Soil Detachment Process Research Articles

Coupling Soil Erosion and Sediment Transport Processes With the Variable Infiltration Capacity Model (VIC‐SED) for Applications Suitable With Coarse Spatial and Temporal Resolutions

AbstractUnderstanding soil erosion and sediment transport from the hillslope scale to the regional scale is crucial for studies on water quality, soil‐water conservation, the lateral carbon cycle, environmental zoning and vulnerability. However, most existing erosion and sediment transport models are only applicable at the hillslope scale or for small watersheds with fine spatial resolutions (typically much less than 1 km). This study presents a process‐based soil erosion and sediment transport model for model applications designed for applications with coarse spatial (e.g., ≥10 km) and temporal (e.g., from hourly to daily) resolutions. This new model, referred to as VIC‐SED, effectively accounts for interactions between erosion and hydrological processes. This is achieved by tightly coupling the erosion processes with a hydrologically based Three‐layer Variable Infiltration Capacity (VIC‐3L) land surface model (LSM) and to a multi‐scale routing (MSR) model. VIC‐SED considers the impacts of (a) the spatio‐temporal variability of rainfall intensity on erosion processes and (b) soil moisture on the soil detachment process. VIC‐SED is evaluated in two watersheds. Results demonstrate that VIC‐SED is capable of reproducing water and suspended sediment discharges at coarse spatial resolutions and varying temporal scales varying from 15‐min to daily intervals. Our study indicates that the VIC‐SED model is a promising tool for studying and assessing the impacts of climate and land cover changes on suspended sediment yields over large regions using coarse spatial and temporal resolutions.

Effects of root traits on soil detachment capacity driven by farmland abandonment

Vegetation restoration significantly affected root traits during farmland abandonment, thereby considerably altering the soil detachment process. However, few studies have investigated the effects of root traits on soil detachment capacity under different ages of farmland abandonment. To examine the effects of root trait changes caused by vegetation restoration on soil detachment capacity by overland flow (Dc) and soil resistance to erosion under different ages of farmland abandonment, flow scouring tests under five shear stress levels (2.80–12.95 Pa) were conducted on 125 undisturbed soil samples collected using an indoor variable slope flume. The results showed that, compared to slope farmland, the root traits of abandoned farmland (5, 12, 24, and 36 years) increased considerably, and the Dc and erodibility decreased by 51.3–93.9 % and 61.8–94.1 %, respectively. The critical shear stress of abandoned farmland gradually increased with the age of farmland abandonment, but remained less than that of slope farmland. Among hydraulic parameters, stream power was the optimal index for predicting Dc, while among root morphological traits, root surface area density (RSAD) was the best indicator for predicting Dc. The median soil particle size (D50) and cohesion are important factors that affect soil detachment. According to the Water Erosion Prediction Project (WEPP) model, Dc can be predicted using the RSAD and soil cohesion (R2 = 0.95, NSE = 0.95). In addition, a new model was established to estimate the soil detachment capacity based on D50 (R2 = 0.97, NSE = 0.97). In the present study, the performance of the model was improved compared with that of previous studies.

Effects of freeze-thaw on soil detachment capacity in the black soil region of Northeastern China

