Process-based Erosion Models Research Articles

THE EFFECT OF WAVE OBLIQUITY ON DUNE EROSION: A FIELD EXPERIMENT

Storm conditions can lead to excessive dune erosion with potential floods as a consequence. Barrier islands and low-lying countries protected by dunes are especially vulnerable to dune erosion. To properly assess the risks these areas face, a clear understanding of the physical processes during dune erosion is required. One of such processes is the effect of wave obliquity on sediment transport in the surf zone. Classic dune erosion models assume that dune erosion volumes decrease under oblique wave attack, because the time-averaged cross-shore undertow decreases in magnitude and with that offshore directed sediment transport decreases (Steetzel, 1993). More recent process-based erosion models predict an increase in erosion quantities, because the generated longshore currents increase surf zone sediment concentrations, and with that offshore directed sediment transport increases (Den Heijer, 2013). The main objective of this study is to analyse the effect of wave obliquity on dune erosion through a field experiment, by quantifying the effect of the decreasing undertow but increasing alongshore current on sediment concentrations in the surf zone.

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.

Interactive effects of rainfall intensity, kinetic energy and antecedent soil moisture regime on splash erosion in the Ultisol region of South China

Splash erosion has been acknowledged as a key process of water erosion, depending on multiple factors including rainfall and soil properties, antecedent soil moisture, and surface runoff. Previous studies mostly focused on the effect of simple factors on splash erosion, but the interactive effects of multiple factors on splash erosion are still obscure. Therefore, we investigated the characteristics of splash erosion under simulated rainfall and quantified the interactive effects of multiple factors on splash erosion using a modified splash pan for measuring directional splash erosion. The results showed that total splash erosion (summation of four directional splash erosion) and net splash erosion (downslope minus upslope splash erosion) ranged from 14.4 to 804.8 g m−2 and 2.0 to 186.5 g m−2, respectively, and both increased with rainfall intensity and rainfall kinetic energy. The splash erosion rates in the dry run were greater than those in the wet run at the earlier stage, whereas after runoff occurred, the splash erosion rates in the dry run were lower than those in the wet run. Furthermore, the calculated variance contributions indicated that before runoff occurrence, rainfall kinetic energy (KE) and antecedent soil moisture (θa) were the dominant factors affecting splash erosion, accounting for 25.3 % and 15.0 % of variance contributions, respectively. After runoff occurrence, the main factors affecting splash erosion were rainfall intensity (RI), rainfall kinetic energy (KE) and their interactions, explaining 98.2 % of the total splash and 95.7 % of the net splash for all variance contributions. In addition, the effect of shallow flow on splash erosion was dependent on the ratio of flow depth and raindrop diameter (h/D). The thresholds of h/D seemingly ranged from 0.63 to 0.86 and 0.77 to 1.02 for the dry and wet runs under 100 mm h−1 rainfall intensity. This study provides a basis for understanding splash erosion mechanisms and improving process-based erosion models.

Adapting the WEPP Hillslope Model and the TLS Technology to Predict Unpaved Road Soil Erosion.

Unpaved road erosion have been recognized as important sediment sources in a watershed. To evaluate where and when road erosion occurs, the soil loss along road segments should be precisely predicted with process-based erosion models. Methods: The hillslope version of the Water Erosion Prediction Project (WEPP) was used to estimate soil loss from 20 typical road segments in the red soil region of South China. Terrestrial laser scanning (TLS)-measured soil losses were used to validate the model simulations. The results showed that the WEPP model could reasonably predict the total soil loss in relatively short (less than 100 m) and gentle (slope gradient lower than 10%) road segments. In contrast, soil loss would be underestimated for long or steep road segments. Detailed outputs along roads revealed that most of the peak soil loss rates were underestimated. It might due to the linear critical shear stress theory in the WEPP model. Additionally, the lack of upstream flow was found to be connected to the relatively low model efficiency. Nevertheless, the WEPP simulation could accurately fit erosion trend and predict the peak soil loss positions along road segments. Conclusions: The WEPP model could be adopted to evaluate the erosion risk of unpaved roads in the red soil region of South China.

