Journal of Hydrology 454–455 (2012) 123–130

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Journal of Hydrology

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Soil erosion processes and sediment sorting associated with transport mechanismson 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 712100, Chinab College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, China

a r t i c l e i n f o s u m m a r y

Article history:Received 24 February 2012Received in revised form 30 May 2012Accepted 4 June 2012Available online 15 June 2012This manuscript was handled byKonstantine P. Georgakakos, Editor-in-Chief,with the assistance of Ana P. Barros,Associate Editor

Keywords:Erosion processesRainfall simulationSediment sizeSuspension–saltationRolling

0022-1694/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jhydrol.2012.06.004

⇑ Corresponding author at: College of ResourcesAgricultural University, Wuhan 430070, China. Tel.: +

E-mail address: shizhihua70@gmail.com (Z.H. Shi)

Sediment size distribution greatly affects sediment transport and deposition. A better understanding ofsediment sorting will improve understanding of erosion and sedimentation processes, which in turn willimprove erosion modeling. To address this issue, a total of 12 rainfall simulation experiments were con-ducted in a 1 m by 5 m box with varying steep slopes (10�, 15�, 20� and 25�), and the simulated rainfalllasted for 1 h at a rate of 90 mm h�1. For each simulated event, runoff and sediment were sampled at3-min intervals, which were performed to study in detail the temporal change in size distribution of theeroded materials. These data were used to interpret the real-time sequence of transport mechanisms act-ing 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 itwas highly correlated to instantaneous rain power (R2 = 0.452, P < 0.01, n = 120). Suspension–saltationtransports the finer than 0.054 mm size sediment was the most important erosion mechanism duringinterrill erosion processes. However, after rill development on hillslopes, bed-load transport by rollingof medium to large-sized sediment particles (coarser than 0.152 mm) became an increasingly importanttransport mechanism, and it were also enhanced by increased slope. Overall, the study supports a strongrelationship between the sediment transport of contact (rolling) load and stream power. The partition ofsoil loss into these more meaningful components appears to be essential both for initial data interpretationand for subsequent use of such data for soil loss prediction.

� 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 ofraindrops and runoff. There is a need to describe sediment sortingto characterize the on-site and off-site effects of soil erosion. A bet-ter understanding of sediment sorting will improve understandingof erosion and sedimentation processes, which in turn will improveerosion modeling (Asadi et al., 2011). Sediment may serve as thedelivery mechanism for soil nutrients and contaminants to streams.Nutrient distribution is non-uniform over different sizes of sedi-ment particles; fine sediments are usually richer in soil-sorbednutrients than are coarse sediments (Palis et al., 1990). Knowledgeof sediment sorting and its dynamics can also provide the basis forunderstanding and modeling the transfer of nutrients and contam-inants from hillslopes to water bodies (Teixeira and Misra, 1997).

Soil erosion by water is commonly divided into rill and interrillcomponents, depending on the source of eroded sediment (Meyer

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and Environment, Huazhong86 27 87288249..

and Wischmeier, 1969; Laflen et al., 1991). Sediment leaving aneroding area is a combination of primary soil particles (sand, siltand clay) and secondary or aggregated soil material (Mitchellet al., 1983). The general agreement is that interrill erosion resultsin selective removal of fine particles, whereas rill erosion is lessselective (or nonselective) after a specific critical flow shear stressis exceeded (Proffitt and Rose, 1991; Durnford and King, 1993;Wan and El-Swaify, 1998; Malam Issa et al., 2006). The selectivetransport of fine sediment produced by interrill erosion has beenattributed to the insufficient ability of interrill overland flow totransport large detached particles (Parsons et al., 1991) or to theselective deposition of coarse sediment (Proffitt and Rose, 1991).Even for this general observation, though, conflicting reports existin the literature regarding sediment sorting. Young and Onstad(1978) and Meyer et al. (1992) found that sediment transportedby interrill erosion was coarser than the in situ soils and the rillsediment. It has also been noted that both the concentration andsize distribution of sediment can change dynamically duringrainfall-driven erosion (Hairsine et al., 1999). Many previous stud-ies have reported that eroded materials are enriched in clay andsilt-sized particles relative to the original soil where the erosionevent commenced. The eroded materials gradually become coarser

