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 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.     

Effects of biological soil crusts on soil detachment process by overland flow in the Loess Plateau of China

Biological soil crusts (BSCs) cover up to 60 to 70% of the soil surface in grasslands after the ‘Grain for Green’ project was implemented in 1999 to rehabilitate the Loess Plateau. However, few studies exist that quantify the effects of BSCs on the soil detachment process by overland flow in the Loess Plateau. This study investigated the potential effects of BSCs on the soil detachment capacity (D_c), and soil resistance to flowing water erosion reflected by rill erodibility and critical shear stress. Two dominant BSC types that developed in the Loess Plateau (the later successional moss and the early successional cyanobacteria mixed with moss) were tested against natural soil samples collected from two abandoned farmland areas. The samples were subjected to flow scouring under six different shear stresses ranging from 7.15 to 24.08 Pa. The results showed that D_c decreased significantly with crust coverage under both moss and mixed crusts. The mean D_c of bare soil (0.823 kg m^?2 s^?1) was 2.9 to 48.4 times greater than those of moss covered soil (0.017–0.284 kg m^?2 s^?1), while it (3.142 kg m^?2 s^?1) was 4.9 to 149.6 times greater than those of mixed covered soil (0.021–0.641 kg m^?2 s^?1). The relative detachment rate of BSCs compared with bare soils decreased exponentially with increasing BSC coverage for both types of BSCs. The D_c value can be simulated by flow shear stress, cohesion, and BSC coverage using a power function (NSE ≥ 0.59). Rill erodibility also decreased with coverage of both crust types. Rill erodibility of bare soil was 3 to 74 times greater than those of moss covered soil and was 2 to 165 times greater than those of mixed covered soil. Rill erodibility could also be estimated by BSC coverage in the Loess Plateau (NSE ≥ 0.91). The effect of crust coverage on critical shear stress was not significant. Copyright © 2015 John Wiley & Sons, Ltd.     

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

Soil 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 & Sons, Ltd.     

Impact of root architecture on the erosion‐reducing potential of roots during concentrated flow

Many studies focus on the effects of vegetation cover on water erosion rates, whereas little attention has been paid to the effects of the below ground biomass. Recent research indicates that roots can reduce concentrated flow erosion rates significantly. In order to predict this root effect more accurately, this experimental study aims at gaining more insight into the importance of root architecture, soil and flow characteristics to the erosion‐reducing potential of roots during concentrated flow. Treatments were (1) bare, (2) grass (representing a fine‐branched root system), (3) carrots (representing a tap root system) and (4) carrots and fine‐branched weeds (representing both tap and fine‐branched roots). The soil types tested were a sandy loam and a silt loam. For each treatment, root density, root length density and mean root diameter (D) were assessed. Relative soil detachment rates and mean bottom flow shear stress were calculated. The results indicate that tap roots reduce the erosion rates to a lesser extent compared with fine‐branched roots. Different relationships linking relative soil detachment rate with root density could be established for different root diameter classes. Carrots with very fine roots (D < 5 mm) show a similar negative exponential relationship between root density and relative soil detachment rate to grass roots. With increasing root diameter (5 < D < 15 mm) the erosion‐reducing effect of carrot type roots becomes less pronounced. Additionally, an equation estimating the erosion‐reducing potential of root systems containing both tap roots and fine‐branched roots could be established. Moreover, the erosion‐reducing potential of grass roots is less pronounced for a sandy loam soil compared with a silt loam soil and a larger erosion‐reducing potential for both grass and carrot roots was found for initially wet soils. For carrots grown on a sandy loam soil, the erosion‐reducing effect of roots decreases with increasing flow shear stress. For grasses, grown on both soil types, no significant differences could be found according to flow shear stress. The erosion‐reducing effect of roots during concentrated flow is much more pronounced than suggested in previous studies dealing with interrill and rill erosion. Root density and root diameter explain the observed erosion rates during concentrated flow well for the different soil types tested. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatial variation in soil resistance to flowing water erosion along a regional transect in the Loess Plateau