Soil detachment capacity (Dc) is an important parameter used to determine erosion intensity in physical-process-based erosion models. Freeze-thaw affects soil detachment processes by altering the mechanical properties of soil; however, due to the compound action of freeze-thaw and runoff on Dc, quantifying the impact of seasonal freeze-thaw on Dc remains challenging. A series of experiments with six freeze-thaw cycles (FTC), six initial soil moisture contents (SMC), three slope gradients, and five flow discharges were conducted to investigate the effect of freeze-thaw and hydrodynamic characteristics on Dc. The results showed that soil shear strength (τm), cohesion (Coh), and internal friction angle (φ) gradually tended to become stable with increasing FTC, indicating that repeated FTC had a cumulative impact on soil mechanical properties, and there was a critical FTC between 5 and 7. When FTC rose from 1 to 15, the reduction in τm, Coh, and φ was 0.03∼23.96%, 2.63∼75.21%, and − 5.70∼19.24%, respectively, which increased with an increasing SMC, suggesting that the deterioration effect of FTC on soil mechanical properties was promoted by increasing SMC. During alternating FTC, the relative range and variation coefficient of Dc were 2.21–2.43 and 67.87–75.72%, respectively, indicating that Dc was highly sensitive to FTC. Furthermore, Dc increased by 2.37–71.22% after 15 FTC. Alternating freeze-thaw weakened the soil resistance to detachment. Moreover, the promoting effect of FTC on Dc intensified with an increasing SMC, indicating that the variation in Dc was strongly affected by SMC during FTC. A prediction model (R2=0.955, RRMSE=14.99%) was established to quantify the influence of freeze-thaw and hydrodynamic characteristics on Dc. The explanation rate of variables in the Dc prediction equation was quantitated: the explanation rate of stream power (64.3%) was higher than that of FTC (10.02%) and SMC (3.92%), suggesting that the impact of freeze-thaw on Dc was covered by hydrodynamic characteristics. Further validation is required for the prediction equations when applied beyond the range of construction conditions.

Effects of physical crust on soil detachment by overland flow in the Loess Plateau region of China

Physical soil crust (PSC), a key component of surface soil structure, exists extensively in loess areas. PSC is considered to have a significant effect on soil detachment processes. However, the long-term effects and the corresponding mechanisms of PSC on soil detachment by overland flow are still not well understood, especially in natural environments. To investigate temporal variation in soil erosion resistance and the underlying factors during PSC formation, an 8 × 8-m soil plot was exposed to natural conditions in the Loess Plateau over a 524-day period spanning two rainy seasons and a winter between them. A flume test was conducted to determine soil detachment capacity (Dc) under six designed flow shear stress levels (5.66–22.11 Pa) using crusted (SC) and non-crusted (NSC) soil samples at different PSC development stages. Subsequently, two soil erosion resistance parameters, rill erodibility (Kr) and critical shear stress (τc), were calculated. Over time, in the SC and NSC treatments, Kr decreased from 0.516 to 0.120 s m−1 and 0.521 to 0.223 s m−1, respectively, while τc increased from 0.49 to 4.42 Pa and 0.26–2.46 Pa, respectively. Variation in soil erosion resistance was rapid in the first one to two months, and then slowed down, with slight fluctuations afterwards. In the SC treatment, Kr was 42% lower and τc was 67% greater than those in the NSC treatment. Soil properties changed greatly for both treatments. SCT increased from 0 to 7.09 mm in the SC treatment. Coh increased from 2.91 to 9.04 kPa and 3.01–4.78 kPa in SC and NSC treatments, respectively. Both soil erosion resistance parameters could be well predicted by SCT and Coh in the SC treatment (R2 ≥ 0.82), while their best predictor was Coh in the NSC treatment (R2 ≥ 0.90). The results demonstrate that PSC formation enhances soil erosion resistance in the soil detachment process in the loess region under natural conditions. Our study revealed the important role and complexity of PSC in the process of soil erosion, and provided theoretical and data support for accurate understanding and prediction of soil erosion.

Effect of roots on the soil detachment process in grassland and shrubland

AbstractPlant roots play an important role in reducing soil detachment and increasing soil erosion resistance. Current studies have mainly focused on the quantification of the soil detachment rate (SDR). However, few studies have been conducted to explore the variation in the SDR and root effects during the soil detachment process. In this study, undisturbed soil samples were collected from four shrublands (with the main species Caragana korshinskii, C. korshinskii mixed with Agropyron cristatum, C. korshinskii mixed with Bothriochloa ischaemum, and C. korshinskii mixed with Artemisia gmelinii) and three grasslands (with the main species B. ischaemum, B. ischaemum mixed with A. cristatum, and B. ischaemum mixed with A. gmelinii) on the Loess Plateau, China. The samples were subjected to flow scouring through hydraulic flume experiments under six different shear stresses. The duration of this experiment was 6 min, and the SDR of each minute was estimated. The results showed that the SDR of shrubland was higher than that of grassland in each detachment process. The dominant factors affecting SDR gradually changed from the root–soil composite properties to the hydrodynamic parameters in this process. The effect of roots on the SDR was stronger than that of soil properties in the root–soil composite. In the early detachment stage, the 0–2 mm fine roots had the strongest effect on the SDR, while it was affected by the combined effect of fine roots and coarse roots in the later stage. When the fibrous roots were mixed with the tap roots, the fibrous roots had a significant negative effect on the SDR and mainly affected the early detachment stage. However, the tap roots had a weak effect in all stages. With the detachment process, the soil erosion resistance first increased and then stabilized, and the variations were mainly concentrated from 0 to 3 min. The roots mainly affected the SDR and soil erosion resistance by influencing rill erodibility parameters.