Soil detachment capacity by rill flow for five typical loess soils on the Loess Plateau of China

The loess region of China is one of the most heavily eroded areas in the world. Soil detachment capacity by rill flow (Dc) is a key parameter for quantifying intensity of rill erosion in many process-based erosion models. However, only a limited number of studies have been devoted to soil detachment capacity for the various types of loess soil such as is found on the Loess Plateau, where there is variation from south to north and in terms of soil particle size composition. The objectives of this study were (1) to discriminate differences in soil detachment capacity by rill flow (Dc) among five loess soils, (2) to investigate the relationship between Dc and hydrodynamic parameters, and the relationship between Dc and soil properties, and (3) to establish an equation to model soil detachment capacity by rill flow for the loess region. Soil detachment capacity by rill flow for five typical loess soils found on the Loess Plateau of China was investigated through a flume experiment by varying five flow discharges and five slope gradients. The results show that Dc of SM sandy loess is the largest with a mean of 2.2145 kg m−2 s−1, followed by YL clay loess, DB sandy loess, AS loess, and CW loess. Stream power is the best hydrodynamic parameter to describe the dynamic process of soil detachment capacity by rill flow for these five loess soils. Soil detachment capacity by rill flow was negatively correlated with soil cohesion and effective silt content (P < 0.05), while it was positively correlated with effective median soil particle size (P < 0.01) and effective sand content (P < 0.05). Soil detachment capacity by rill flow for various hydraulic and soil conditions in the loess region could be modeled using a quaternary power function of slope gradient, flow discharge, soil cohesion and effective median particle size (NSE = 0.96), or it could be modeled by a ternary power function which calculates the variation of soil detachment capacity with stream power, soil cohesion and effective median size (NSE = 0.96). The results of this study reveal the mechanism of soil detachment by rill flow and advance development of a physically-based rill erosion model. Future research should focus on the impact of effective particle size on Dc to ensure a full understanding of soil erosion processes.

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.

The Effects of Tillage Practices on Interrill Erosion in the Red Soil Region of China

HighlightsConservation practices (straw mulch, manure, peanut-orange intercrop) reduced soil and water loss vs. tillage.Reductions in soil loss were greatest under high rainfall intensity or low soil moisture.Measured interrill erodibility was relatively high in dry run tests and decreased with the final infiltration rate.ABSTRACT.Tillage practices have been adopted extensively to control runoff and soil loss from croplands. To quantify the effects of tillage practices on erosion processes in interrill areas, microplot rainfall simulation experiments were conducted in peanut fields with five tillage practices in the red soil region of China. The results showed that conservation tillage practices, including straw mulch cover (PC), manure fertilizer (PM), and peanut-orange intercropping (PO), could significantly conserve water and reduce soil loss rates by more than 50% compared with traditional furrow (PF) and peanut-radish rotation (PR) treatments. The variations in the hydrologic and soil loss indexes among the treatments were larger in the dry run tests than in the wet run tests. Accordingly, the efficiencies of the conservation practices in reducing soil loss were reflected mainly under a high rainfall intensity or low soil moisture in this study. Additionally, the runoff generation indexes were significantly correlated with the soil loss rate in all five treatments. The measured interrill erodibility was highest in the PR treatment (944,124 kg s-1 m-4) and lowest in the PO and PM treatments (571,797 and 584,238 kg s-1 m-4, respectively), as the organic matter content and aggregate stability were relatively high in the two conservation treatments. Influenced by the dynamics of loose materials and possible surface sealing, the interrill erodibility was relatively high and decreased with the final infiltration rate when measured under the dry run conditions. These results could guide cropland soil management and improve process-based erosion models. Keywords: Interrill erodibility, Interrill erosion, Rainfall simulation, Red soil region, Tillage practices.

Soil detachment capacity of shallow overland flow in Earth-Rocky Mountain Area of Southwest China

In process-based erosion models, soil detachment capacity (Dc) is a pivotal parameter that is typically estimated by hydraulic parameters. Purple soil in Earth-Rocky Mountain Area of Southwest China is a unique soil resource with characteristics including fast physical weathering, soft texture, and rich nutrient reserves. However, the prediction accuracy of Dc and its relationship with certain hydraulic parameters remains poorly understood for purple soil, which is defined as an Entisol in the US Soil Taxonomy system. The objective of this study was to investigate the relationship between Dc of shallow overland flow and hydraulic parameters to establish an accurate model for purple soil. A series of 825 flume experiments were conducted in a 12 m × 0.3 m hydraulic flume. Flow discharge was 8.33 × 10−3 to 41.67 × 10−3 m2 s−1, slope gradient was 8.7% to 39.1%, and rill length was 0.5 m to 10 m. Results showed that flow discharge and slope, stream power, and effective stream power were effective predictors of Dc. However, flow depth, flow velocity, shear stress and unit stream power were relatively poor predictors. Stream power was the optimal parameter for predicting the Dc of shallow overland flow. Flow discharge, slope and effective stream power were also useful parameters for predicting Dc. Linear equation between Dc and stream power was the optimal model for predicting Dc, which was recommended when flow depth and flow velocity data were available; however, power equation among Dc, flow discharge and slope also showed good performance in predicting Dc, which was recommended when only flow discharge and slope data were available. The results of this study provide theoretical guidance for predicting sediment yield in Earth-Rocky Mountain Area of Southwest China as well as a decision-making basis for controlling soil and water loss in Earth-Rocky Mountain Area of Southwest China.