The factors influencing soil erosion may vary with scale. It remains unclear whether the spatial variation in soil erosion resistance is controlled by regional variables (e.g. precipitation, temperature, and vegetation zone) or by local specific variables (e.g. soil properties, root traits, land use, and farming operations) when the study area enlarges from a hillslope or catchment to the regional scale. This study was performed to quantify the spatial variations in soil erosion resistance to flowing water under three typical land uses along a regional transect on the Loess Plateau and to identify whether regional or local specific variables are responsible for these changes. The results indicated that the measured soil detachment capacities (D_c) of cropland exhibited an irregular trend along the regional transect. The D_c of grassland increased with mean annual precipitation, except for two sites (Yijun and Erdos). The measured D_c of woodland displayed an inverted ‘U’ shape. The changes in rill erodibility (K_r) of three land uses were similar to D_c, whereas no distinguishable trend was found for critical shear stress (τ_c). No significant correlation was detected between D_c, K_r and τ_c, and the regional variables. The spatial variation in soil erosion resistance could be explained reasonably by changes in soil properties, root traits, land use, and farming operations, rather than regional variables. The adjustment coefficient of K_r for grassland and woodland could be well simulated by soil cohesion and root mass density (R² = 0.70, P < 0.01), and the adjustment coefficient of critical shear stress could be estimated with aggregate stability (R² = 0.57, P < 0.01). The results are helpful for quantifying the spatial variation in soil detachment processes by overland flow and to develop process‐based erosion model at a regional scale. Copyright © 2015 John Wiley & Sons, Ltd.     

Quantifying the surface covering,binding and bonding effects of biological soil crusts on soil detachment by overland flow

Biological soil crusts (BSCs) have impacts on soil detachment process through surface covering, and binding and bonding (B&B) mechanisms, which might vary with successional stages of BSCs. This study was conducted to quantify the effects of surface covering, binding and bonding of BSCs on soil detachment capacity by overland flow in a 4 m long hydraulic flume with fixed bed. Two dominant BSC types, developed well in the Loess Plateau (the early successional cyanobacteria and the later successional moss), were tested using natural undisturbed soil samples collected from the abandoned farmlands. Two treatments of undisturbed crusts and one treatment of removing the above‐ground tissue of BSCs were designed for each BSC type. For comparison, bare loess soil was used as the baseline. The collected soil samples were subjected to flow scouring under six different shear stresses, ranging from 6.7 to 21.2 Pa. The results showed that soil detachment capacity (D_c) and rill erodibility (K_r) decrease with BSC succession, and the presence of BSCs obviously increased the critical shear stress, especially for the later successional moss crust. For the early successional cyanobacteria crust, D_c was reduced by 69.2% compared to the bare loess soil, where 37.7% and 31.5% are attributed to the surface covering and B&B, respectively. For the later successional moss crust, D_c decreased by 89.8% compared to the bare loess soil, where 68.9% and 20.9% contributed to the surface covering and B&B, respectively. These results are helpful in understanding the influencing mechanism of BSCs on soil erosion and in developing the process‐based erosion models for grassland and forestland. Copyright © 2017 John Wiley & Sons, Ltd.     

Predicting physical equations of soil detachment by simulated concentrated flow in Ultisols (subtropical China)

Soil detachment in concentrated flow is due to the dislodging of soil particles from the soil matrix by surface runoff. Both aggregate stability and shear strength of the topsoil reflect the erosion resistance of soil to concentrated runoff, and are important input parameters in predicting soil detachment models. This study was conducted to develop a formula to predict soil detachment rate in concentrated flow by using the aggregate stability index (As), root density (R_d) and saturated soil strength (σ_s) in the subtropical Ultisols region of China. The detachment rates of undisturbed topsoil samples collected from eight cultivated soil plots were measured in a 3.8 m long, 0.2 m wide hydraulic flume under five different flow shear stresses (τ = 4.54, 9.38, 15.01, 17.49 and 22.54 Pa). The results indicated that the stability index (As) was well related with soil detachment rate, particularly for results obtained with high flow shear stress (22.54 Pa), and the stability index (As) has a good linear relationship with concentrated flow erodibility factors (K_c). There was a positive linear relationship between saturated soil strength (σ_s) and critical flow shear stress (τ_c) for different soils. A significant negative exponential relationship between erodibility factors (K_c) and root density (R_d) was detected. This study yielded two prediction equations that allowed comparison of their efficiency in assessing soil detachment rate in concentrated flow. The equation including the root density (R_d) may have a better correlation coefficient (R² = 0.95). It was concluded that the formula based on the stability index (As), saturated soil strength (σ_s) and root density (R_d) has the potential to improve methodology for assessing soil detachment rate in concentrated flow for the subtropical Chinese Ultisols. Copyright © 2012 John Wiley & Sons, Ltd.     