Investigating the effects of herbaceous root systems on the soil detachment process at the species level

Plant root growth significantly affect soil detachment process, whereas the mechanism of how roots affect the soil detachment process by overland flow at species level is not fully understood. This study was conducted to investigate the soil detachment rate responds to plant-induce soil properties and root traits at species level. Two typical herbaceous plants, Bothriochloa ischcemum (Linn.). Keng (BI; fibrous root system) and Artemisia vestita Wall. ex Bess (AG; tap root system), from the Loess Plateau were studies for one year under six planted densities of 5 plants m−2, 10 plants m−2, 15 plants m−2, 20 plants m−2, 25 plants m−2, and 30 plants m−2. In total, 24 steel tanks were planted, and two plots were used as bare soil controls. Their soil detachment rates were tested under a constant overland flow (1.5 l s−1) on a 26.2 % slope. The results showed that soil detachment rate under the six planted densities ranged from 0.034 kg m2 s−1 to 0.112 kg m2 s−1 for BI and was ranged from 0.053 kg m2 s−1 to 0.132 kg m2 s−1 for AG, which all greatly reduced soil detachment rate and were 68.17 % to 92.33 % and 69.20 % to 87.27 % less than that of the control. In general, BI was more effective in reducing soil detachment rate than AG, achieving a mean soil detachment rate that was 23.75 % lower. With increasing plant density, soil detachment rate decreased as a power function. The overland flow hydraulic characteristics, soil properties and root traits influenced by plant density were positively or negatively correlated with soil detachment rate. Specifically, soil detachment rate decreased with velocity, bulk density, root length density, and increased with shear stress and Darcy–Weisbach friction factor as power or exponential functions. On this basis, the soil detachment rate (Dr) can be satisfactorily estimated by overland flow velocity (v), soil bulk density (BD) and root length density (RLD) as a power function (Dr = 63.03v0.174 × BD−20.712 × RLD−0.233R2 = 0.65; NSE = 0.60; p < 0.01).

Quantifying the Effects of Root and Soil Properties on Soil Detachment Capacity in Agricultural Land Use of Southern China

Unsustainable agricultural land use will lead to an increased risk of soil erosion and soil degradation. However, few studies have investigated impacts of changes in root and soil properties on the soil detachment process. Therefore, we investigated the effects of agricultural land use on the relative contribution of root and soil properties to soil detachment capacity. Soil samples were collected from six different land use types and subjected to flow scouring under six shear stresses ranging from 4.98 to 16.37 Pa. Agricultural land use influenced root distribution and soil properties in the soil surface layer. Root length density, root surface area density, and root volume density in orchards with no cover, orchards with grass cover, and farmland were less than those of grassland samples. Different land use types affected soil detachment capacity. Bare land, farmland, and orchards with no cover were more vulnerable to erosion, while forest, orchard with grass cover, and grassland showed little soil detachment. Soil detachment capacity decreased exponentially with increasing soil bulk density, aggregate stability, organic matter, and root mass density. The root mass density and aggregate stability had the greatest contribution to the soil detachment capacity. Agricultural land use increases the risk of soil erosion; a groundcover management strategy, such as planting grass in orchard, could effectively increase the fine root distribution and aggregate stability to control soil erosion.

