Rill Detachment Research Articles

Rill Erosion Due to Wildfire or Deforestation in Forestlands of Northern Iran

Rill erosion, mostly affecting steep and long hillslopes, is one of the most severe effects of deforestation and wildfires in natural ecosystems. Specific monitoring and accurate but simple models are needed to assess the impacts of these forest disturbances on the rill detachment process. To address this need, this study has simulated the rill detachment capacity (Dc) through flume experiments on samples of soils collected in hillslopes after deforestation and severe burning. The associations between Dc and organic matter (OM) and the aggregate stability of soil (WSA), two key parameters influencing the rill detachment process, have also been explored under the two soil conditions (deforested and burned soils) using multivariate statistical techniques. Finally, linear regression models to predict Dc from these soil parameters or the hydraulic and morphological variables (water flow rate, WFR, and soil slope, S), set in the flume experiments, have been proposed for both soil conditions. Higher Dc in samples from deforested sites compared to the burned soils (+35%) was measured. This Dc increase was associated with parallel decreases in OM (−15%) and WSA (−34%) after deforestation compared to the wildfire-affected sites. However, the discrimination in those soil properties between the two soil conditions was not sharp. Accurate linear equations (r2 > 0.76) interpolating Dc and the shear stress (τ) have been set to estimate the rill erodibility (Kr) to evaluate soil resistance in erosion models to be applied in deforested or burned sites.

Changes in rill detachment capacity after deforestation and soil conservation practices in forestlands of Northern Iran

Rill erosion is particularly intense after deforestation on hillslopes with steep and long profiles. Since the rill detachment capacity (“Dc”) shows a noticeable variability resulting from different soil properties and vegetation characteristics, there is a need to explore the changes in key soil properties and rill erosion rates under different natural or planted species and a variety of soil conservation practices after deforestation. This study evaluates the changes in four key soil properties (organic carbon content, bulk density and water-stable aggregates of soil, and weight density of plant roots) and rill detachment capacity in forest sites with natural tree species as well as in areas subjected to reforestation and soil conservation treatments in comparison to deforested sites. To this aim, 2000 soil samples have been collected in forestlands of Northern Iran. Sampling was done in deforested areas (assumed as reference condition), and in forest sites 14 plant species (natural or after reforestation). Deforested samples were subjected to five soil conservation treatments (using additives or hydromulching). On all soil samples, Dc has been measured in a laboratory flume at five water flow rates (0.27 to 0.69 L/m s−1) and five soil slopes (5.9 to 31.7 %). All soil properties, when compared to the reference condition, were significantly different (between −50 % to 124 %) among the three soil conditions (natural forests, reforested soils and deforested and treated sites), the natural forests and the treated sites showing a better quality compared to planted forests. Dc was noticeably lower in all conditions (0.022 ± 0.021 kg m−2 s−1) compared to the deforested and untreated sites (0.046 ± 0.023 kg m−2 s−1). Natural and planted forests showed a similar decrease in rill erodibility (−70 to 80 %), while a much lower reduction in Dc (−36 %) was measured for the treated areas. Overall, the study demonstrates that the changes in soil properties due to plant species and soil management as well as the associated variability in rill detachment are noticeably site-specific, and greatly depend on soil conservation treatments. The results of this study may give landscape planners clear indications about the relationships between rill detachment and the associated soil properties among treated and untreated (natural or reforested) soils.

Exploring the accuracy of Random Forest and Multiple Regression models to predict rill detachment in soils under different plant species and soil treatments in deforested lands

Exploring the accuracy of Random Forest and Multiple Regression models to predict rill detachment in soils under different plant species and soil treatments in deforested lands

Modelling effects of human-caused fires on rill detachment capacity based on surface burning of soils in forest lands

