Journal of Hydrology

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1 Journal of Hydrology (2012) Contents lists available at SciVerse ScienceDirect Journal of Hydrology journal homepage: Soil erosion processes and sediment sorting associated with transport mechanisms on steep slopes Z.H. Shi a,b,, N.F. Fang a, F.Z. Wu b, L. Wang a, B.J. Yue a, G.L. Wu a a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling, Shanxi , China b College of Resources and Environment, Huazhong Agricultural University, Wuhan , China article info summary Article history: Received 24 February 2012 Received in revised form 30 May 2012 Accepted 4 June 2012 Available online 15 June 2012 This manuscript was handled by Konstantine P. Georgakakos, Editor-in-Chief, with the assistance of Ana P. Barros, Associate Editor Keywords: Erosion processes Rainfall simulation Sediment size Suspension saltation Rolling Sediment size distribution greatly affects sediment transport and deposition. A better understanding of sediment sorting will improve understanding of erosion and sedimentation processes, which in turn will improve erosion modeling. To address this issue, a total of 12 rainfall simulation experiments were conducted in a 1 m by 5 m box with varying steep slopes (10, 15, 20 and 25 ), and the simulated rainfall lasted for 1 h at a rate of 90 mm h 1. For each simulated event, runoff and sediment were sampled at 3-min intervals, which were performed to study in detail the temporal change in size distribution of the eroded materials. These data were used to interpret the real-time sequence of transport mechanisms acting in response to the simulated rainfall. Total soil loss is the sum of suspended, saltating and contact loads. The proportion of sediment <0.002 mm showed little temporal fluctuation (generally 12 14%), although it was highly correlated to instantaneous rain power (R 2 = 0.452, P < 0.01, n = 120). Suspension saltation transports the finer than mm size sediment was the most important erosion mechanism during interrill erosion processes. However, after rill development on hillslopes, bed-load transport by rolling of medium to large-sized sediment particles (coarser than mm) became an increasingly important transport mechanism, and it were also enhanced by increased slope. Overall, the study supports a strong relationship between the sediment transport of contact (rolling) load and stream power. The partition of soil loss into these more meaningful components appears to be essential both for initial data interpretation and for subsequent use of such data for soil loss prediction. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Soil erosion by water involves the processes of detachment, transport and deposition of soil materials by the erosive forces of raindrops and runoff. There is a need to describe sediment sorting to characterize the on-site and off-site effects of soil erosion. A better understanding of sediment sorting will improve understanding of erosion and sedimentation processes, which in turn will improve erosion modeling (Asadi et al., 2011). Sediment may serve as the delivery mechanism for soil nutrients and contaminants to streams. Nutrient distribution is non-uniform over different sizes of sediment particles; fine sediments are usually richer in soil-sorbed nutrients than are coarse sediments (Palis et al., 1990). Knowledge of sediment sorting and its dynamics can also provide the basis for understanding and modeling the transfer of nutrients and contaminants from hillslopes to water bodies (Teixeira and Misra, 1997). Soil erosion by water is commonly divided into rill and interrill components, depending on the source of eroded sediment (Meyer Corresponding author at: College of Resources and Environment, Huazhong Agricultural University, Wuhan , China. Tel.: address: shizhihua70@gmail.com (Z.H. Shi). and Wischmeier, 1969; Laflen et al., 1991). Sediment leaving an eroding area is a combination of primary soil particles (sand, silt and clay) and secondary or aggregated soil material (Mitchell et al., 1983). The general agreement is that interrill erosion results in selective removal of fine particles, whereas rill erosion is less selective (or nonselective) after a specific critical flow shear stress is exceeded (Proffitt and Rose, 1991; Durnford