124 Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130

over time, and at steady-state, its composition becomes very sim-ilar to that of the original soil (Asadi et al., 2011). Loch and Donno-llan (1983) and Asadi et al. (2007b) found that the mass fraction ofdifferent sediment sizes is distributed bimodally. They concluded abimodal distribution of sediment resulted from the different trans-port mechanisms of suspension, saltation and rolling, which eachact predominantly on particles of different size classes. There arestill some conflicting and unexplained results regarding sedimentsorting during erosion processes. Sediment size distributions ap-pear to depend on many factors such as rainfall characteristics,vegetation cover, hydraulic flow type (sheet or rill), soil propertiesand slope. More information on sediment sorting and its dynamicsis needed to better understand the behavior and interaction of thedifferent components that make up erosion processes.

Approximately 800 million people worldwide depend directlyon steeplands for their sustenance (Drees et al. 2003). Knowledgeof the predominant erosion mechanisms that occur under steepslope conditions is essential if conservation measures are to beproperly planned. The literature reveals that slopes in erosionexperiments are usually less than 25%, and many studies have onlyevaluated the effects of slope steepness on wash loss from shortand unrilled slopes (Fu et al., 2011). To address these issues, a clayloam soil in the Loess Plateau was chosen in this study. The tempo-ral change in size distribution of the exiting sediment resultingfrom 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 atthe State Key Laboratory of Soil Erosion and Dryland Farming onthe Loess Plateau. Rainfall intensities were adjusted through nozzlesizes and water pressure. Calibrations of rainfall intensities wereconducted prior to the experiments. Experimental plots were con-structed with metal sheets of 5 m (length) � 1 m (width) � 0.5 m(depth). The plots were placed on movable platforms for ease oftransport. A metal runoff collector was set at the bottom of eachplot to direct runoff into a container. The plot was electronicallyadjusted to a desired slope between 0� and 30�. The soil used inthe experiments was a clay loam soil collected from Yangling inShanxi Province, China. The natural consolidated soil has a bulkdensity between 1.2 and 1.4 g/cm3, with an organic matter contentof 0.6%. The soil texture information is listed in Table 1.

2.2. Rainfall simulation experiments

Soil samples were air dried, crushed to pass through a 10.0-mmsieve and mixed thoroughly. The soil was packed 30 cm deep ineach plot (in three 10-cm layers) to achieve a 1.2–1.4 g cm�3 bulkdensity. Additionally, each soil layer was raked lightly before thenext layer was packed to diminish the discontinuity between

Table 1Textural characteristics of the soil used in all experiments (mean values and standarderrors for three replicates).

Particle-size fraction

Clay(0–2 lm)

Fine silt(2–20 lm)

Coarse silt(20–50 lm)

Fine sand(50–250 lm)

Coarsesand(>250 lm)

Mean values(%)

31 39 25 4 1

Standarderrors

4.2 6.3 1.9 0.2 0

layers. To prevent ponding of water at the lower end of the soiltray, soils were glued onto the wall of the tray so that the packedsoil samples were coherent with the wall. Four slope gradients(10�, 15�, 20� and 25�) were applied. The 25� slope correspondedwith the maximum slope for cultivated land according to the Chi-nese Soil and Water Conservation Act. The soil samples were wet-ted from the top with deionized water (EC = 4.8 ls/cm) applied asmist. Once the soils reached full saturation, the plots were exposedto a simulated rainstorm of 90 mm of deionized water. The rainfallintensity studied was 90 ± 3.2 mm h�1, Rainfall dynamics weremonitored with six electric pluviographs around each plot. Theseinstruments were connected to a control data logger (CR10,Campbell Scientific Inc., USA) operating at a 20 s time step. Thechosen rainfall intensity of 90 mm h�1 is typical of intense stormsin sub-humid climate regions of China that are dominated by mon-soon climate conditions (Chen, 1987; Cai et al., 1998). Each treat-ment was tested in three replicates.