Soil erodibility as impacted by vegetation restoration strategies on the Loess Plateau of China

Near soil surface characteristics change significantly with vegetation restoration, and thus, restoration strategies likely affect soil erodibility. However, few studies have been conducted to quantify the effects of vegetation restoration strategies on soil erodibility in regions experiencing rapid vegetation restoration. This study was conducted to evaluate the effects of vegetation restoration strategies on soil erodibility, reflected by soil cohesion (Coh), penetration resistance (PR), saturated conductivity (K_s), number of drop impacts (NDI), mean weight diameter of soil aggregates (MWD), and soil erodibility K factor on the Loess Plateau. One slope farmland and five 25-year-restored lands covered by old world bluestem, korshinsk peashrub, shrub sophora, sea-buckthorn, and black locust were selected as test sites. The old world bluestem was restored via natural succession, while the other four lands were restored by artificial planting. A comprehensive soil erodibility index (CSEI) was produced by a weighted summation method to quantify the effects of vegetation restoration strategies on soil erodibility completely. The results showed that Coh, K_s, NDI, and MWD of the five restored lands were greater than those of the slope farmland. However, the PR and K of the five restored lands were less than those of the slope farmland. CSEI varied greatly under different restoration strategies, from 1 to 0.214. Compared with the control, these indices decreased on average by 68.2%, 78.6%, 72.7%, 75.8%, and 62.8% for old world bluestem, korshinsk peashrub, shrub sophora, sea-buckthorn, and black locust, respectively. The variation in soil erodibility was significantly influenced by biological crust thickness, bulk density, organic matter content, plant litter density, and root mass density. Shrub-lands via artificial planting, especially korshinsk peashrub, were considered the most effective restoration strategies to reduce soil erodibility on the Loess Plateau. The results are helpful for selecting vegetation restoration strategies and asking their benefits in controlling soil erosion. © 2018 John Wiley & Sons, Ltd.     

Modelling increased soil cohesion due to roots with EUROSEM

As organic root exudates cause soil particles to adhere firmly to root surfaces, roots significantly increase soil strength and therefore also increase the resistance of the topsoil to erosion by concentrated flow. This paper aims at contributing to a better prediction of the root effects on soil erosion rates in the EUROSEM model, as the input values accounting for roots, presented in the user manual, do not account for differences in root density or root architecture. Recent research indicates that small changes in root density or differences in root architecture considerably influence soil erosion rates during concentrated flow. The approach for incorporating the root effects into this model is based on a comparison of measured soil detachment rates for bare and for root‐permeated topsoil samples with predicted erosion rates under the same flow conditions using the erosion equation of EUROSEM. Through backwards calculation, transport capacity efficiencies and corresponding soil cohesion values can be assessed for bare and root‐permeated topsoils respectively. The results are promising and present soil cohesion values that are in accordance with reported values in the literature for the same soil type (silt loam). The results show that grass roots provide a larger increase in soil cohesion as compared with tap‐rooted species and that the increase in soil cohesion is not significantly different under wet and dry soil conditions, either for fibrous root systems or for tap root systems. Power and exponential relationships are established between measured root density values and the corresponding calculated soil cohesion values, reflecting the effects of roots on the resistance of the topsoil to concentrated flow incision. These relationships enable one to incorporate the root effect into the soil erosion model EUROSEM, through adapting the soil cohesion input value. A scenario analysis shows that the contribution of roots to soil cohesion is very important for preventing soil loss and reducing runoff volume. The increase in soil shear strength due to the binding effect of roots on soil particles is two orders of magnitude lower as compared with soil reinforcement achieved when roots mobilize their tensile strength during soil shearing and root breakage. Copyright © 2008 John Wiley & Sons, Ltd.     