and King, 1993; Wan and El-Swaify, 1998; Malam Issa et al., 2006). The selective transport of fine sediment produced by interrill erosion has been attributed to the insufficient ability of interrill overland flow to transport large detached particles (Parsons et al., 1991) or to the selective deposition of coarse sediment (Proffitt and Rose, 1991). Even for this general observation, though, conflicting reports exist in the literature regarding sediment sorting. Young and Onstad (1978) and Meyer et al. (1992) found that sediment transported by interrill erosion was coarser than the in situ soils and the rill sediment. It has also been noted that both the concentration and size distribution of sediment can change dynamically during rainfall-driven erosion (Hairsine et al., 1999). Many previous studies have reported that eroded materials are enriched in clay and silt-sized particles relative to the original soil where the erosion event commenced. The eroded materials gradually become coarser /$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

2 124 Z.H. Shi et al. / Journal of Hydrology (2012) over time, and at steady-state, its composition becomes very similar to that of the original soil (Asadi et al., 2011). Loch and Donnollan (1983) and Asadi et al. (2007b) found that the mass fraction of different sediment sizes is distributed bimodally. They concluded a bimodal distribution of sediment resulted from the different transport mechanisms of suspension, saltation and rolling, which each act predominantly on particles of different size classes. There are still some conflicting and unexplained results regarding sediment sorting during erosion processes. Sediment size distributions appear to depend on many factors such as rainfall characteristics, vegetation cover, hydraulic flow type (sheet or rill), soil properties and slope. More information on sediment sorting and its dynamics is needed to better understand the behavior and interaction of the different components that make up erosion processes. Approximately 800 million people worldwide depend directly on steeplands for their sustenance (Drees et al. 2003). Knowledge of the predominant erosion mechanisms that occur under steep slope conditions is essential if conservation measures are to be properly planned. The literature reveals that slopes in erosion experiments are usually less than 25%, and many studies have only evaluated the effects of slope steepness on wash loss from short and unrilled slopes (Fu et al., 2011). To address these issues, a clay loam soil in the Loess Plateau was chosen in this study. The temporal change in size distribution of the exiting sediment resulting from erosion under simulated rainfall was measured in detail, and these data were used to interpret likely transport mechanisms. 2. Materials and methods 2.1. Experiment facilities The experiments were conducted under simulated rainfall at the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau. Rainfall intensities were adjusted through nozzle sizes and water pressure. Calibrations of rainfall intensities were conducted prior to the experiments. Experimental plots were constructed with metal sheets of 5 m (length) 1 m (width) 0.5 m (depth). The plots were placed on movable platforms for ease of transport. A metal runoff collector was set at the bottom of each plot to direct runoff into a container. The plot was electronically adjusted to a desired slope between 0 and 30. The soil used in the experiments was a clay loam soil collected from Yangling in Shanxi Province, China. The natural consolidated soil has a bulk density between 1.2 and 1.4 g/cm 3, with an organic matter content of 0.6%. The soil texture information is listed in Table Rainfall simulation experiments Soil samples were air dried, crushed to pass through a 10.0-mm sieve and mixed thoroughly. The soil was packed 30 cm deep in each plot (in three 10-cm layers) to achieve a g cm 3 bulk density. Additionally, each soil layer was raked lightly before the next layer was packed to diminish the discontinuity between Table 1 Textural characteristics of the soil used in all experiments (mean values and standard errors for three replicates). Particle-size fraction Clay (0 2 lm) Fine silt (2 20 lm) Coarse silt (20 50 lm) Fine sand ( lm) Mean values (%) Standard errors