It is well known that forest fires affect physico-chemical characteristics of soils. However, few investigations have explored the effects of human-caused fires on soil erosion processes, particularly for the parameters used in rill erosion on steep slopes. To fill this gap, this study has evaluated the effects of human-caused fires on the rill detachment capacity (Dc) and rill erodibility (Kr) in the burned soils, compared to the unburned soils, as control soils, in a forest of Guilan province (northern Iran). For measuring Dc, the slopes (3.9, 9.1, 15.3, 23.9, and 32.7%) and water discharges (0.28, 0.38, 0.46, 0.54, and 0.63 L/m s−1) were adjusted to the desired values using a laboratory flume. Moreover, three properties of soils including the organic matter, bulk density and aggregate stability were determined. The results showed that Dc and Kr were significantly higher in the burned soils, compared to the unburned soils (both more than twice), as the consequence of the significant changes in organic matter content, bulk density and aggregate stability (p < 0.01). Compared to unburned soils, when soil organic matter and aggregate stability decrease, Dc increases in burned soils and these differences led to rill development. These changes in soil properties were confirmed by principal component analysis, which did show a clear discrimination between the burned and unburned sites. The model developed can predict the rill detachment capacity according to a simple linear relationship of the selected soil properties and may be used for an appropriate hydrological management of forest soils.

Modified Numerical Method for Improving the Calculation of Rill Detachment Rate

A rational calculation of the rill detachment rate (RDR) and an accurate simulation of the rill detachment process are important for determining the model parameters of hillslope erosion. Here, we found a difference between RDRs calculated using different methods that cannot be ignored. This study proposes a modified numerical method based on the dataset of the measured sediment concentrations along the rill length over a saturated loess soil slope to improve the calculation of RDR. For the saturated loess soil slope, the modified numerical RDR reduced the relative error from 58.3% to 4.6%, thereby demonstrating the efficiency of the modified numerical method. Furthermore, datasets of previous studies on different soil types and rill width verified the accuracy and applicability of the modified numerical method. A measurement strategy with more sampling points set at the forepart of the rill is proposed to enhance the calculation accuracy of RDR in accordance with the absolute error distribution between numerical and modified numerical RDRs. This study contributes to the literature by correcting previous data, improving data for subsequent measurements, and supplying a basis for the accurate estimation of RDR for rill erosion modeling.

Effects of silica nanoparticles on root characteristics of Zoysia grass and rill detachment capacity in soils treated with hydromulch

It is well known that hydromulching is an appropriate method for controlling runoff and soil erosion. Also, the addition of silica nanoparticles (Nsi) in soil in various ways has been found to be a beneficial technique to improve root characteristics. However, few studies have explored the effects of silica nanoparticles produced from rice husk (as a crop that is exceedingly plentiful in many Asian countries) on soil erosion processes, particularly rill erosion due to overland flow in deforested lands. To fill this gap, this study has evaluated the impacts of root growth induced by silica nanoparticles (Nsi) on the rill detachment capacity (Dc) and erodibility (Kr) in the hydromulched soil samples, compared to the control soil samples in deforested lands of Northern Iran. Dc has been measured by a hydraulic flume under four bed slopes (7.3, 13.9, 18.5, and 23.4%) and five water discharges (0.25, 0.36, 0.47, 0.59, and 0.72 L m−1 s−1) with six replications per experiment. Moreover, after the application of hydromulch on soil surface and the growth of grass, root traits (root diameter, root length, root biomass and root weight density) were measured. The results show that rill detachment capacity was lower (on average −59%) in the hydromulched soil with the presence of silica nanoparticles compared to the untreated plot (P < 0.01). Dc was negatively correlated with the root length, root biomass and root weight density. Conversely, as the root diameter increased, the Dc increased. Rill erodibility, obtained by regressing Dc on shear stress, was noticeably lower (-86%) in the hydromulched soil with the presence of silica nanoparticles compared to the bare plot. Overall, the results of this study confirm that the effects of silica nanoparticles on growth development of root traits and the reduction of soil erosion appear to be impactful, especially on steep slopes.

