated sediment regime was found under filed and laboratory conditions. • An increase in sediment concentration of inflow decreased downslope soil erosion rates. • Laboratory simulations verified the field monitoring results for extreme rainfall events. Ephemeral gully (EG) is an important linear erosion feature, often occurring at mid-slope position, that can be greatly influenced by upslope inflow and its sediment transport capacity. How sediment concentration in upslope inflow influences downslope EG erosion is still unclear. Thus, field monitoring on a natural hillslope and indoor simulations on a dual-box system were conducted to better understand their relationship. The results showed that both field monitoring and indoor simulation displayed the additional sediment delivery caused by upslope sediment-laden flow, verifying the transport-dominated sediment regime in EG systems. In field observations, the sediment increment coefficient (SIC , ratio of net sediment delivery caused by upslope sediment-laden flow to the total sediment delivery) on an event scale varied from 4.6% to 88.6%. In indoor simulations, the SIC changed from 24.9% to 87.5%. Indoor simulation experiments with a range of 0–378.2 g L−1 sediment concentration supplied by a feeder box, showed that SIC linearly decreased as sediment concentration of upslope inflow increased. Field monitoring showed complicated phenomenon because of natural random variations. For rainfall events with PI 30 (the product of precipitation and maximum 30-minute rainfall intensity) larger than 400 mm2 h−1 (similar to the designed extreme rainfall in laboratory simulations), SIC also generally decreased as sediment concentration of upslope inflow increased, especially when sediment concentration of upslope inflow was smaller than 400 g L−1. Thus, laboratory simulations verified the field monitoring results of extreme rainfall events with large rainfall amounts and intensities. This study illustrated the necessity of in-situ field monitoring and indicated that design of indoor simulation experiments needs more comprehensive considerations on complicated hydrological and sediment conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • A theoretical framework was used to explain dynamic scale effect on soil erosion. • Scale effect on soil erosion varies with erosion types, rainfall, and soil properties. • Slope length factor of USLE/RUSLE performs better on rill erosion dominant slopes. The variation of soil erosion across scales remains a controversial issue. A theoretical framework, coupling the normalized Green-Ampt equation for infiltration, 1D kinematic wave model for overland flow, and WEPP erosion modeling approaches for soil erosion, was used to explain and quantify the direct effect of scale on the soil erosion process. The results show that the relation between interrill erosion and slope length accords with a power-law decreasing trend, while the relation of rill erosion versus slope length shows a power-law increasing trend. Moreover, the power-law scaling of interrill erosion becomes more prominent with an increase of rainfall duration and intensity but is nearly independent of the interrill erodibility factor. The rainfall duration, soil type, soil critical shear stress, sediment transport coefficient, and rill erodibility factor substantially influence the magnitude of scale effect on rill erosion, while rainfall intensity, slope gradient, and Manning's roughness have minor effects. It suggests that the scale effect on soil erosion is variable and dynamic for different erosion types, rainfall and soil characteristics, which can explain why the controversial scaling relationships of soil erosion exist in previous studies. The physically based soil erosion model WEPP is compared to the empirical models USLE and RUSLE at various scales. Only when rill erosion plays a dominant role in soil loss do all these models exhibit consistent scale effect on erosion. These results provide insights into the essence of scaling in the hillslope soil erosion process, which are expected to benefit the development of upscaling techniques for the large-scale hydrological, geomorphological, and ecological processes. [ABSTRACT FROM AUTHOR]