Process-based soil erodibility estimation for empirical water erosion models

ABSTRACTFunctional relationships between soil erodibility equations of empirically-based revised universal soil loss equation and process-based water erosion prediction project models were investigated using new datasets from rainfall simulation experiments to overcome conceptual differences of models in estimating soil erodibility. Erodibility potentials of two different soils were quantified for three different initial soil moisture conditions, and relations between the process-based erodibility, partitioned as interrill erodibility, rill erodibility and critical shear stress, and empirically-based erodibility were examined. A process-based soil erodibility assessment within the universal soil loss equation was attempted. Statistically significant differences are found when considering the effects of surface hydrologic conditions on soil erodibility. Process-based soil erodibility estimates under dry conditions were found to be comparable with original water erosion prediction project datasets. The results showed that procedure could be useful for tapping into the large number of datasets available and building the next generation of process-based erosion models.

Gaining insights into interrill erosion processes using rare earth element tracers

Increasing interest in developing process-based erosion models requires better understanding of the relationships among soil detachment, transportation, and deposition. The objectives are to 1) identify the limiting process between soil detachment and sediment transport for interrill erosion, 2) understand the dynamic relationships between transport and deposition by tracking sediment fate with multiple tracers, and 3) verify the effects of slope length on interrill soil erosion. Five rare earth element (REE) tracers were applied in five bands or segments to track sediment dynamics on a 10%, 4×4-m long uniform slope under simulated rainfall. A silt loam soil with 8% clay and 87% silt was used. Six rain events were applied, the first four with 60mmh−1 and the last two with 90mmh−1. Sediment in runoff and deposition along the slope were measured. Results confirmed that interrill soil erosion was controlled by the transport process. Thus soil erodibility estimated with many interrill erosion models was actually sediment transportability. As the slope length increased, soil erosion rates initially increased and then decreased, indicating that the upper section was dominated by erosion and the lower section by transport. Raindrop-driven creeping prevailed in the upper section while flow-driven rolling dominated in the lower section. Sediment influx from upslope clearly suppressed soil detachment downslope. Moreover, the steady state sediment discharge from a segment was positively correlated to the amount of sediment deposited downslope from that segment, implying that 1) re-detachment, transport, and deposition occurred simultaneously in the system and 2) sediment was transported in a form of bedload, mainly rolling or creeping on interrill areas. Sediment delivery ratios of each segment tended to slightly increase downslope, suggesting the flow-driven transport was more efficient than the raindrop-driven transport. The sequence of sediment discharged from different slope positions was inconsistent with the transport-distance theory. However, interrill erosion processes vary with soil, slope, and rainfall properties, and the findings here need to be verified in different conditions.

The abrasion of soil aggregate under different artificial rough beds in overland flow

During slope erosion, the travel and abrasion of aggregate during transport may produce finer and more transportable particles, causing a significant effect on the sediment transport capacity and erosion intensity. Little information is available on the effect of roughness on aggregate abrasion in overland flow. This study investigated the potential effects of slope gradient, flow discharge, and roughness on aggregate abrasion and hydraulic characteristics in overland flow. These factors were studied with a series of aggregate abrasion experiments in a 3.8m long flume at four slope gradients (8.7%, 17.6%, 26.8%, 36.4%), five discharges (0.4Ls−1, 0.6Ls−1, 0.8Ls−1, 1.0Ls−1, 1.2Ls−1), and five surfaces of artificial rough beds. The results indicated that the different roughness conditions (with contributions of 63.5%) had obvious effects on the abrasion of aggregate compared to the slope gradient and discharge. Regardless of the discharge and slope gradient, the abrasion degree of aggregate gradually increased as the roughness increased. The changes in the abrasion degree of the aggregate were not the same at the different combinations of discharge and slope for each artificial rough bed. Flow depth can well explain the extent of abrasion of the aggregate for the single fixed rough bed. However, the abrasion degree of the aggregate had no significant variation with increases in the flow depth from all the data of the five artificial rough beds. The ratio between the residual weight and initial weight of soil aggregates (Wr/Wi) decreased in a power function with increase in the friction factor and shear stress from all the data of the five artificial rough beds. The friction factor and shear stress should be appropriate indicators to reflect aggregate abrasion under various roughness conditions in overland flow. The information of the analysis of the abrasion of soil aggregate under different artificial rough beds can be useful for the development of soil process-based erosion models.