Effects of rice husk biochar on rill detachment capacity in deforested hillslopes

Biochar addition to soil has been found to be a useful practice to control erosion. However, few studies have explored the effects of biochar produced from husk of rice on rill erosion, due to overland flow in deforested areas. This study has evaluated the rill detachment capacity (Dc) and erodibility (Kr) in soil samples treated with rice husk biochar in comparison to untreated sites (control), collected in deforested hillslopes of Northern Iran. Dc was measured in a lab-scale hydraulic flume at four-bed slopes (9.7, 14.3, 19.5, and 24.2%) and five water discharges (0.24, 0.36, 0.47, 0.59, and 0.69 L m−1 s−1). Moreover, organic carbon, aggregate stability and bulk density of soils were measured. Dc was lower (on average − 32%) in the treated soil compared to the control. Dc was not correlated to the other physico-chemical properties of soil. Kr, estimated by linear regressions between Dc and shear stress, was noticeably lower (−79%) in the treated soil compared to the control. These results evidence that rice husk biochar is effective at controlling and mitigating soil detachment, and at improving organic matter content and aggregate stability in deforested hillslopes. The proposed values of rill erodibility and shear stress of deforested hillslopes (treated or untreated) are helpful to model soil resistance to rill erosion in process-based erosion models.

Response of Soil Detachment Rate to Sediment Load and Model Examination: A Key Process Simulation of Rill Erosion on Steep Loessial Hillslopes.

The rate of soil detachment by water flow indicates soil erosion intensity directly. The exact relation between soil detachment rate and actual sediment load in water flow, however, is still unclear, and the existing relationships have not been adequately tested. The aims of the present study were to investigate the response of soil detachment rate to sediment load using rill flume data with loessial soil and to quantitatively examine the soil detachment equations in the WEPP and EUROSEM soil erosion models. Six slopes were combined with seven flow discharges to measure detachment rates under seven sediment loads using a rill flume with a soil-feeding hopper. Significant differences were found among the soil detachment rate by different sediment loads in low sediment load levels, but an insensitive response of soil detachment rate to sediment load was found under high levels of sediment load. The soil detachment rate was proved to be negatively linearly correlated with sediment load. The rill detachment equation in the WEPP model predicted the soil detachment rate by rill flow very well under our experiment condition. The soil detachment equation in the EUROSEM model underestimated the detachment rates under controlled conditions, but removing the setting velocity from the equation greatly improved prediction. Further experiments that could reflect the dynamic convective detachment and deposition process need to be conducted to compare with the present examination results and to further understand rill erosion processes.

Reduction in soil detachment capacity by inoculation of bacillus polymyxa strain BcP26 in deforested lands

The beneficial effects of rhizobacteria in increasing plant growth, nutrient uptake and rhizosphere soil aggregation are well known. However, few investigations have explored the impacts of these bacteria on soil erosion processes, particularly for the parameters used in rill erosion. To fill this gap, this investigation has evaluated the effects of growth of Zoysia grass roots induced by bacillus polymyxa strain BcP26 on the rill detachment capacity (Dc) and rill erodibility (Kr) in the treated soil, compared to the control soil by a paired sample t-test with six replications for each experiment in deforested lands of Northern Iran. For measuring Dc, a hydraulic flume was used at five slopes (4.3%–38.2%) and five water discharges (0.29–0.69 l m−1 s−1). Three characteristics of roots including the length, biomass and weight density of roots were measured. Dc was lower (on average - 24% for the average slopes) in the treated soil with the bacillus strain (PB) compared to the control soil (AB). Rill erodibility (Kr), obtained by the relationship between Dc and the shear stress, also was significantly lower (−60%) in the PB soil compared to the control soil. A principal component analysis provided a significant combined factor (the first principal component) that revealed an obvious discrimination in rill detachment capacity between treated and control soils. This value of rill erodibility for deforested lands is contributory for modelling purposes of soil resistance to rill erosion. It is concluded that, thanks to the bacillus strain used as one of plant growth promoting rhizobacteria, soil erosion is significantly (at p < 0.01) reduced, and this strain contributes to reduce the risk of soil degradation.

Soil physical degradation and rill detachment by raindrop impact in semi-arid region