2.3. Measurements

2.3.1. Runoff and sediment measurementsFor each rainfall event, runoff was volumetrically measured and

sampled at 1-min intervals for sediment concentration. Collectedsamples were deposited, separated from the water, dried in aforced-air oven at 105 �C until constant mass was achieved andweighed. The sediment concentration was determined as the ratioof dry sediment mass to sampled runoff volume, while soil losswas defined as the total sediment load present in runoff water thatexits a specified area. After rill initiation in the plot, the rill widthswere frequently measured with a millimeter-scale ruler at numer-ous locations. A fluorescent dye was used for flow velocity measure-ment (Gilley et al., 1990), and a millimeter-scale ruler was used todetermine flow width. Visual observations of sediment transportand soil surface conditions were made and recorded both duringand after simulated storms. During rainfall, runoff and sedimentwere also collected in a bucket at 3-min intervals for sediment size.The particle size distribution of the collected samples was sievedwith 2.0, 1.0, and 0.5 mm pore openings within 5 min. Sizes lessthan 0.5 mm were determined using a Malvern Mastersizer 2000laser diffraction device (Malvern Instruments Ltd., Malvern, UK).

2.3.2. Measurements of non-dispersed soil size distributionThe size distribution of soil aggregates or particles was mea-

sured by wet sieving with three replicates. Ten grams of the origi-nal soil samples were immersed in distilled water, and the particlesize distribution was measured by wet sieving with 2.0, 1.0, and0.5 mm pore openings. Each sample was sieved for duration of10 min at a frequency of 35 RPM and 5 cm amplitude of the move-ment (Asadi et al., 2007b). Sizes less than 0.5 mm were also mea-sured by laser diffraction above.

2.3.3. Kinetic energy associated with rainfallWhen 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, W m�2) is the time derivative of the kineticenergy per unit area, and it is calculated with the equation devel-oped by Gabet and Dunne (2003):

R ¼ qIv2ð1� CvÞ cos h2

ð1Þ

where q is the density of water (assumed to have a constant valueof 1000 kg m�3 at 25 �C), I (m s�1) is the rainfall intensity, v israindrop velocity (m s�1), Cv represents the proportion of area cov-ered by ground-level vegetation, and h is slope gradient.

Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130 125

2.3.4. Energy associated with runoffStream power represents the energy of the runoff water flowing

over the soil surface, some or all of which may be available to re-move and transport soil particles from the erosion surface (Teixeiraand Misra, 1997). Stream power (X, W m�2) was calculated as:

x ¼ qgSQ=W ð2Þ

where q is the density of water (assumed to have a constant valueof 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 dis-charge rate (m3 s�1) and W is the plot or rill width (m).

2.4. Data treatment

Data from the wet sieving of the original soil were subdividedinto 10 size classes, each having an equal mass fraction. The frac-tion of each size class in the outflow sediment at different timesduring each experiment was then obtained using the subdivisionof equal classes obtained for the original soil as described in Asadiet al. (2007a). It is illustrated for a particular experiment in Fig. 1.Using the same size class boundaries obtained from subdivision ofthe original soil into equal mass classes, the fraction of erodedmaterials in the outflow from the plot was determined for eachsize class. The outflow sediment concentration in each size classof eroded materials was then obtained by multiplying the totalsediment concentration by the fraction of each size class.

The size distribution of the eroded material was also expressedas the mean weighted diameter (MWD), calculated using the fol-lowing formula (Le Bissonnais, 1996):

MWD ¼X10

i¼1

xi �wi ð3Þ

where xi is the mean diameter of the ith size class, wi is the weightfraction of particles of the ith size class, and i represents 10 sizeclasses.

3. Results and discussion

3.1. Runoff and soil loss

Table 2 shows the results from all the rainfall simulation exper-iments under different slopes. Steady runoff rate and time to startrunoff did not vary significantly between the slopes (Tukey’s testwith a = 0.05) most likely due to the pre-saturation of the soil

Fig. 1. Subdivision of particle size distribution of the original soil into 10 classes of equa(experiment of 25� slope at 51–54 min) using the size boundaries obtained from origina

bed before the experiment. Significant differences between theslopes were identified when applying the same test to soil loss rate.But variation in slope from 15� to 20� did not have a significant ef-fect on the soil loss rate (2.24 and 2.61 kg m�2 min�1, at 15� and20�, respectively); they fell within the same group. The time to rillinitiation at the 10� slope was significantly longer than that of allother three slopes (15�, 20�and 25�). The average and maximumsediment concentrations were higher for the steeper slope.