Coarse sand (>250 lm) layers. To prevent ponding of water at the lower end of the soil tray, soils were glued onto the wall of the tray so that the packed soil samples were coherent with the wall. Four slope gradients (10, 15, 20 and 25 ) were applied. The 25 slope corresponded with the maximum slope for cultivated land according to the Chinese Soil and Water Conservation Act. The soil samples were wetted from the top with deionized water (EC = 4.8 ls/cm) applied as mist. Once the soils reached full saturation, the plots were exposed to a simulated rainstorm of 90 mm of deionized water. The rainfall intensity studied was 90 ± 3.2 mm h 1, Rainfall dynamics were monitored with six electric pluviographs around each plot. These instruments were connected to a control data logger (CR10, Campbell Scientific Inc., USA) operating at a 20 s time step. The chosen rainfall intensity of 90 mm h 1 is typical of intense storms in sub-humid climate regions of China that are dominated by monsoon climate conditions (Chen, 1987; Cai et al., 1998). Each treatment was tested in three replicates Measurements Runoff and sediment measurements For each rainfall event, runoff was volumetrically measured and sampled at 1-min intervals for sediment concentration. Collected samples were deposited, separated from the water, dried in a forced-air oven at 105 C until constant mass was achieved and weighed. The sediment concentration was determined as the ratio of dry sediment mass to sampled runoff volume, while soil loss was defined as the total sediment load present in runoff water that exits a specified area. After rill initiation in the plot, the rill widths were frequently measured with a millimeter-scale ruler at numerous locations. A fluorescent dye was used for flow velocity measurement (Gilley et al., 1990), and a millimeter-scale ruler was used to determine flow width. Visual observations of sediment transport and soil surface conditions were made and recorded both during and after simulated storms. During rainfall, runoff and sediment were also collected in a bucket at 3-min intervals for sediment size. The particle size distribution of the collected samples was sieved with 2.0, 1.0, and 0.5 mm pore openings within 5 min. Sizes less than 0.5 mm were determined using a Malvern Mastersizer 2000 laser diffraction device (Malvern Instruments Ltd., Malvern, UK) Measurements of non-dispersed soil size distribution The size distribution of soil aggregates or particles was measured by wet sieving with three replicates. Ten grams of the original soil samples were immersed in distilled water, and the particle size distribution was measured by wet sieving with 2.0, 1.0, and 0.5 mm pore openings. Each sample was sieved for duration of 10 min at a frequency of 35 RPM and 5 cm amplitude of the movement (Asadi et al., 2007b). Sizes less than 0.5 mm were also measured by laser diffraction above Kinetic energy associated with rainfall When a drop of rain strikes a patch of soil, the kinetic energy of the drop is transferred to soil particles and to water on the surface, detaching soil particles and displacing water (Gabet and Dunne, 2003). Rain power (R, Wm 2 ) is the time derivative of the kinetic energy per unit area, and it is calculated with the equation developed by Gabet and Dunne (2003): R ¼ qiv2 ð1 C v Þ cos h 2 where q is the density of water (assumed to have a constant value of 1000 kg m 3 at 25 C), I (m s 1 ) is the rainfall intensity, v is raindrop velocity (m s 1 ), C v represents the proportion of area covered by ground-level vegetation, and h is slope gradient. ð1þ

3 Z.H. Shi et al. / Journal of Hydrology (2012) Energy associated with runoff Stream power represents the energy of the runoff water flowing over the soil surface, some or all of which may be available to remove and transport soil particles from the erosion surface (Teixeira and Misra, 1997). Stream power (X, Wm 2 ) was calculated as: x ¼ qgsq=w where q is the density of water (assumed to have a constant value of 1000 kg m 3 at 25 C), g is the gravitational acceleration (9.8 m s 2 ), S is the sine of the erosion surface slope, Q is the discharge rate (m 3 s 1 ) and W is the plot or rill width (m) Data treatment Data from the wet sieving of the original soil were subdivided into 10 size classes, each having an