Modelling sediment transport capacity of rill flow for loess sediments on steep slopes

Sediment transport is an important aspect of soil erosion, and sediment transport capacity (Tc) is a key to establishing process-based erosion models. A lot of studies exist that have determined Tc for overland flow, however, few studies have been conducted to determine Tc for loess sediments on steep slopes. Experimental data for this region are thus needed. The objectives of this study are to formulate new equations to describe Tc and evaluate the suitability of these equations for loess sediments on steep slopes. The slope gradients in this study ranged from 10.51% to 38.39%, and flow discharges per unit width varied from 1.11×10−3m2s−1 to 3.78×10−3m2s−1. Results showed that Tc increased as a power function with flow discharge and slope gradient, with R2=0.99 and Nash–Sutcliffe model efficiency (NSE)=0.99. Tc was more sensitive to flow discharge than slope gradient. Tc increased as a power function with mean flow velocity, which was satisfied to predict Tc with R2=0.99 and NSE=0.99. Shear stress (R2=0.89, NSE=0.88) was also a good predictor of Tc, and stream power (R2=0.96, NSE=0.96) was a better predictor of Tc than shear stress. However, unit stream power was not a good predictor to estimate Tc in our study, with R2=0.63 and NSE=0.62. These findings offer a new approach for predicting Tc for loess sediments on steep slopes.

A study of initial motion of soil aggregates in comparison with sand particles of various sizes

Critical shear stress and threshold stream power are two important soil characteristics controlling detachment of soil particles by runoff and have been used in process-based erosion models such as WEPP, GUEST and EUROSEM. In this research, laboratory experiments were conducted in a 20×350cm flume to study the effects of particle size and density on initial motion. Two contrasting soil samples, a well-aggregated forest soil and non-cohesive fluvial sand, were used to provide particles with different densities. Each sample was divided into six size classes. Flow bed in the flume was roughed according to testing area for each size class using a plate which sand particles from each size class were glued on it. The initial motion of the particles was determined by two methods. In the first method, slope was increased gradually for a given constant discharge until particles start to move from every point of the testing area. In the second method, flume slope was set to a given steepness and discharge was gradually increased until particles start to move. Three different discharges and three slopes were tested in the first and second methods, respectively. Each test replicated two times. Analysis of the data showed that the particle size and density and also their interaction significantly affect (P<0.001) critical shear stress and threshold stream power. The critical shear stress and threshold stream power increased with increasing particle size and density, but the impact of particle density is higher on the coarser particles than the finer ones. Threshold values measured for the sand particles were about 2.3 times of those measured for soil particles in the three coarser classes, this difference decreased to about 65% (1.65 times) in the three finer classes, and even the difference between the two types of particles was not significant for the finest class (0.125–0.053).

Improved Hillslope Erosion Module for the Digital Yellow-River Model

AbstractSevere soil erosion in the Loess Plateau region and associated large sediment discharge downstream in China have been of great concern for years. A physically based distributed model, the Digital Yellow River model (DYRIM), was developed in 2007 for land management at the basin scale in this region. In DYRIM, an explicit sediment yield equation was derived based on simple assumptions where sediment discharge would increase nonlinearly with the slope length, leading to unrealistic results as the spatial scale increases. The main aims of this study were to reformulate the module of DYRIM in comparison with other process-based erosion models; to develop a generic framework for erosion prediction and simulation of erosion and sediment delivery processes within storm events; to validate the new hillslope erosion module for improved performance across a range of spatial scales; and to evaluate parameter consistency among different runoff events. Observed flow and sediment discharge data for 16 runoff ev...