Insight into the dynamics of soil erosion is provided through thevariations in runoff rate, and sediment concentration with time forthe 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 experimentindicates limited differences between the slopes (Fig. 2 and Table2). The sediment concentrations were highly variable betweenand within the slope treatments. There was a clear and similar pat-tern in the sediment concentration over time for different slopes(Fig. 2). The sediment concentrations for all runs initially showeda sharp increase and then experienced a rapid decrease. Theincreasing sediment concentration at the early stages of the rainfallevent indicates that the erosion process is characterized by a trans-port-limited sediment regime. This may include raindrop detach-ment followed by a raindrop-induced flow transport system, assuggested by Kinnell (2005); this system is always transport-lim-ited. However, the transition from interrill to rill processes is crit-ical for both sediment concentrations and erosion rates. In alltreatments, the flow sediment concentrations increased rapidlyafter rill initiation, and the sediment concentration at any givenmoment during rainfall was higher for plots with steeper slopes(Fig. 2). These results are in agreement with the conclusion ofKinnell (2000) that the sediment concentration increases withslope gradient, particularly when the gradient exceeds 10%. Withthe increase of slope gradient, the shear forces applied by the run-off flow velocity increase while the depth of the runoff decreases.These factors increase erosion, either by enhancing soil detach-ment or by limiting the protective effect of the thin layer of mate-rial moving with the runoff flow (Kinnell, 1990; Huang, 1998; Foxand Bryan, 1999). In addition, Fig. 2 shows that fluctuations in thesediment concentration were enhanced by increased slope.

3.2. Changes in non-dispersed sediment size with time

Fig. 3 shows the temporal variations in sediment size from thefour slope experiments. The sediment sizes were classified as clay

l mass, and an example of the mass fractions of each of 10 size classes in sedimentl soil.

Table 2Results from rainfall simulations, Tukey test groupings and maximum rill length/depth.

Slope(�)

Time to startrunoff (min)

Time to rillinitiation (min)

Maximum rill length/depth (mm/mm)

Steady runoff rate(L m�2 min�1)

Sedimentconcentration(kg L�1)

Maximum sedimentconcentration (kg L�1)

Soil loss rate(kg m�2 min�1)

10 1.55a 28.75a 2008/75 1.46a 0.089a 0.144a 1.24a15 1.90a 16.02b 2830/115 1.44a 0.163b 0.218b 2.24b20 1.60a 13.38c 3010/127 1.42a 0.187b 0.302c 2.61b25 1.85a 12.56c 3940/140 1.39a 0.251c 0.392d 3.29c

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. Eachvalue of sediment concentration is averaged.

126 Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130

(<0.002 mm), fine silt (0.002–0.02 mm), coarse silt (0.02–0.05mm), fine sand (0.05–0.25 mm) and coarse sand (>0.25 mm). Rilldevelopment in plots made the concentrations of different sizefractions undergo drastic change in almost all the simulations; thistrend became clearer with increasing slope.

Clay-sized particles are commonly associated with aggregationby rearrangement and flocculation (Bronick and Lal, 2005). Despiteclay 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%. Thesedata suggested relatively little clay dispersion occurred and thatmost of the clay in the sediments were in the form of aggregates.This is consistent with field observations of deposited sedimentfrom similar soils (Zhang et al., 2011). The content of <0.002 mmsediment in runoff provides an indication of the forces that actedon aggregates during detachment and transport by the erosiveagent (Loch and Donnollan, 1983). The percentage of <0.002 mmsediment stayed almost unchanged during the rainfall events forall runs. This means the dispersed clay in sheet-flow that reachedrills did not increase during subsequent transport in the rill. Thisimplies that clay dispersion did not occur in the rills but rather oc-curred mainly during raindrop-impacted flow transport to the rills.As shown in Fig. 4, the percentage of clay in the sediment wasmoderately correlated with the instantaneous rain power. Thiscan be attributed largely to raindrop impact occurring either onbare soil or on soil covered by shallow overland flow. In contrast,the flow depth in rills was commonly >15 mm (Table 2), and muchof the raindrop energy would have been absorbed by the water(Morgan et al., 1998). Therefore, collisions between entrained soilaggregates with the rill bed might be the major energy source foraggregate breakdown in rills. From comparison of the raindropvelocity (6.5 m s�1) and the rill flow velocity (<0.3 m s�1), it ap-pears that flow energy would be small relative to raindrop impacts.