Variation in the vertical zonality of erodibility and critical shear stress of rill erosion in China's Hengduan Mountains

Soil erodibility is an essential parameter used in soil erosion prediction. This study selected the Liangshan town watershed to quantify variation in the vertical zonality of rill erodibility (k_r) in China's ecologically fragile Hengduan Mountains. Soil types comprised of yellow–brown (soil A), purple (soil B), and dry-red (soil C) in a descending order of occurrence from the summit to the valley, which roughly corresponds to the vertical climate zone (i.e. cool-high mountain, warm-low mountain, and dry-hot valley sections) of the study area. With elevation, vertical soil zonality varied in both soil organic matter (SOM) content and soil particle-size fractions. A series of rill erosion-based scour experiments were conducted, using water discharge rates of 100, 200, 300, 400, 500, and 600 mL min^-1. Additionally, detachment rates (Dr) were measured under three hydrological conditions (the drainage, saturation, and seepage treatments). Results show that both Dr and flow shear stress (Ʈ) values increased as discharge increased. As elevation increased, the k_r values decreased, while the vertical zonality of critical shear stress (Ʈ_c) values showed no obvious variation. The highest k_r values were observed during the seepage treatment, followed by the saturation treatment then drainage treatment, indicating that variation in vertical hydraulic gradients could significantly alter k_r values. This study also found that land-use types could also alter k_r and Ʈ_c values. Further research, however, is necessary to better quantify the effects of subsurface hydrological conditions and land-use types on k_r under different soil zonalities in China's Hengduan Mountains. © 2018 John Wiley & Sons, Ltd.     

The effects of tillage induced surface roughness,slope and discharge rate on soil detachment by concentrated flow: An experimental study

Soil erosion in sloping cropland is a key water and soil conservation issue in the Loess Plateau region, China. How surface roughness influences soil detachment remains unclear due to the inconsistent results obtained from existing studies. The objectives of the present study were to evaluate the effects of tillage practices on soil detachment rate in sloping cropland and establish an accurate empirical model for the prediction of soil detachment rates. A series of movable bed experiments were conducted on sloping surfaces under three different tillage practices (manual dibbling, manual hoeing, and contour drilling), with a smooth surface (non-tillage) as a control. The research indicated that soil detachment rate significantly increased with roughness (p < 0.05) since the average soil detachment rate was the highest under the contour drilling treatment (6.762 g m⁻² s⁻¹), followed by manual hoeing (4.180 g m⁻² s⁻¹), and manual dibbling (3.334 g m⁻² s⁻¹); the lowest detachment rate was observed under the non-tillage treatment (3.214 g m⁻² s⁻¹). Slope gradient and unit discharge rate were positively correlated with soil detachment rate and proved to be more influential than soil surface roughness. Four composite hydraulic parameters were introduced to estimate soil detachment rate on tilled surfaces. Regression analyses revealed that stream power was the most effective predictor of soil detachment rate compared with unit length shear force, shear stress, and unit stream power. By integrating surface roughness as a variable, the detachment rate could be accurately described as a nonlinear function of stream power and surface roughness. The results of the present study indicate that tillage practice could influence soil loss on sloping cropland, considering the higher soil detachment rates under all tillage practices tested compared with non-tillage. The results are attributed mainly to concentrated flow caused by the high water storage levels on tilled surfaces, which could damage surface microtopography and, subsequently, the development of headcuts.     

The vertical heterogeneity of soil detachment by overland flow on the water-level fluctuation zone slope in the Three Gorges Reservoir,China

Periodic submersion and exposure due to the operation of the Three Gorges Reservoir (TGR) alter the soil properties and plant characteristics at different elevations within the water level fluctuation zone (WLFZ), possibly influencing the soil detachment capacity (D_c), but the vertical heterogeneity of this effect is uncertain. Soil samples were taken from 6 elevation segments (5 m per segment) along a slope profile in the WLFZ of the TGR to clarify the vertical heterogeneity of D_c. Scouring experiments were conducted at 5 slope gradients (17.6%, 26.8%, 36.4%, 46.6%, and 57.7%) and 5 flow rates (10, 15, 20, 25, and 30 L min⁻¹) to determine D_c. The results indicate that the soil properties and biomass parameters of the WLFZ exhibit strongly vertical heterogeneity. D_c fluctuates with increasing elevation, with maximum and minimum average values at elevations of 145–150 m and 165–170 m, respectively. Linear equations accurately describe the relationships between D_c and hydrodynamic parameters, for which the shear stress (τ), stream power (ω), and unit energy of water-carrying section (E) perform much better than the unit stream power (U). Furthermore, a clear improvement is achieved when using a general index of flow intensity to estimate D_c. Furthermore, D_c is significantly and negatively correlated with the mean weight diameter (MWD, p < 0.05) and organic matter content (p < 0.01) but not significantly correlated with other soil properties (p > 0.05). The rill erodibility at elevations of 145–150 m and 170–175 m is greater than that at other elevations. The critical hydraulic parameters were highest in the 165–170 m segments. Both the rill erodibility and the critical parameters fluctuate vertically along the sloping surface. This research highlights the vertical heterogeneity of D_c and is helpful for better understanding the mechanisms responsible for soil detachment in the WLFZ of the TGR.     