Soil Detachment Capacity by Overland Flow for Soils of the Beijing Region

Soil type may influence soil detachment process by overland flow, but few studies have quantified the effect fully and systematically. This study was undertaken to quantify the effects of soil type on soil detachment capacity using undisturbed soil samples collected from 11 soil types from the Beijing Region. The soil samples were placed in a 5.0-m long and 0.38-m wide hydraulic flume and eroded by overland flow under three slope gradients (17.4%–34.2%) and three unit discharges (1.32–5.26 × 10−3 m2 sec−1). The results showed that soil detachment capacities were significantly affected by soil types. Aeolian Sandy Soil was the most easily detached, whereas Wet Meadow Soil was the hardest. Soil detachment capacity was significantly affected by soil properties. For soil texture, loamy sand had the largest detachment capacity, followed by sandy loam, loam, and silt loam. Furthermore, detachment capacity was negatively correlated to clay and silt content and positively related to sand content and median diameter of soil particles. Both shear strength and organic matter content were negatively correlated to detachment capacity. Detachment capacity could be predicted reasonably well by stream power, clay content, and organic matter content (r2 = 0.743) with a coefficient of Nash-Sutcliffe efficiency of 0.644. An equation was developed to estimate detachment capacity based on stream power and soil properties (r2 = 0.704; Nash-Sutcliffe efficiency, 0.702). Rill erodibility ranged from 0.73 to 85.22 × 10−3 sec m−1, and the critical shear stress changed from 0.683 to 7.978 Pa. The results are helpful to understand the mechanism of soil detachment process under different soil types and to develop the process-based erosion models.

Particle size and shape variation of Ultisol aggregates affected by abrasion under different transport distances in overland flow

Particle size variation or distribution is a recognized factor influencing crust characteristics, water infiltration and sediment transport during slope erosion. Limited studies have been conducted about particle size and shape variation resulting from the abrasion of soil aggregate in overland flow. This experiment was based on aggregate abrasion in a 3.8m long flume with a fixed bed, and the particle size and shape variations were studied using dry sieving method and a Laser particle size and shape analyzer. Results indicated that the high abrasion rate of large particles (particles larger than 5mm) caused the mass percentage of particles in intermediate size ranges (2mm–1mm, 1mm–0.5mm and 0.5mm–0.25mm) increased with transport distance. The D50 values of Average Feret Diameter of smaller abraded aggregates (0.0385mm–0.25mm) increased first and then decreased, and their shape became more regular and rounder with the transport distance. There were significant negative correlations between RMI (relative mechanical breakdown index) of initial aggregates with MWD (Mean weight diameter) for particles larger than 0.25mm, and with Cirm (median value of Circularity Factor) for particles smaller than 0.25mm in all the five transport distances (p<0.05). These indicated that the initial aggregates had low susceptibility to mechanical breakdown, resulting in weak abrasion of particles larger than 0.25mm and regular shape of particles smaller than 0.25mm under certain transport distance in overland flow. A multiple regression equation was established, relating Cirm to RMI and transport distance (x) for estimating the particle shape of smaller abraded aggregates in overland flow. The information of the analysis of particle size and shape variation can be useful for the development of soil process-based erosion models.

Evaluation of GUEST and WEPP with a new approach for the determination of sediment transport capacity

Summary Sediment transport capacity (Tc) is a key parameter for all process-based erosion models. The objective of this study was to develop a new approach for accurate determination of Tc under infiltration conditions based on rill morphology and the sediment feedback relationship. This would allow the way in which sediment transport capacity is computed in WEPP and GUEST models. Six-m long rills in three soils were subjected to constant inflow rates (50, 75 and 122 ml s−1) and three flume slope gradients (2%, 4% and 6%). Cross-sections of rills were measured precisely at 0.5 m intervals using a profile-meter prior to and after each experiment. The cumulative net rates of erosion at the end of each section were converted to sediment load along the rill length. The sediment feedback relationship was applied to fit the sediment load values and Tc was determined asymptotically under equilibrium conditions. The results showed that under infiltration conditions, both models tended to under-estimate Tc, however, Tc predicted using the GUEST model had better agreement with the measured Tc values than that predicted with WEPP. Using the proposed value of F = 0.15, the performance of GUEST in predicting Tc could be further improved. Sidewall slumping was found to be a plausible explanation for underestimation of Tc by WEPP, which is not explicitly modeled. In contrast, GUEST takes into account any observed changes in rill morphology. The results also indicated that Tc increased as a function of flow discharge and slope gradient, but was more sensitive to slope gradient than to flow discharge. Moreover, stream power was found to be a better predictor of Tc than shear stress. Yet, regression equations using unit discharge and slope gradient as independent variables, did not achieve any better estimation of Tc than did the GUEST (when F = 0.15). Accurate determination of parameter(s) used in a process-based model such as GUEST will lead to improved Tc estimations. Furthermore, Tc is a process-based parameter which should not ideally be estimated with regression equations.