Tillage along slopes creates furrows similar to rills which exposes the soil surface directly to raindrop impact (RI) especially in semi-arid regions. In order to quantify the effect of RI on soil physical degradation and rill detachment we determined the change of soil aggregate stability, bulk density, surface resistance and rill detachment in semi-arid soils. Toward these some dominant soils including silt, silty clay, clay, sandy clay and loamy sand were exposed under rainfall impact at four slope gradients (5, 10, 15 and 20%). The laboratory experiments were set up at two soil surface conditions: with and without RI in a flume under simulated rainfalls with an intensity of 90 mm h−1. Four rills with parabolic cross sections, 10 cm width and 4 m length were installed along the flume. Results indicated that the soils are physically degraded (aggregate breakdown, soil compaction and surface resistance) by RI. The rate of physical degradation processes was significantly influenced by the soil type and slope gradient. The soils with higher water-stable aggregates such as sandy clay showed lower physical degradation symptoms. The susceptibility of these soils to physical degradation processes increased with increasing slope gradient. Soil detachment through concentrated flow under RI was affected by the soil physical degradation rate, which in turn was controlled by the two variables i.e. soil type and slope gradient. However the direct effect of RI act in rill detachment (rainfall-driven force) increased rill detachment. As a conclusion, the rill detachment showed higher dependency to RI than soil physical degradation processes. This study revealed that the importance of maintaining the soil surface cover for controlling the negative effects of RI in the soil physical degradation as well as rill detachment especially in semi-arid soils with lower aggregate stability and under steep slopes.

Variability of rill detachment capacity with sediment size, water depth and soil slope in forest soils: A flume experiment

Rill detachment is the most important erosive process in steep slopes and its comprehension and prediction accuracy is important to properly develop soil conservation practices in forest areas. This process is largely influenced by sediment size, soil slope and water flow characteristics, but the results of the studies that have explored these influences are contrasting. This study has simulated in an experimental flume the rill detachment capacity (Dc) of soil with five particle sizes (0–0.25, 0.25–0.5, 0.5–1, 1–2, and 2–3 mm) at five water flow rates (0.26, 0.35, 0.45, 0.56, and 0.67 L m−1 s−1) and five bed slopes (3.5%, 9.1%, 19.2%, 29.1%, and 38.3%) on samples collected in a forestland of Northern Iran. Dc was significantly higher (by 70%) for sediments size over 1 mm compared to the other soil fractions and increased primarily with soil slope and secondarily with water depth. A modelling approach has shown that the unit stream power is the best predictor of Dc using power equations (NSE over 0.87). Linear regression models between Dc and shear stress (τ) were very accurate (r2 over 0.80 with few exceptions) in predicting both rill erodibility and critical shear stress, when developed separately for each particle size class. Sediments with size higher than 1 mm had lower resistance to rill erosion compared to the finer fractions.Overall, the study helps to better understand particle detachment of the erosion process, on which particle size is a key parameter. The modelling activity proposes to land planners values of the rill erodibility and critical shear stress of forested areas for applications in process-based erosion models.

Improving and calibrating channel erosion simulation in the Water Erosion Prediction Project (WEPP) model

The Water Erosion Prediction Project (WEPP) model has been widely used to assess the impacts of management practices and climate change on runoff and soil loss at both hillslope and watershed scales. However, the representation of channel erosion processes in WEPP has not been changed significantly since it was released. The current (WEPP v2012.8) and previous WEPP versions assume that channel input erodibility parameters are constant through time, which may lead to erroneous channel detachment predictions, especially for cropland with substantial tillage operations. In this research, the temporally constant values of channel erodibility and critical shear stress were replaced by daily updated values, using the same temporal erodibility and critical shear stress adjustments that are applied in hillslope profile simulations for rill detachment. Observed watershed-scale runoff and soil erosion data from six agricultural watersheds were used to calibrate and compare the WEPP model performance in simulating channel runoff volumes and soil losses before and after the modification. The research showed both WEPP v2012.8 and the modified WEPP model (WEPP_CE) could satisfactorily simulate event-based hydrology and soil erosion at the watershed outlets after calibration. The WEPP_CE model with temporally varying channel erodibility and critical shear stress values demonstrated improved representation of the physical processes in channel soil detachment. Continued improvement in the representation of channel erosion processes in WEPP and other process-based models is needed. The improved WEPP model can be used to evaluate the effectiveness of soil conservation practices on hydrology and erosion in further research.