Soil erosion resistance effects on rill and gully initiation points and dimensions

Recent research has indicated the large spatial and temporal variation in soil erosion resistance against concentrated flow (SER). This study analyzes this variability in relation to rill and gully initiation locations on slopes and the downslope eroded volumes. The soil erodibility (Kc) and critical flow shear stress (τ_cr), were estimated from topsoil properties and correlated to eroded rill and gully volumes and their initiation points on slopes in the Belgian loess belt. Therefore, concentrated flow paths and topsoil properties were measured in their vicinity. The results show that rill and gully initiation points, and hence the lengths of concentrated flow paths, depend on τ_cr, which is controlled by soil surface conditions and can be predicted from saturated soil shear strength. Soil erosion control measures that increase soil shear strength (e.g. thalweg compaction), can therefore decrease rill and gully lengths. Once a rill or an ephemeral gully is initiated, its cross‐section was found to depend on Kc, which can be estimated from the soil water content, dry bulk density, and the dry density of roots and crop residues incorporated in the topsoil. 74% of the variation in the channel cross‐sectional area measured in the study area could be predicted from the combined effect of flow intensity and these three soil properties, whereas flow intensity alone could only account for 31% of the variation. Soil conservation measures affecting one of the soil properties that control Kc (e.g. double drilling of the thalweg, conservation tillage) can therefore decrease the cross‐sections of the concentrated flow paths. These findings also indicate that rill and gully initiation points are not only topographically controlled but also depend on the SER, which in turn determines the dimensions of these concentrated flow paths. Hence, knowledge of the variability in SER is indispensable. Copyright © 2009 John Wiley & Sons, Ltd.     

Impacts of different vegetation restoration options on gully head soil resistance and soil erosion in loess tablelands

Vegetation restoration is identified as an effective approach to control soil erosion and affects soil detachment and resistance to concentrated flow on the Loess Plateau. However, the effects of vegetation restoration at gully heads in loess-tableland remains unclear. This study was performed to investigate the effects of nine vegetation restoration types at gully heads on soil detachment rate (Dr) and soil resistance to concentrated flow (i.e. soil erodibility, K_r and critical shear stress, τ_c). Undisturbed soil samples were collected from nine vegetation-restored lands and one slope cropland (as the control) and were subjected to a hydraulic flume to obtain Dr values of gully heads under six inflow discharges (0.5–3.5 L s^-1). The results showed that the Dr values of nine revegetated gully heads were 77.11% to 95.81% less than that of slope cropland, and the grassland dominated by Cleistogenes caespitosa and the shrubland dominated by Hippophae rhamnoides had a relatively greater decrease in Dr than those of other seven restoration types. The Dr value of nine revegetated gully heads could be better simulated by stream power than by flow velocity and shear stress and was also significantly affected by soil disintegration rate (positively), soil bulk density, saturated hydraulic conductivity, organic matter content, and water-stable aggregate stability (negatively). Additionally, roots with diameters of 0 to 0.5 mm showed a greater effect on Dr than those with larger diameters. Compared to cropland, the nine restored types reduced K_r by 76.26% to 94.26% and improved τ_c by 1.51 to 4.68 times. The decrease in K_r and the increase in τ_c were significantly affected by organic matter content, water-stable aggregate, mean weight diameter of aggregate and root mass density. The combination of grass species (Cleistogenes caespitosa) and shrub (Hippophae rhamnoides) could be considered the best vegetation restoration types for improving soil resistance of gully heads to concentrated flow. © 2019 John Wiley & Sons, Ltd.     