equal mass fraction. The fraction of each size class in the outflow sediment at different times during each experiment was then obtained using the subdivision of equal classes obtained for the original soil as described in Asadi et al. (2007a). It is illustrated for a particular experiment in Fig. 1. Using the same size class boundaries obtained from subdivision of the original soil into equal mass classes, the fraction of eroded materials in the outflow from the plot was determined for each size class. The outflow sediment concentration in each size class of eroded materials was then obtained by multiplying the total sediment concentration by the fraction of each size class. The size distribution of the eroded material was also expressed as the mean weighted diameter (MWD), calculated using the following formula (Le Bissonnais, 1996): MWD ¼ X10 x i w i i¼1 where x i is the mean diameter of the ith size class, w i is the weight fraction of particles of the ith size class, and i represents 10 size classes. 3. Results and discussion 3.1. Runoff and soil loss Table 2 shows the results from all the rainfall simulation experiments under different slopes. Steady runoff rate and time to start runoff did not vary significantly between the slopes (Tukey s test with a = 0.05) most likely due to the pre-saturation of the soil ð2þ ð3þ bed before the experiment. Significant differences between the slopes were identified when applying the same test to soil loss rate. But variation in slope from 15 to 20 did not have a significant effect on the soil loss rate (2.24 and 2.61 kg m 2 min 1,at15 and 20, respectively); they fell within the same group. The time to rill initiation at the 10 slope was significantly longer than that of all other three slopes (15, 20 and 25 ). The average and maximum sediment concentrations were higher for the steeper slope. Insight into the dynamics of soil erosion is provided through the variations in runoff rate, and sediment concentration with time for the four slopes (Fig. 2). The runoff rate quickly increased with time, approaching steady state at about 7 10 min after runoff initiation. Examination of the steady state discharge during each experiment indicates limited differences between the slopes (Fig. 2 and Table 2). The sediment concentrations were highly variable between and within the slope treatments. There was a clear and similar pattern in the sediment concentration over time for different slopes (Fig. 2). The sediment concentrations for all runs initially showed a sharp increase and then experienced a rapid decrease. The increasing sediment concentration at the early stages of the rainfall event indicates that the erosion process is characterized by a transport-limited sediment regime. This may include raindrop detachment followed by a raindrop-induced flow transport system, as suggested by Kinnell (2005); this system is always transport-limited. However, the transition from interrill to rill processes is critical for both sediment concentrations and erosion rates. In all treatments, the flow sediment concentrations increased rapidly after rill initiation, and the sediment concentration at any given moment during rainfall was higher for plots with steeper slopes (Fig. 2). These results are in agreement with the conclusion of Kinnell (2000) that the sediment concentration increases with slope gradient, particularly when the gradient exceeds 10%. With the increase of slope gradient, the shear forces applied by the runoff flow velocity increase while the depth of the runoff decreases. These factors increase erosion, either by enhancing soil detachment or by limiting the protective effect of the thin layer of material moving with the runoff flow (Kinnell, 1990; Huang, 1998; Fox and Bryan, 1999). In addition, Fig. 2 shows that fluctuations in the sediment concentration were enhanced by increased slope Changes in non-dispersed sediment size with time Fig. 3 shows the temporal variations in sediment size from the four slope experiments. The sediment sizes were classified as clay Fig. 1. Subdivision of particle size distribution of the original soil into 10 classes of equal mass, and an example of the mass fractions of each of 10 size classes in sediment (experiment of 25 slope at min) using the size boundaries obtained from original soil.