Vegetation-induced soil stabilization in coastal area: An example from a natural mangrove forest

Mangrove forests provide essential ecosystem services in tropical and semitropical regions by supporting their natural regeneration and other biosystem processes, offering livelihood for local communities, and contributing significantly to the natural resources. Systematic analyses on the protective role of mangrove forests and their effects on reducing coastal erosion have been carried out rarely. Mangroves form a complex ecosystem that increases substrate stabilization and dissipates wave energy favouring the deposition of fine material. This study focuses on assessing the role of white mangrove (Avicennia marina (Forssk.) Vierh.) roots in stabilizing the coastline. In a study site located in Southern Iran, a series of field and laboratory measurements were conducted on root systems that were collected from transects perpendicular to the coastline. Root samples were collected from soil cores at fixed distances from tree stems in three layers at seaward and landward positions. Moreover, Root tensile tests were conducted to estimate their biomechanical characteristics which provided the data of parameters of root reinforcement models. Also, the spatial distribution of root reinforcement and the intrinsic-variability of stabilizing components in relation to horizontal and vertical distances from the tree stem were calculated. Three models of Wu & Waldron (W&W), Fiber Bundle (FBM), and Root Bundle Weibull (RBMw) were applied. results showed that the Root Volume Ratio (RVR) and the Number of Roots (NoR) decreased with distance from the tree stem. Root tensile forces increased with root diameter. Finally, calculated root reinforcements at 0.75 m distance associated with the highest value while the lowest value was observed at 1.50 m from the tree stem. The differences between seaward and landward positions were minor. Soil detachment ratio (SDR) was approximately 10% higher at landwards positions than seaward, due to different geomorphological conditions that affected the soil detachment process. The similartity of root reinforcement values among root systems at seaward and landward positions suggest that stem density may not be an important parameter in managing mangrove forests as a coastal protection measure. Yet, RVR at different distances and NoR increasing significantly with soil depth and being different at seaward and landward positions, could improve their potential role as a nature-based solution for shoreline protection.

Temporal variation in soil detachment processes under litter incorporation effects in typical grassland on the Loess Plateau of China

Plant litter incorporation into soil is a widespread phenomenon in the natural environment. Accurate estimation of the soil detachment capacity (Dc) driven by overland flow under litter incorporation effects is crucial for improving soil erosion prediction. However, the effects of litter incorporation on soil detachment processes are often ignored, and the temporal variation of Dc under litter incorporation effects remains unclear for grassland in the Loess Plateau of China. In this study, two litter incorporation soil treatments (Bothriochloa ischaemum (L.) Keng. litter, and Artemisia sacrorum Ledeb. litter) and one bare soil treatment (control) were set. Flume tests with flow shear stress ranging from 5.66 to 22.11 Pa were conducted to simulate overland flow scouring and determine the temporal variation of Dc between May 2017 and October 2018 (nine times, a 524-day period). Furthermore, the key factors affecting Dc were identified. Our results showed that the temporal variation in Dc was consistent across the different soil treatments (two litter incorporation treatments and one bare soil control), showing a rapid decline and then fluctuating at a low level, with Dc ranging from 0.115 to 6.876 kg m−2 s−1. Incorporation of litter effectively reduced Dc, with the Dc of soil treatments incorporating litter being 15% to 29% lower than that of bare soil (2.110 kg m−2 s−1). Further analysis showed that the temporal variation in Dc was mainly affected by the development of a physical soil crust. Dc can be successfully estimated using a nonlinear equation incorporating flow shear stress and soil cohesion (R2 = 0.77, NSE = 0.65), which represent the erosive force and soil erosion resistance, respectively. Our study reveals the important role of plant litter in the soil detachment process and aids the understanding of soil erosion pathways. Further studies are needed to investigate the effects of a physical soil crust on the soil detachment process driven by overland flow.