The effect of conservation tillage on runoff erosivity and soil erodibility during concentrated flow

Crop residues in conservation tillage systems are known to cause both a reduction in the erosive runoff power and an increase in the topsoil erosion resistance. In this study, the relative importance of both mechanisms in reducing soil loss by concentrated flow erosion is examined. Therefore, a method to calculate the effective flow shear stress responsible for soil detachment in the presence of a residue cover is applied. The determination of effective flow shear stress is based on the recalculation of the hydraulic radius for residue treatments. The method was tested in a laboratory flume by comparing soil detachment rates of identical pairs of soil samples that only differ in the presence or absence of crop residues. This shear stress partitioning approach and a soil detachment correction were then applied to a dataset of soil detachment measurements on undisturbed topsoil samples from a no‐till field plot on a loess‐derived soil, sampled during one growing season. Results indicate that only a small fraction (10% on average) of the difference in soil detachment rate between conventional and conservation tillage can be attributed to the dissipation of shear forces on the residues. The remaining decrease in soil detachment during concentrated runoff after a two‐year application of conservation tillage can be explained by the increased dry bulk density and root and crop residue content in the topsoil that reduces soil erodibility. After correcting for the presence of residues, the temporal variability in soil detachment rates (D_r) during concentrated flow for a given flow shear stress (τ) for both treatments can be predicted fairly well (R² = 0·87) from dry soil bulk density (DBD, representing consolidation effects), soil moisture content (SMC, representing antecedent rainfall conditions), the dry mass of organic material (OM, representing root growth and residue decomposition) and saturated soil shear strength σ_{s, sat} using an equation of the form: This study is the first to show that the effect of conservation tillage on soil detachment rates is a result of soil property modifications affecting soil erodibility, rather than a result of the surface residue decreasing flow erosivity. Copyright © 2007 John Wiley & Sons, Ltd.     

Soil detachment by overland flow on steep cropland in the subtropical region of China

Accurate prediction of soil detachment capacity is fundamental to establish process-based erosion models and improve soil loss assessment. Few studies were conducted to reveal the mechanism of detachment process for yellow soil on steep cropland in the subtropical region of China using field experiments. This study was performed to determine soil detachment characteristics and explore the relationships between soil detachment capacity (D _c) and flow rate, slope gradient, mean velocity, shear stress, stream power and unit stream power. Field experiments were conducted on intact soil with flow rates ranging from 0.2 × 10⁻³ to 0.5 × 10⁻³ m⁻³ s⁻¹ and slope gradients varying from 8.8 to 42.4%. The results showed the following. (a) D _c of yellow soil was smaller than other soils because of its high clay content. (b) D _c was more susceptible to flow than to slope gradient. Power functions were derived to depict the relationship between D _c and the flow rate and slope gradient (R² = 0.91). (c) D _c was better simulated by power functions of the stream power (R² = 0.83) than functions of the shear stress or the unit stream power. (d) Considering its accuracy, simplicity and accessibility, the power function based on flow rate and slope gradient is recommended to predict D _c of yellow soil in the field. The results of this study provide useful support for revealing soil detachment mechanism and developing process-based soil erosion models for the subtropical region of China.     

Rill erosion processes on a constantly saturated slope

Rill erosion processes on saturated soil slopes are important for understanding erosion hydrodynamics and determining the parameters of rill erosion models. Saturated soil slopes were innovatively created to investigate the rill erosion processes. Rill erosion processes on saturated soil slopes were modelled by using the sediment concentrations determined by sediment transport capacities (STCs) measurement and the sediment concentrations at different rill lengths. Laboratory experiments were performed under varying slope gradients (5°, 10°, 15°, and 20°) and unit-width flow rates (0.33, 0.67, and 1.33 × 10⁻³ m³ s⁻¹ m⁻¹) to measure sediment concentrations at different rill lengths (1, 2, 4, and 8 m) on saturated soil slopes. The measured sediment concentrations along saturated rills ranged from 134.54 to 1,064.47 kg/m³, and also increased exponentially with rill length similar to non-saturated rills. The model of the rill erosion process in non-saturated soil rills was applicable to that in saturated soil rills. However, the sediment concentration of the rill flow increased much faster, with the increase in rill length, to considerably higher levels at STCs. The saturated soil rills produced 120–560% more sediments than the non-saturated ones. Moreover, the former eroded remarkably faster in the beginning section of the rills, as compared with that on the non-saturated soil slopes. This dataset serves as the basis for determining the erosion parameters in the process-based erosion models on saturated soil slopes.     