Hydromulch roots reduce rill detachment capacity by overland flow in deforested hillslopes

Deforestation, which removes soil protection by plant leafs and roots, causes severe soil erosion, since the beneficial effects of plant cover and root actions on soil erodibility are lost. Hydromulching has been found to be a suitable technique for erosion control, but little research has been carried out to evaluate the effectiveness of hydromulch roots in controlling rill erosion in deforested areas due to overland flow. This study has evaluated rill detachment capacity (Dc) and erodibility (Kr, which is the slope of the equation regressing Dc and critical shear stress) in hydromulched and bare plots (1.3-m long and 0.5-m wide) on soils sampled in deforested hillslopes of Northern Iran; the most important root parameters of the grass used for hydromulching were also measured, such as root density and diameter. Dc has been measured in a laboratory flume under four bed slopes (10, 15, 20, and 25%) and five water discharges (0.26, 0.35, 0.45, 0.56, and 0.67 L m−1 s−1) with five replications per experiment. Dc was lower (on average − 44%, with a minimum reduction of −40% at a slope of 25% and a maximum −50% at a slope of 15%) in the hydromulched soils compared to the untreated plot. Dc was positively and negatively correlated, with diameter and density of hydromulch roots, respectively. Rill erodibility was noticeably lower (−81%) in the hydromulched soil compared to the bare plot. By regressing Dc on shear stress, rill erodibility and critical shear stress for deforested hillslopes (treated with hydromulching or left bare) were given. These parameters are useful to hydrologists in applications of physically-based erosion models.

Saturation effect on the distribution of rill detachment rate

AbstractDetachment rate in rill erosion is an important indicator of soil erosion speed and serves as the basis for estimation of soil erosion using the process‐oriented model. Rill detachment rates on saturated soil slopes were numerically calculated from the dataset of the rill erosion process. The datasets of sediment concentrations were obtained from laboratory experiments, along saturated soil rills 1, 2, 4 and 8 m under slope gradients of 5°, 10°, 15° and 20° and flow rates of 2, 4 and 8 L min−1, and were used to compute rill detachment rates through a numerical algorithm. The resulting rill detachment rates on saturated soil slopes ranged from 0.006 to 0.994 kg m−2 s−1. Exponential functions fitted well the rill detachment rates with rill lengths, whereas linear functions fitted well those with sediment concentrations. The rill detachment rate patterns of saturated soil slopes exhibited similar rill erosion–sediment concepts to those of non‐saturated soil slopes. However, saturated soil slopes were detached considerably faster than the non‐saturated soil slopes. Detachment rates of saturated soil rills were initially and considerably more rapid and decreased faster with rill length than those of non‐saturated soil rills. The saturated soil rills detached 1.627 to 2.246 times the sediments of the non‐saturated soil rills. This phenomenon indicates that faster rill detachments in saturated soil rills make sediment loads in water approach the sediment transport capacities faster than those in non‐saturated soil rills. The computed detachment rates and the functional relationships in this work can provide a basis for determining rill erosion model parameters on saturated soil slopes.Highlights Detachment rate of saturated rill can be computed similarly to non‐saturated rill. Rill detachment rate is well correlated to sediment concentration and rill length. Soil particles in saturated rill are detached considerably faster than non‐saturated rill.

Evaluating the effects of forest tree species on rill detachment capacity in a semi-arid environment

The beneficial effects of plant roots in decreasing soil detachment in forest ecosystems exposed to rill erosion are well known. However, these effects vary largely between different plant species. There has been lots of research into the relationship between root-soil systems and rill erodibility with a particular focus on grass species. Conversely, fewer studies are available for tree species, especially in forests of semi-arid or arid environments. Greater knowledge is therefore needed to identify the most effective tree species against rill erosion in these ecosystems, where water availability is the limiting factor for vegetation growth and afforestation is often the only solution to control erosion. To fill this gap, this study evaluates the rill detachment capacity of soils with four tree species (Parrotia persica, Carpinus betulus, Quercus castaneifolia, and Pinus taeda) in a semi-arid forest ecosystem in Northern Iran. These species are typical of these forests, but they also grow in other environmental contexts. The rill detachment was simulated in a laboratory flume at five slope gradients (1% to 5%) and five flow rates (0.22 to 0.69 L m−1 s−1) on soil samples with each of the tree species. The specific goal of the study was to evaluate which tree species with its specific root characteristics is most effective at reducing the rill detachment capacity. Moreover, simple prediction models are proposed to evaluate if it is possible to estimate the rill detachment capacity and rill erodibility (Kr) from the unit stream power for the investigated tree species. The soils with Parrotia persica and Carpinus betulus showed the lowest and the highest rill detachment capacity, respectively. The greater root system biomass of Parrotia persica could have played a binding effect on the soil, thus improving its aggregate stability thanks to the action of plant's root system. Based on these results, Parrotia persica is able to provide a higher soil protection capacity against erosion compared to the other tree species. A logarithmic function was accurate in predicting the rill detachment capacity from the unit stream power at water flow rates over 0.0025 m s−1. By a regression between the rill detachment capacity and the shear stress of the soil, rill erodibility and critical shear stress of soils were estimated for the four tree species; the rill erodibility and critical shear stress are important input parameters for physically-based erosion models. Overall, the results of this study can support land planners in the choice of tree species most indicated for soil conservation as well as in the extensive application of erosion prediction models.