Soil detachment by sediment-laden rill flow interpreted using three experimental design methods

Soil detachment by rill flow is a key sub-process of rill erosion on hillslope areas; however, many studies have focused on soil detachment by clear rather than by sediment-laden rill flow and most studies have used the time- and labor-consuming full-factorial design. This study used three experimental design methods, including the full-factorial design, orthogonal design, and Taguchi method for indoor flume experiments to explore the process of soil detachment by sediment-laden rill flow. We analyzed and quantified the effects of the slope, flow discharge, and sediment load on the soil detachment rate by sediment-laden rill flow, and compared the results of the three experimental design methods to evaluate the applicability of the orthogonal design and Taguchi method in indoor flume experiments. The results showed that the flow discharge and slope were positively correlated with the soil detachment rate, while the sediment load was negatively correlated with it. The main effects of slope, flow discharge, and sediment load on the soil detachment rate, which were obtained from the three methods, had the same trends. The contribution rates of the flow discharge, slope, and sediment load were 46.20–47.83, 34.42–40.88, and 7.45–12.87%, respectively. The contribution rates of each factor obtained from the three methods were slightly different, but the overall ranked order was the same. The Taguchi method and the orthogonal design can effectively reduce the number of tests and ensure low relative errors. The prediction of the soil detachment rate from the Taguchi method was closer to the full-factorial design than the orthogonal design. Therefore, the Taguchi method proved was most efficient for flume experiments on soil detachment processes by the sediment-laden rill flow.

Investigating the Effects of Herbaceous Root Types on the Soil Detachment Process at the Species Level

Investigating the Effects of Herbaceous Root Types on the Soil Detachment Process at the Species Level

Detachment rates of purple soil rills affected by subsurface water flow depth

Subsurface water flow, considered as a specific hydrological condition, can couples with overland flow, which likely lead to notable effects on soil detachment process in rills. Clarifying the change in soil detachment rate under different subsurface flow conditions is beneficial to understand the mechanisms of soil erosion and improve process-based erosion model. In this study, the maximum sediment delivery rates at sediment transport capacity were used to determine the conditions when soil detachment rates reached zero. Then, a numerical method (NM) and modified numerical method (MNM) were utilized to estimate the detachment rates through sediment delivery amounts along a rill affected by subsurface water flow depths (5 cm, 10 cm, and 15 cm). The experiments were performed under combined conditions of four slope gradients (5°, 10°, 15°, and 20°) and three flow rates (2, 4, and 8 L min−1). The results showed that the detachment rates could be fitted by a linear function of the sediment concentration computed by the NM and MNM under different subsurface water flow depths. When the depths of subsurface water were 5 cm, 10 cm and 15 cm below the soil surface, soil detachment rates estimated by the NM were approximately 1.23, 1.31 and 1.12 times those obtained by the MNM, respectively. The MNM yielded more reasonable detachment rates than the NM. This method is applicable to estimate detachment rates affected by different subsurface water flow depths. Soil detachment rates of subsurface water flow 5 cm below the surface were similar to those of saturated soil with a 20° slope gradient and a flow rate of 8 L min−1. Soil detachment rate for subsurface water flow at 15 cm below the surface were similar to those of unsaturated soil under low hydrodynamic conditions. The results of this study are helpful to understand the mechanisms of soil erosion affected by subsurface flow and provide a theoretical basis for improving the soil erosion model.

Effects of moss-dominated biocrusts on soil detachment by overland flow in the Three Gorges Reservoir Area of China

Biological soil crusts (biocrusts) are important landscape components that exist in various climates and habitats. The roles of biocrusts in numerous soil processes have been predominantly recognized in many dryland regions worldwide. However, little is known about their effects on soil detachment process by overland flow, especially in humid climates. This study quantified the effects of moss-dominated biocrusts on soil detachment capacity (Dc) and soil erosion resistance to flowing water in the Three Gorges Reservoir Area which holds a subtropical humid climate. Potential factors driving soil detachment variation and their influencing mechanism were analyzed and elucidated. We designed five levels of coverage treatments (1%–20%, 20%–40%, 40%–60%, 60%–80%, and 80%–100%) and a nearby bare land as control in a moss-dominated site. Undisturbed soil samples were taken and subjected to water flow scouring in a hydraulic flume under six shear stresses ranging from 4.89 to 17.99 Pa. The results indicated that mean Dc of moss-covered soil varied from 0.008 to 0.081 kg m−2 s−1, which was 1.9 to 21.0 times lower than that of bare soil (0.160 kg m−2 s−1). Rill erodibility (Kr) of moss-covered soil ranged from 0.0095 to 0.0009 s m−1, which was 2 to 20 times lower than that of bare soil (0.0187 s m−1). Both relative soil detachment rate and Kr showed an exponential decay with increasing moss coverage, whereas the critical shear stress (τc) for different moss coverage levels did not differ significantly. Moss coverage, soil cohesion, and sand content were key factors affecting Dc, while moss coverage and soil bulk density were key factors affecting Kr. A power function of flow shear stress, soil cohesion, and moss coverage fitted well to estimate Dc (NSE=0.947). Our findings implied that biocrusts prevented soil detachment directly by their physical cover and indirectly by soil properties modification. Biocrusts could be rehabilitated as a promising soil conservation measure during ecological recovery to enhance soil erosion resistance in the Three Gorges Reservoir Area.