Temporal variability in rill erodibility for two types of grasslands

The temporal variability in rill erodibility (Kr) and its influencing factors are not fully quantified in grasslands. This study was conducted to detect temporal variation and quantify the potential factors causing changes in rill erodibility by using natural, undisturbed soil samples collected from two grasslands and one bare soil near Beijing, China. Sampling was at ~20-day intervals from April to October 2011. Soil detachment capacity by concentrated flow was measured in a hydraulic flume with the fixed bed under six different flow shear stresses to determine rill erodibility. Root mass density was measured to analyse potential effects on temporal variability in rill erodibility. Mean rill erodibility of bare soil was 13.2 and 19.6 times greater than under switchgrass (Panicum virgatum) and smooth bromegrass (Bromus inermis). The temporal variability in rill erodibility under grasslands differed significantly from that of bare soil. Distinctive temporal variation patterns were found throughout the growing season. Rill erodibility declined as root density increased, and the rill erodibility of grassland could be well estimated from the measured erodibility of bare soil and root density (R2 ≥ 0.92). The results of this study aid understanding of soil erosion mechanisms and development of process-based erosion models to simulate the seasonal variation in soil detachment by concentrated flow for grassland.

Effects of sediment size on transport capacity of overland flow on steep slopes

Sediment transport capacity is a key concept in determining rates of detachment and deposition in process-based erosion models, yet limited studies have been conducted on steep slopes. We investigated the effects of sediment size on transport capacity of overland flow in a flume. Unit flow discharge ranged from 0.66 to 5.26 × 10-3 m2 s-1, and slope gradient varied from 8.7 to 42.3%. Five sediment size classes (median diameter, d 50, of 0.10, 0.22, 0.41, 0.69 and 1.16 mm) were used. Sediment size was inversely related to transport capacity. The ratios of average transport capacity of the finest class to those of the 0.22, 0.41, 0.69 and 1.16 mm classes were 1.09, 1.30, 1.55 and 1.92, respectively. Sediment transport capacity increased as a power function of flow discharge and slope gradient (R2 = 0.98), shear stress (R2 = 0.95), stream power (R2 = 0.94), or unit stream power (R2 = 0.76). Transport capacity generally decreased as a power function of sediment size (exponent = −0.35). Shear stress and stream power predicted transport capacity better than unit stream power on steep slopes when transport capacity was <7 kg m-1 s-1. Sediment transport capacity increased linearly with mean flow velocity. Critical or threshold velocity increased as a power function of sediment size (R2 = 0.93). Further studies with fine soil particles are needed to quantify the effects of sediment size on transport capacity of overland flow on steep slopes. Citation Zhang, G.-H., Wang, L.-L., Tang, K.-M., Luo, R.-T. & Zhang, X.C. (2011) Effects of sediment size on transport capacity of overland flow on steep slopes. Hydrol. Sci. J. 56(7), 1289–1299.

A transport‐distance approach to scaling erosion rates: 2. sensitivity and evaluation of Mahleran

AbstractIn the first paper in this series, we demonstrated that most process‐based erosion models have a series of in‐built assumptions that led us to question their true process basis. An alternative soil‐erosion model (Mahleran – Model for Assessing Hillslope‐Landscape Erosion, Runoff And Nutrients) based upon particle‐travel distance has been presented in the first paper in this series and this paper presents the first of two evaluations of the model. Here, a sensitivity analysis shows that the numerical model is consistent with the analytical model of Parsons et al. (2004) and demonstrates that downslope patterns of sediment flux on hillslopes are a complex interaction of rainfall intensity, duration and pattern; hillslope gradient; surface roughness and sediment size. This result indicates that the spatial scaling of sediment transfers on hillslopes is a non‐trivial problem and will vary from point to point and from event to event and thus from year to year. The model is evaluated against field data from a rainfall‐simulation experiment on an 18 m × 35 m plot for which there are sub‐plot‐scale data on runoff hydraulics and sediment flux. The results show that the model is capable of reproducing the sedigraph with an overall normalized root‐mean‐square error of 18·4% and Nash–Sutcliffe efficiency of 0·90. Spatial and temporal patterns of particle‐size distributions of the eroded sediment are also reproduced very well, once erosion parameters have been optimized for the specific soil conditions. Copyright © 2008 John Wiley &amp; Sons, Ltd.