The sediment was principally composed of silt (0.002–0.05 mm)which accounted for about 65–80% of the sediment load (Fig. 3).The percentage of silt declined slightly (about 3–5%) with time inall runs. The fraction of fine sand in sediment varied over a narrowrange (generally 5–8%) during runs under all four slope gradients.The percentage of coarse sand increased with time and slope in allthe simulations. It also significantly fluctuated with increasingslope angle, especially after rill development. The coefficient ofvariation was 3.5%, 2.9%, 12.2% and 21.9% for the 10�, 15�, 20�and 25� slopes, respectively. Young (1980) suggested that soilswith more than 33% silt content usually generate sediments inthe silt-size range (mostly in the range 20–35 lm). He also sug-gested that the most erodible size ranges include particles andaggregates between 20 and 200 lm. The author postulated thatparticles with a size larger than 200 lm have enough mass to limittheir movement, whereas for particles below 20 lm, cohesiveforces impede particle detachment. Therefore, according to Young,soil texture is the main factor behind differences in sediment sizedistribution. Moreover, Durnford and King (1993) reported thatwhen rainfall energy is high enough to break soil aggregates, claybecame available for transport. The relative proportions of the dif-

ferent size classes thereby depend on rainfall and runoff properties.According to the results in Figs. 3 and 4, the conclusions in thesetwo studies may not be in contradiction but rather may reflectthe different combinations of rainfall energy, runoff energy and soilproperties being investigated in these studies. An increased pro-portion of coarse sand with time suggests that sediment detach-ment by runoff is active after rill development (Bryan, 2000). Inthe present study, an increase in flow energy after rill developmentresulted in an elevated proportion of sand-sized particles, whichwas more pronounced in the runs with the steeper slope. Underthese conditions, flow detachment prevailed and thus coarse mate-rials, representative of the soil matrix, dominated sediment output.

3.3. Sediment sorting

In order to clarify understanding the processes of interrill andrill erosion, the individual mass fractions of each of the 10 size clas-ses in the outflow at a sampling time of 3–6 min after runoff havebeen drawn to represent interrill erosion (Fig. 5a) and at 51–54 min to represent the combination of rill and interrill erosion(Fig. 5b). The original intact soil consisted of 10 equal mass frac-tions in each size class, indicated in Fig. 5 by a uniform 10% in eachsize class. Thus, any size class with a mass fraction of greater than10% in sediment can be said to be preferentially transported.

In samples taken early in the erosion process, 87–95% of the to-tal sediment loss consisted of particles or aggregates finer than0.054 mm for all slopes. The mass fraction of particles in size clas-ses >0.054 mm was less than 6% and decreased gradually with size(Fig. 5a). After rill development, the percentage of particles finerthan 0.054 mm decreased to 76–81%, and the sediment size devel-oped a multimodal distribution with lows at 0.003–0.005 mm,0.152–0.495 mm and >1.15 mm (Fig. 5b). The sediment size distri-bution for two sampling times was used to calculate the MWD ofthe sediments (Table 3). There was a significant difference in theMWD for different slopes. The ratio between the MWD of the sed-iments for the 25� and 10� slopes was 2.6 for early samples and 1.6for late samples. Analysis of variance also showed significant dif-ference in the MWD with sampling time: sediment size becomescoarser after rill development.

The sediment size distribution could be influenced by the fol-lowing factors: (a) the particle size distribution of the original soil,(b) aggregate breakdown during erosion and (c) the settling veloc-ity of different size classes of particles or aggregates (Loch andDonnollan, 1983; Proffitt and Rose, 1991; Rose et al., 2007; Asadiet al., 2007b). This does not necessarily account for grainsize differ-ences in the original soils, since a one soil was used for all theexperiments. The main mechanisms of aggregate breakdown dur-ing water erosion processes are slaking by fast wetting andmechanical breakdown due to raindrop impact (Legout et al.,2005; Shi et al., 2010). In all runs, the soil bed was pre-saturatedbefore each experiment. Aggregate breakdown due to raindrop im-pact is likely to be a major factor affecting size distribution duringexperiments. Interrill processes are confined to the soil surface and

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.

Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130 127

are strongly influenced by raindrop energy. Sediment may havebeen splashed into the air a number of times before reaching areasof flow and being transported off the plots (Loch and Donnollan,1983; Kinnell, 1990). Rill processes involve more concentratedflow that can detach soil from depths of up to 10 cm, which could

not be directly affected by raindrops (Bryan, 2000). Many experi-mental results have shown that during rainfall-driven erosion,sediment is enriched with finer particles at early times (Mosset al., 1979; Proffitt and Rose, 1991). A theory of erosion processesthat assumes no breakdown in soil structure during rainfall-driven

Fig. 4. Relationship between percentage of clay in sediment and rain power.

Table 3Mean weight diameter (mm) of sediments as influenced by slope treatments and rilldevelopment.

Slope (�) MWD of the sediments

3–6 min 51–54 min

10 0.038Aa 0.124Ba15 0.060Ab 0.143Bb20 0.081Ac 0.184Bc25 0.092Ac 0.197Bc

Values for different slopes in a column followed by the same lowercase letter andvalues for different sampling time at a slope in a row followed by the sameuppercase letter are not significantly different at p < 0.05, n = 3.

128 Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130

erosion predicts that the particle size distribution at the steadystate will be the same as for the original soil (Hairsine et al.,1999). However, in the present study, this distribution is unevenbetween size classes; this could indicate either structural break-down due to raindrop impacts, uneven or selective transport of dif-ferent size classes, or a combination of these effects.

Fig. 6. A hypothetical diagram showing the possible effects of suspension–saltationand bed load mechanisms in transporting of various size classes of soil particlesfollowing development of a bed-load component (from Asadi et al., 2007b).

3.4. Sediment transport mechanisms

Moss et al. (1979) noted that sediment transport can be dividedinto suspended, saltating and contact (rolling) loads, each normallybeing broadly associated with particular sediment size ranges. Asa-di et al. (2007b) found that the bimodal distribution of sedimentsize resulted from two different transport mechanisms, rollingand suspension/saltation, each acting predominantly on particlesof different size classes. Fig. 6 provides a plausible suggestion forhow these two erosion mechanisms may overlap and complementeach other. Size class with minimum transport rate can be used toestimate an approximate cutoff between suspension–saltation andbed load transport. Loch and Donnollan (1983) suggested a transi-

Fig. 5. Mass fractions of the 10 size classes in outflow sedimen

tion from saltating to contact load in the size range of 0.125–0.250 mm. Asadi et al. (2011) indicated that the boundaries forcontact (rolling) load exist for size classes between 0.18 and0.38 mm in fluvial sand and between 0.5 and 1.0 mm in forest soil.The boundary between suspension/saltation and bed load domi-nance as transport mechanisms depend on both soil type and flowhydraulic characteristics.

t for two sampling times: (a) 3–6 min and (b) 51–54 min.

Table 4Relative importance (%) of suspension–saltation and bed load in sediment.

Slope (�) Suspension–saltation Bed load

3–6 min 51–54 min 3–6 min 51–54 min

10 95.2Aa 83.7Ba 4.8Aa 16.3Ba15 91.5Ab 81.5Bab 8.5Ab 18.5Bab20 89.4Abc 78.4Bbc 10.6Abc 21.6Bbc25 87.1Ac 77.6Bc 12.9Ac 22.4Bc

Values for different slopes followed by the same lowercase letter and values fordifferent sampling time at a slope followed by the same uppercase letter are notsignificantly different at p < 0.05, n = 3.

Fig. 7. Relationship between stream power and relative effect of contact (rolling)load in sediment transport.

Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130 129

Fig. 5 indicated 0.054–0.152 mm as the approximate particlesize beyond which bed-load transport for the studied soil startsto become a significant additional mechanism of sediment trans-port. The relative importance of each mechanism in sediment lossat two sampling times is calculated and presented in Table 4. Table4 shows that during interrill erosion processes, more than 87% ofsoil particles were transported by suspension–saltation. However,after rill development on the hillslope, rolling appears to becomean active mechanism, as approximately 20% of the sediment trans-ported is in the coarsest fraction. The concentration of size frac-tions associated with rolling transport often showed short-termfluctuations (Fig. 3). This can be attributed to both the intermittentnature of some sediment inputs (e.g., rill bank collapse) and theintermittent nature of the movement involved. Sediment transportby contact (rolling) load significantly increased with slope in all thesimulations. This could mainly be attributed to the higher streampower under steep slope conditions. There is a strong relationship(Fig. 7) between stream power and the relative effect of sedimenttransport by contact (rolling) load.