Effects of root morphological traits on soil detachment for ten herbaceous species in the Loess Plateau

Plant root systems can greatly reduce soil loss, and their effects on soil erosion differ across species due to their varied root traits. The purpose of this study was to determine the effects of root morphology traits of herbaceous plants on the soil detachment process. Ten herbaceous plants (dominant species) in the Loess Plateau were selected, and 300 undisturbed soil samples (including living roots from the selected herbages) were scoured with flowing water to measure their soil detachment capacities under six levels of shear stress (4.98 to 16.37 Pa). Then, the root traits of each soil sample were measured, and the rill erodibility and critical shear stress were estimated based on the Water Erosion Prediction Project (WEPP) model. The results showed that root morphology traits varied greatly among the ten selected herbages. Accordingly, resulting variations in soil detachment capacity (0.030 to 3.297 kg m−2 s−1), rill erodibility (0.004 to 0.447 s m−1), and critical shear stress (4.73 to 1.13 Pa) were also observed. Plants with fibrous roots were more effective than those with tap roots in reducing soil detachment. Their mean soil detachment capacity and rill erodibility were 93.2% and 93.4% lower, respectively, and their mean critical shear stress was 1.15 times greater than that of the herbaceous plants with tap root systems. Of all the root traits, root surface area density (RSAD) was the primary root trait affecting the soil detachment, and it estimated the soil detachment capacity well (R2 = 0.91, normalized squared error (NSE) = 0.82). Additionally, an equation with few factors (soil aggregate and RSAD) was suggested to simulate the soil detachment capacity when the plant root parameters and soil properties were limited.

Evaluating the Contribution of the Physical and Biochemical Effects of Root on Detachment for the Coarse-Textured Soil from the Three Gorges Reservoir, China

Plant roots have great effect on the process of soil detachment, but the contribution of the physical and biochemical effects of root on soil detachment have not been sufficiently evaluated. For the purpose to distinguish the relative contribution of the root physical effect and biochemical effect to soil erosion resistance, three kinds of treatment, soil samples without roots, dead roots and live roots, were chosen to simulate the condition of no root effect, the root’s physical effect and the root’s whole effect on the soil detachment, respectively. The Cynodon dactylon and purple soil in Three Gorges Reservoir Area in China were selected to conduct the scouring experiment under condition of five slope gradients (17.63%, 26.79%, 36.40%, 46.63% and 57.73%) and three runoff rates (1, 2 and 3 L min-1). The soil detachment process under different treatment conditions was analyzed. The effects of slope gradient and the runoff rate on soil detachment rate were also estimated for different treatment conditions and the contributions of the physical and biochemical effects of root on soil detachment were also evaluated. The results indicated that detachment rate showed a dramatically decrease initially, followed by gradual decrease until to a stable stage for all treatments. The soil detachment rate can be effectively described with the slope gradient and the runoff rate by using the power function, which was more influenced by slope gradient than by runoff rate for all the treatment conditions in this study. Plant roots could effectively diminish the soil detachment rate, and the contributions of physical and biochemical effects of root on detachment were 59.64%–82.09% and 17.91%–40.36%, with mean values of 68.92% and 31.08%, respectively. The root’s physical effect is the main reason strengthens soil erosion resistance.