Using fluidized bed and flume experiments to quantify cohesion development from aging and drainage

Temporal variations in soil erosion resistance are often the result of decreased soil cohesion due to physical disruption followed by a regain of soil strength through a process called aging, stabilization or consolidation. The goal of this study was to quantify changes in soil cohesion due to aging and subsurface hydrologic condition using a fluidized bed method. A flume experiment was also used to verify that findings from the fluidized bed experiment translated into measurable changes in soil erodibility. Tests were performed on three different soils (a Miami soil, a Cecil soil and Crosby–Miami soil complex). Changes in soil cohesion due to aging and drainage state were successfully detected by the fluidized bed technique. For all soils tested, cohesion developed in a two‐stage process where an increase in cohesion with aging duration immediately after the soil was rewetted, was followed by a decrease in cohesion which often started after 24 h of aging. When soils were aged at field capacity, the resulting cohesion measured by the fluidized bed method was on average 3.13 times higher than that measured when aging was performed at saturation. Trends in soil rill erodibility K_r with aging duration measured in the flume experiment were consistent with the two‐stage pattern observed in soil cohesion estimates but the legacy effect of suction applied at field capacity faded after 72 h of aging. Copyright © 2013 John Wiley & Sons, Ltd.     

Temporal variation in soil detachment under different land uses in the Loess Plateau of China

Measurements of temporal variations in soil detachability under different land uses are badly needed to develop new algorithms or evaluate the existing ones for temporal adjustment of soil detachability in continuous soil erosion models. Few studies have been conducted in the Loess Plateau to quantify temporal variations in detachment rate of runoff under different land uses. The objectives of this study were to investigate the temporal variations of soil detachment rate under different land uses and to further identify the potential factors causing the change in detachment rate in the Loess Plateau. Undisturbed soil samples were collected in the fields of arable land (millet, soybean, corn, and potato), grassland, shrub land, wasteland, and woodland and tested in a laboratory flume under a constant hydraulic condition. The measurements started in mid‐April and ended in early October, 2006. The results showed that soil detachment rate of each land use fluctuated considerably over time. Distinctive temporal variation in detachment rate was found throughout the summer growing season of measurement in each land use. The maximum detachment rates of different land uses varied from 0·019 to 0·490 kg m^–2 s^–1 and the minimum detachment rates ranged from 0·004 to 0·092 kg m^–2 s^–1. Statistical analysis using a paired‐samples t‐test indicated that variations in soil detachment rate differed significantly at the 0·05 level between land uses in most cases. The major factors responsible for the temporal variation of soil detachment were tillage operations (such as planting, ploughing, weeding, harvesting), soil consolidation, and root growth. The influence of tillage operations on soil detachment depended on the degree of soil disturbance caused by the operations. The consolidation of the topsoil over time after tillage was reflected by increases in soil bulk density and soil cohesion. As soil bulk density and cohesion increased, detachment rate decreased. The impact of root density was inconclusive in this study. Further studies are needed to quantify the effects of root density on temporal variations of soil detachment. This work provides useful information for developing temporal adjustments to soil detachment rate in continuous soil erosion models in the Loess Plateau. Copyright © 2009 John Wiley & Sons, Ltd.     

Factors affecting connectivity and sediment yields following wildfire and post-fire salvage logging in California's Sierra Nevada

Sediment delivery following post-fire logging is a concern relative to water quality. While studies have assessed the effect of post-fire logging on sediment yields at different spatial scales, none have explicitly identified sediment sources. Our goal was to quantify post-fire and post-salvage logging sediment yields and use rill patterns to identify sediment sources. We measured the extent and type of logging disturbance, length of rills per unit area or “rill density”, ground cover, and sediment yields in nine logged and five control small catchments or “swales”, 0.09 to 0.81 ha, for 5 years after the 2013 Rim Fire in California's Sierra Nevada. The logged swales had a mean ground disturbance of 31%. After the first wet season following logging, there was no difference in either mean rill density (0.071 and 0.088 m m⁻², respectively) or mean transformed, normalized sediment yields between the control and logged swales. Untransformed mean sediment yields across three sites ranged from 0.11–11.8 and 1.1–3.2 Mg ha⁻¹ for the controls and salvage-logged swales, respectively. Rill density was strongly related to sediment yield and increased significantly with the amount of high-traffic skid trail disturbance in logged swales. Rill density was not significantly related to the amount of bare soil despite a significant relationship between sediment yields and bare soil. Rills usually initiated in bare soil and frequently connected high traffic skid trails to the drainage network after being diverted by waterbars. Rill connectivity and sediment yields decreased in control and logged swales where vegetation or other surface cover was high, suggesting this cover disconnected rills from the drainage network. Increasing ground cover on skid trails and between areas disturbed by post-fire logging and stream channels may reduce sediment yields as well as the hydrologic connectivity between hillslopes and the drainage network.