4 126 Z.H. Shi et al. / Journal of Hydrology (2012) Table 2 Results from rainfall simulations, Tukey test groupings and maximum rill length/depth. Slope ( ) Time to start runoff (min) Time to rill initiation (min) Maximum rill length/ depth (mm/mm) Steady runoff rate (L m 2 min 1 ) Sediment concentration (kg L 1 ) Maximum sediment concentration (kg L 1 ) a 28.75a 2008/ a 0.089a 0.144a 1.24a a 16.02b 2830/ a 0.163b 0.218b 2.24b a 13.38c 3010/ a 0.187b 0.302c 2.61b a 12.56c 3940/ a 0.251c 0.392d 3.29c Soil loss rate (kg m 2 min 1 ) Note: Means in a column followed by the same letter are not significantly different (a = 0.05). Maximum rill length/depth was measured at the end of an experiment. Each value of sediment concentration is averaged. (<0.002 mm), fine silt ( mm), coarse silt ( mm), fine sand ( mm) and coarse sand (>0.25 mm). Rill development in plots made the concentrations of different size fractions undergo drastic change in almost all the simulations; this trend became clearer with increasing slope. Clay-sized particles are commonly associated with aggregation by rearrangement and flocculation (Bronick and Lal, 2005). Despite clay contents of 31% for the studied soil, 9 16% (generally 12 14%) of the total sediment was <0.002 mm, with a mean of 13%. These data suggested relatively little clay dispersion occurred and that most of the clay in the sediments were in the form of aggregates. This is consistent with field observations of deposited sediment from similar soils (Zhang et al., 2011). The content of <0.002 mm sediment in runoff provides an indication of the forces that acted on aggregates during detachment and transport by the erosive agent (Loch and Donnollan, 1983). The percentage of <0.002 mm sediment stayed almost unchanged during the rainfall events for all runs. This means the dispersed clay in sheet-flow that reached rills did not increase during subsequent transport in the rill. This implies that clay dispersion did not occur in the rills but rather occurred mainly during raindrop-impacted flow transport to the rills. As shown in Fig. 4, the percentage of clay in the sediment was moderately correlated with the instantaneous rain power. This can be attributed largely to raindrop impact occurring either on bare soil or on soil covered by shallow overland flow. In contrast, the flow depth in rills was commonly >15 mm (Table 2), and much of the raindrop energy would have been absorbed by the water (Morgan et al., 1998). Therefore, collisions between entrained soil aggregates with the rill bed might be the major energy source for aggregate breakdown in rills. From comparison of the raindrop velocity (6.5 m s 1 ) and the rill flow velocity (<0.3 m s 1 ), it appears that flow energy would be small relative to raindrop impacts. The sediment was principally composed of silt ( mm) which accounted for about 65 80% of the sediment load (Fig. 3). The percentage of silt declined slightly (about 3 5%) with time in all runs. The fraction of fine sand in sediment varied over a narrow range (generally 5 8%) during runs under all four slope gradients. The percentage of coarse sand increased with time and slope in all the simulations. It also significantly fluctuated with increasing slope angle, especially after rill development. The coefficient of variation was 3.5%, 2.9%, 12.2% and 21.9% for the 10, 15, 20 and 25 slopes, respectively. Young (1980) suggested that soils with more than 33% silt content usually generate sediments in the silt-size range (mostly in the range lm). He also suggested that the most erodible size ranges include particles and aggregates between 20 and 200 lm. The author postulated that particles with a size larger than 200 lm have enough mass to limit their movement, whereas for particles below 20 lm, cohesive forces impede particle detachment. Therefore, according to Young, soil texture is the main factor behind differences in sediment size distribution. Moreover, Durnford and King (1993) reported that when rainfall energy is high enough to break soil aggregates, clay became available for transport. The relative proportions of the different size classes thereby depend on rainfall and runoff properties. According to the results in Figs. 3 and 4, the conclusions in these two studies may not be in contradiction but rather may reflect the different combinations of rainfall energy, runoff energy and soil properties being investigated in these studies. An increased proportion of coarse sand with time suggests that sediment detachment by runoff is active after rill development (Bryan, 2000). In the present study, an increase in flow energy after rill development resulted in an elevated proportion of sand-sized particles, which was more pronounced in the runs with the steeper slope. Under these conditions, flow detachment prevailed and thus coarse materials, representative of the soil matrix, dominated sediment output Sediment sorting In order to clarify understanding the processes of interrill and rill erosion, the individual mass fractions of each of the 10 size classes in the outflow at a sampling time of 3 6 min after runoff have been drawn to represent interrill erosion (Fig. 5a) and at min to represent the combination of rill and interrill erosion (Fig. 5b). The original intact soil consisted of 10 equal