Impacts of near soil surface factors on soil detachment process in benggang alluvial fans

Soil detachment processes are greatly impacted by vegetation recovery. However, the understanding of these relationships on bengggang alluvial fans in southeast China is limited. In this research, the impacts of near soil surface factors (litter stems, biological crusts, and roots) on soil detachment is quantitated by overland flow with two introduced grass species (Paspalum mandiocanum and Pennisetum giganteum) in the alluvial fans. Flow scouring was performed on soil samples obtained from two grasslands under nine forms of shear stress ranging from 2.70 to 10.51 Pa. Paspalum mandiocanum and Pennisetum giganteum significantly reduced the soil detachment capacity of alluvial fans by 94% and 98%, respectively. The roots of the two kinds of grass were most effective in reducing soil detachment (48% and 63%), the situation of >80% was caused by root binding; besides, the contribution of biological crusts (37% and 20%) was higher than that of litter-stems (9% and 15%). Compared with bare land, roots and biological crusts under Paspalum mandiocanum and Pennisetum giganteum decreased rill erodibility by 43% and 40%, 60% and 20%, respectively; meanwhile, they increased the critical shear stress by 0.92- to 3.02-fold, 1.15- to 3.34-fold, respectively. Roots of the two plants increased soil resistance more than the biological crusts mainly by root binding. Overall, Pennisetum giganteum (a deep roots plant) showed a slightly better performance in controlling soil detachment than Paspalum mandiocanum (a shallow roots plant); among them, the roles of roots and litter-stems of Pennisetum giganteum were greater than those of Paspalum mandiocanum while the role of biological crusts of Paspalum mandiocanum was greater than that of Pennisetum giganteum. These findings highlight the significance of litter-stems, biological crusts and roots in controlling soil erosion and are helpful for selecting suitable grasses for vegetation recovery in alluvial fans of benggang.

Detachment of dilatant soil due to high hydraulic shear stresses explained

When soil surfaces are exposed to high shear stresses induced by high-velocity water flows, they erode. Erosion consists of the detachment, entrainment and transport of soil particles. When exposed to high shear stresses the process of soil detachment is no longer given by the pick-up of individual particles but by the continuous shear failure of layers of soil. The understanding of how soil properties influence the detachment rate of soil is lacking. This paper presents how the bed shear stress, initial and critical porosity, hydraulic conductivity, and density influence the detachment rate of dilatant soils which are subjected to high-velocity flows. Based on the constitutive mass and momentum balance equations it is hypothesized that the exchange of mass and momentum during soil detachment corresponds with that situation for which the dilatancy induced shear resistance of the soil is maximum. The hypothesis is validated against high-velocity erosion experiments on sand.

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

AbstractPlant litter can be incorporated into topsoil by a natural process, affecting the soil erosion process. This is a widespread phenomenon in erosion‐prone areas. This study was conducted to investigate the effect of litter incorporation on the process of soil detachment on the Loess Plateau, China. Four common plant litters (Bothriochloa ischaemum L. Keng., Artemisia sacrorum Ledeb., Setaria viridis L. Beauv., and Artemisia capillaris Thunb.) were collected, then incorporated into the silt loam soil at five rates (0.1, 0.4, 0.7, 1.0, and 1.3 kg m−2) on the basis of our field investigation. Twenty litter–soil treatments and one bare soil control were prepared. After 50 days of natural stabilization, 30 soil samples of each treatment were collected. We used a flume test to scour the soil samples under six flow shear stress conditions (5.66, 8.31, 12.21, 15.55, 19.15, and 22.11 Pa). The results showed that the different incorporated litter masses and morphological characteristics, such as litter tissue density (ranging from 0.52 to 0.68 g cm−3), length density (2.34 to 91.00 km m−3), surface area density (LSAD; 27.9 to 674.2 m2 m−3), and volume ratio (0.003 to 0.050 m3 m−3), caused varied soil detachment capacities (0.043 to 4.580 kg·m−2·s−1), rill erodibilities (0.051 to 0.237 s m−1), and critical shear stresses (2.02 to 6.83 Pa). The plant litter incorporated within the soil reduced the soil detachment capacities by 38%–59%, lowered the rill erodibilities by 32%–46%, and increased the critical shear stresses by 98%–193% compared with the bare soil control. The soil containing B. ischaemum (L.) Keng. litter was more resistant to erosion. By comparing different parameters, we found that the contact area between the litter and soil was the main factor affecting the soil detachment process. The soil erosion resistance increased with the increasing contact area between the soil and litter. Furthermore, the litter incorporation effect on rill erodibility can be comprehensively reflected by LSAD (R2 = .93; Nash–Sutcliffe efficiency = 0.79), which could be used to adjust the rill erodibility parameter in physical process‐based soil erosion models.