Rill Erosion and Soil Quality in Forest and Deforested Ecosystems with Different Morphological Characteristics

Rill detachment capacity is a key parameter in concentrated flow erosion. Rill erosion generally turns into gully erosion with severe environmental impacts. Changes in land use and human activities can have heavy effects in rill formation, particularly in forests subject to deforestation; soil morphology plays a significant role in these effects. However, literature reports few studies about rill detachment rates and their implications on soil quality in forest and deforested soils with different morphological characteristics. To fill these gaps, this study has evaluated the rill detachment capacity (Dc) and the main soil quality indicators in three areas (upper, middle and lower slope) of forest and deforested (for 10 years) hillslopes exposed to the north and south in Northern Iran. The variations of Dc have been measured on soil samples under laboratory conditions through a flume experiment at three slope gradients (12 to 19%) and five flow rates (0.22 to 0.67 L m−1 s−1) with four replications. The large and significant (p &lt; 0.05) difference (about 70%) detected for Dc between forest and deforested hillslopes was associated to the higher organic matter content of forest areas; as a consequence, these areas also showed higher aggregate stability, porosity, root weight density, microbial respiration and available water. In the deforested hillslopes exposed to the south, the soil erodibility was higher by 12% compared to those exposed to the north. The differences in the monitored soil quality indicators were instead less noticeable and not always significant (p &lt; 0.05). Conversely, Dc did not significantly change (p &lt; 0.05) among the upper, middle and lower hillslope areas investigated in this study. Simple but accurate models to predict the rill detachment capacity, erodibility and critical shear stress of soils from indicators of soil quality or the unit stream power using regression equations are suggested. Overall, the results can support land planners in prioritizing the actions for soil conservation in deforested hillslopes exposed to the south as well as in the extensive application of the proposed equations in erosion prediction models.

Assessing and Modeling Soil Detachment Capacity by Overland Flow in Forest and Woodland of Northern Iran

Land use has significant effects on the erosion process, since it influences the soil detachment capacity by causing an overland flow (Dc). The effects of different land uses on the rill detachment capacity have not been explained in depth, and the hydraulic parameters providing accurate estimates of this soil property have not been completely identified. This study quantifies Dc at low flow rates in woodland and forestland, compared to two other land uses (cropland and grassland), in the Saravan watershed (Northern Iran), and develops prediction models of Dc and rill erodibility (Kr). Dc was measured on undisturbed soil samples, collected in the four land uses, and characterized in terms of the main physico-chemical properties in a flume experiment, simulating five slopes and five shallow water flows. The results showed that Dc was significantly lower in woodland and forestland compared to cropland and grassland, as the consequence of the changes in the main soil properties and the more developed vegetation cover and structure. Dc was positively correlated to clay and silt contents of soils, and negatively correlated to sand content, aggregate stability, root density, and organic matter. The stream power and unit stream power were found to be very accurate predictors of Dc in woodland and forestland, respectively. Kr values, which assumed the lowest values in woodland and forestland, were provided by interpolating Dc and the shear stress of water flow. Overall, this study has confirmed that vegetation cover and improved soil properties in forestland and woodland may help to reduce erosion in delicate environment ecosystems, such as the forests of Northern Iran.