mass fractions in each size class, indicated in Fig. 5 by a uniform 10% in each size class. Thus, any size class with a mass fraction of greater than 10% in sediment can be said to be preferentially transported. In samples taken early in the erosion process, 87 95% of the total sediment loss consisted of particles or aggregates finer than mm for all slopes. The mass fraction of particles in size classes >0.054 mm was less than 6% and decreased gradually with size (Fig. 5a). After rill development, the percentage of particles finer than mm decreased to 76 81%, and the sediment size developed a multimodal distribution with lows at mm, mm and >1.15 mm (Fig. 5b). The sediment size distribution for two sampling times was used to calculate the MWD of the sediments (Table 3). There was a significant difference in the MWD for different slopes. The ratio between the MWD of the sediments for the 25 and 10 slopes was 2.6 for early samples and 1.6 for late samples. Analysis of variance also showed significant difference in the MWD with sampling time: sediment size becomes coarser after rill development. The sediment size distribution could be influenced by the following factors: (a) the particle size distribution of the original soil, (b) aggregate breakdown during erosion and (c) the settling velocity of different size classes of particles or aggregates (Loch and Donnollan, 1983; Proffitt and Rose, 1991; Rose et al., 2007; Asadi et al., 2007b). This does not necessarily account for grainsize differences in the original soils, since a one soil was used for all the experiments. The main mechanisms of aggregate breakdown during water erosion processes are slaking by fast wetting and mechanical breakdown due to raindrop impact (Legout et al., 2005; Shi et al., 2010). In all runs, the soil bed was pre-saturated before each experiment. Aggregate breakdown due to raindrop impact is likely to be a major factor affecting size distribution during experiments. Interrill processes are confined to the soil surface and

5 Z.H. Shi et al. / Journal of Hydrology (2012) Fig. 2. Temporal variation in sediment concentration and runoff rate under different slope. Fig. 3. Changes in percentage of the non-dispersed sediment particles with time. are strongly influenced by raindrop energy. Sediment may have been splashed into the air a number of times before reaching areas of flow and being transported off the plots (Loch and Donnollan, 1983; Kinnell, 1990). Rill processes involve more concentrated flow that can detach soil from depths of up to 10 cm, which could not be directly affected by raindrops (Bryan, 2000). Many experimental results have shown that during rainfall-driven erosion, sediment is enriched with finer particles at early times (Moss et al., 1979; Proffitt and Rose, 1991). A theory of erosion processes that assumes no breakdown in soil structure during rainfall-driven

6 128 Z.H. Shi et al. / Journal of Hydrology (2012) Table 3 Mean weight diameter (mm) of sediments as influenced by slope treatments and rill development. Slope ( ) MWD of the sediments 3 6 min min Aa 0.124Ba Ab 0.143Bb Ac 0.184Bc Ac 0.197Bc Values for different slopes in a column followed by the same lowercase letter and values for different sampling time at a slope in a row followed by the same uppercase letter are not significantly different at p < 0.05, n = 3. Fig. 4. Relationship between percentage of clay in sediment and rain power. erosion predicts that the particle size distribution at the steady state will be the same as for the original soil (Hairsine et al., 1999). However, in the present study, this distribution is uneven between size classes; this could indicate either structural breakdown due to raindrop impacts, uneven or selective transport of different size classes, or a combination of these effects Sediment transport mechanisms Fig. 6. A hypothetical diagram showing the possible effects of suspension saltation and bed load mechanisms in transporting of various size classes of soil particles following development of a bed-load component (from Asadi et al., 2007b). Moss et al. (1979) noted that sediment transport can be divided into suspended, saltating and contact (rolling) loads, each normally being broadly associated with particular sediment size ranges. Asadi et al. (2007b) found that the bimodal distribution of sediment size resulted from two different transport mechanisms, rolling and suspension/saltation, each acting predominantly on particles of different size classes. Fig. 6 provides a plausible suggestion for how these two erosion mechanisms may overlap and complement each other. Size class with minimum transport rate can be used to estimate an approximate cutoff between suspension saltation and bed load transport. Loch and Donnollan (1983) suggested a transition from saltating to contact load in the size range of mm. Asadi et al. (2011) indicated that the boundaries for contact (rolling) load exist for size classes between 0.18 and 0.38 mm in fluvial sand and between 0.5 and 1.0 mm in forest soil. The boundary between suspension/saltation and bed load dominance as transport mechanisms depend on both soil type and flow hydraulic characteristics. Fig. 5. Mass fractions of the 10 size classes in outflow sediment for two sampling times: (a) 3 6 min and (b) min.