Modelling the process of soil detachment by rill flow on steep loessial hillslopes

AbstractSoil detachment by rill flow is a key process of rill erosion, modelling this process can help in understanding rill erosion mechanisms. However, many soil detachment models are established on conceptual assumptions rather than experimental data. The objectives of this study were to establish a model of soil detachment by rill flow based on flume experimental data and to quantitatively verify the model. We simulated the process of soil detachment by rill flow in flume experiments with a soil‐feeding hopper using loessial soil on steep slopes. Seven flow discharges, six slopes and five sediment loads were combined. Soil detachment capacity, sediment transport capacity, and soil detachment rate by rill flow under different sediment loads were measured. The process of soil detachment by rill flow can be modelled by a dual power function based on soil detachment capacity and transport capacity deficit as variables. The established model exhibited high credibility (NSE=0.97; R2=0.97). The contributions of soil detachment capacity and transport capacity deficit to soil detachment rate by rill flow reached 60% and 36%, respectively. Soil detachment capacity exerted more influence on soil detachment rate than did transport capacity deficit. The performance of the WEPP rill erosion equation is also favourable (NSE=0.95; R2=0.97). The two power exponents in the model we established strengthen the role of soil detachment capacity in soil detachment rate and weaken that for transport capacity deficit. Soil detachment capacity and transport capacity deficit played important roles in the determination of soil detachment rate by rill flow. The results can be applied to implement the numerical modeling and prediction of rill erosion processes on steep loessial hillslopes. © 2019 John Wiley &amp; Sons, Ltd.

Effects of soil-incorporated plant litter morphological characteristics on the soil detachment process in grassland on the Loess Plateau of China

The Loess Plateau of China is one of the most eroded areas in the world. In the past 20 years, effective vegetation restoration measures have significantly changed the near-surface characteristics of soil. In natural conditions, plant litter is widespread in the topsoil. The effects of litter incorporated into soil on the process of soil detachment, which is closely related to plant litter morphology, are still not well known. This study aimed to detect the variation of litter morphological characteristics and quantify their effects on soil detachment capacity and rill erodibility when litter is incorporated into the soil. Four plant litters (Bothriochloa ischaemum (L.) Keng., Artemisia sacrorum Ledeb., Setaria viridis (L.) Beauv., and Artemisia capillaris Thunb.) with five length levels (<0.5, 0.5–2, 2–4, 4–6, and 6–8 cm) were incorporated into soil (idle for 50 days) at the same litter biomass rate (0.7 kg m−2). Then the soil was sampled and cores were subjected to overland flow under six flow shear stress levels (5.66, 8.31, 12.21, 15.55, 19.15 and, 22.11 Pa) using a hydraulic flume (4.0 m × 0.35 m). The results showed that the litter morphological characteristics of litter length density (LLD), litter surface area density (LSAD) and litter volume ratio (LVR) differed in plant species at the same litter biomass rate. Correspondingly, soil detachment capacity (ranging from 0.414 to 2.179 kg m−2 s−1) and rill erodibility (ranging from 0.037 to 0.177 s m−1) varied significantly and mean values from the Bothriochloa ischaemum (L.) Keng. treatments were the minimums, which were 28% to 37% and 23% to 35% less than that of the other treatments, respectively. The soil detachment capacity and rill erodibility were significantly correlated with the litter morphological characteristics of LLD, LSAD and LVR (p < 0.01). The contact area between litter and soil was the most critical factor affecting soil detachment. Incorporated plant litter residue litter effect on rill erodibility could be well estimated by LSAD.