Quantifying effects of root systems of planted and natural vegetation on rill detachment and erodibility of a loessial soil

The objectives of this study were to evaluate different root parameters for estimating rill detachment, and to compare and quantify the effectiveness of planted and natural vegetation on soil physical characteristics and in reducing rill detachment and erodibility. Laboratory-concentrated-flow flume tests were performed under a flow discharge of 3 L min−1 and a slope of 15° with intact soil samples. A silt loam soil (Calcic Cambisol, in Loess Plateau in China) was sampled in 2015 from laboratory with planted ryegrass (Lolium perenne L.) and in 2014 from natural vegetation fields under 12 to 36-a restoration. The results indicated that root length density was the best parameter for estimating the effects of roots on rill detachment and erodibility for both planted ryegrass and natural vegetation, with exponential reduction equations. The exponents were more negative for natural vegetation (−0.411) than for planted species (−0.217) because of the existence of more complex root networks under more diverse natural vegetation. Rill erodibility of soil samples with roots decreased exponentially from the baseline erodibility of the root-free soil as RLD increased. Root systems of the restored natural vegetation ameliorated soil characteristics (bulk density, aggregate stability and organic matter content) more profoundly than those of the laboratory tests due to stronger bonding and binding effects, resulting in a lower absolute soil detachment rate for a given root density. There existed a vertical decay distribution pattern of root densities with soil depth for natural vegetation and planted ryegrass, and this distribution of root systems reached a steady state after 12 years natural vegetation restoration. More vegetation types under more different hydraulic conditions should be evaluated in future studies to provide better scientific bases for modelling their impacts on rill detachment and erodibility.

Impacts of sediment load and size on rill detachment under low flow discharges

Rigorous validation of the first-order detachment-transport coupling concept using different sediment sizes is nonexistent in the literature. We hypothesized that particle sizes in a sediment-laden flow can affect soil detachment. The objectives of this study were to investigate the relationship between detachment rates by rill flows and hydraulic parameters under low flow discharges, and to quantify the responses of soil detachment rates to sediment loads and median sizes. A concentrated flow flume test was conducted under three low flow discharges (0.00033, 0.00067 and 0.00100 m2·s−1) and three sediment concentrations (50, 100 and 200 kg·m−3) with five median sizes (0.233, 0.204, 0.169, 0.115 and 0.078 mm). The results showed that the relationship between soil detachment capacity and shear stress fits the Water Erosion Prediction Project (WEPP) model well (R2 = 0.89). Stream power, compared with shear stress, is a better hydraulic parameter in simulating soil detachment rates under different sediment concentrations. The soil detachment rates by sediment-laden rill flows were negatively related to both sediment loads and sediment median sizes, validating the first-order sediment feedback relationships used in the WEPP model. The overall responses of soil detachment rates to sediment loads and sediment median sizes followed multivariate linear relationships, further demonstrating the negative feedbacks of sediment load and size on soil detachment by rill flow.

Understanding erosion processes using rare earth element tracers in a preformed interrill-rill system

Tracking sediment source and movement is essential to fully understanding soil erosion processes. The objectives of this study were to identify dominant erosion process and to characterize the effects of upslope interrill erosion on downslope interrill and rill erosion in a preformed interrill-rill system. A coarse textured soil with 2% clay and 20% silt was packed into a physical model of a scaled small watershed, which was divided into eight topographic units and was tagged with eight rare earth element (REE) oxides. Three 30-min rains were made at the sequential intensities of 60, 90, and 120mmh−1, and runoff and sediment were collected every 2min at the outlet. REE concentration in sediment was measured and used to estimate source contributions after fine-enrichment correction. Results showed that interrill erosion rate and sediment concentration increased with downslope distance, indicating that sediment transport might have controlled interrill erosion rates. In contrast, rill erosion rate was limited by rill detachment and development process. Rill erosion contributed most soil loss; however, the proportion decreased from 78 to 61% as rainfall intensity increased and rill network matured over three rains. Interrill erosion was more sensitive than rill erosion to rainfall intensity increases. The former was mostly affected by rainfall intensity in this experimental setup, while the latter was controlled by flow discharge, gradient, and rill evolution stage. The greatest sediment concentration and delivery rate occurred in the stage of the fastest rill development. The increased sediment delivery from interrill areas appeared to suppress rill detachment by concentrated flow. This study enhanced our understanding of interrill and rill erosion processes and provided the scientific insights for improving soil erosion models.