7 Z.H. Shi et al. / Journal of Hydrology (2012) Table 4 Relative importance (%) of suspension saltation and bed load in sediment. Slope ( ) Suspension saltation Bed load 3 6 min min 3 6 min min Aa 83.7Ba 4.8Aa 16.3Ba Ab 81.5Bab 8.5Ab 18.5Bab Abc 78.4Bbc 10.6Abc 21.6Bbc Ac 77.6Bc 12.9Ac 22.4Bc Values for different slopes followed by the same lowercase letter and values for different sampling time at a slope followed by the same uppercase letter are not significantly different at p < 0.05, n =3. relative importance of these two types of sediment transport mechanisms was related to stream power. While the relative importance of suspension saltation decreased with increasing stream power, rolling became more important at higher stream powers. Total soil loss is the sum of suspended, saltating and contact loads. The data show that each of these loads is detached and transported at different rates and by different mechanisms. The partition of soil loss into these more meaningful components appears to be essential both for initial data interpretation and for subsequent use of such data for soil loss prediction. Acknowledgments Financial support for this research was provided by the National Natural Science Foundation of China ( ) and the Program for New Century Excellent Talents in University (NCET ). References Fig. 7. Relationship between stream power and relative effect of contact (rolling) load in sediment transport. Fig. 5 indicated mm as the approximate particle size beyond which bed-load transport for the studied soil starts to become a significant additional mechanism of sediment transport. The relative importance of each mechanism in sediment loss at two sampling times is calculated and presented in Table 4. Table 4 shows that during interrill erosion processes, more than 87% of soil particles were transported by suspension saltation. However, after rill development on the hillslope, rolling appears to become an active mechanism, as approximately 20% of the sediment transported is in the coarsest fraction. The concentration of size fractions associated with rolling transport often showed short-term fluctuations (Fig. 3). This can be attributed to both the intermittent nature of some sediment inputs (e.g., rill bank collapse) and the intermittent nature of the movement involved. Sediment transport by contact (rolling) load significantly increased with slope in all the simulations. This could mainly be attributed to the higher stream power under steep slope conditions. There is a strong relationship (Fig. 7) between stream power and the relative effect of sediment transport by contact (rolling) load. 4. Conclusions Dynamic changes in sediment size distribution were measured for a clay loam soil under rainfall-driven erosion over a range of steep slopes. The results suggest that suspension/saltation, which affects fine particles, is the main erosion mechanism at work during interrill erosion processes. However, after rill development on the hillslope, suspension/saltation becomes less dominant, and bed-load transport by rolling of medium to large-sized sediment particles becomes an increasingly important transport mechanism. Concentration of rolling size fractions often showed short-term fluctuations due to both the intermittent nature of some sediment inputs and the intermittent nature of the movement involved. The Asadi, H., Ghadiri, H., Rose, C.W., Rouhipour, H., 2007a. Interrill soil erosion processes and their interaction on low slopes. Earth Surf. Proc. Land. 32, Asadi, H., Ghadiri, H., Rose, C.W., Yu, B., Hussein, J., 2007b. An investigation of flowdriven soil erosion processes at low stream powers. J. 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