4. Conclusions

Dynamic changes in sediment size distribution were measuredfor a clay loam soil under rainfall-driven erosion over a range ofsteep slopes. The results suggest that suspension/saltation, whichaffects fine particles, is the main erosion mechanism at work dur-ing interrill erosion processes. However, after rill development onthe hillslope, suspension/saltation becomes less dominant, andbed-load transport by rolling of medium to large-sized sedimentparticles becomes an increasingly important transport mechanism.Concentration of rolling size fractions often showed short-termfluctuations due to both the intermittent nature of some sedimentinputs and the intermittent nature of the movement involved. The

relative importance of these two types of sediment transportmechanisms was related to stream power. While the relativeimportance of suspension–saltation decreased with increasingstream power, rolling became more important at higher streampowers. Total soil loss is the sum of suspended, saltating and con-tact loads. The data show that each of these loads is detached andtransported at different rates and by different mechanisms. Thepartition of soil loss into these more meaningful components ap-pears to be essential both for initial data interpretation and forsubsequent use of such data for soil loss prediction.

Acknowledgments

Financial support for this research was provided by the NationalNatural Science Foundation of China (41071190) and the Programfor New Century Excellent Talents in University (NCET-10-0423).

References

Asadi, H., Ghadiri, H., Rose, C.W., Rouhipour, H., 2007a. Interrill soil erosionprocesses and their interaction on low slopes. Earth Surf. Proc. Land. 32, 711–724.

Asadi, H., Ghadiri, H., Rose, C.W., Yu, B., Hussein, J., 2007b. An investigation of flow-driven soil erosion processes at low stream powers. J. Hydrol. 342, 134–142.

Asadi, H., Moussavi, A., Ghadiri, H., Rose, C.W., 2011. Flow-driven soil erosionprocesses and the size selectivity of sediment. J. Hydrol. 406, 73–81.

Bronick, C.J., Lal, R., 2005. Soil structure and management, a review. Geoderma 124,3–22.

Bryan, R.B., 2000. Soil erodibility and processes of water erosion on hillslope.Geomorphology 32, 385–415.

Cai, Q.G., Wang, G.P., Chen, Y.Z., 1998. Process of Sediment Yield in a SmallWatershed of the Loess Plateau and its Modeling. Science Press, Beijing, pp. 92–103 (in Chinese).

Chen, Y.Z., 1987. A review: The study of soil erosion on Loess Plateau. GeographicalRes. 6, 76–85 (in Chinese).

Drees, L.R., Wilding, L.P., Owens, P.R., Wu, B., Perottoa, H., Sierra, H., 2003. Steeplandresources: characteristics, stability and micromorphology. Catena 54, 619–636.

Durnford, D., King, P., 1993. Experimental study of processes and particle sizedistributions of eroded soil. J. Irrig. Drain. Eng. 119, 383–398.

Fox, D.M., Bryan, R.B., 1999. The relationship of soil loss by interrill erosion to slopegradient. Catena 38, 211–222.

Fu, S.H., Liu, B.Y., Liu, H.P., Xu, L., 2011. The effect of slope on interrill erosion atshort slopes. Catena 84, 29–34.

Gabet, E.J., Dunne, T., 2003. Sediment detachment by rain power. Water Resour. Res.39, 1002. http://dx.doi.org/10.1029/2001WR000656.

Gilley, J.E., Kottwiz, E.R., Simanton, J.R., 1990. Hydraulic characteristics of rills.Trans. ASAE 33, 1900–1906.

Hairsine, P.B., Sander, G.C., Rose, C.W., Parlange, J.Y., Hogarth, W.L., Lisle, I.,Rouhipour, H., 1999. Unsteady soil erosion due to rainfall impact: a model ofsediment sorting on the hillslope. J. Hydrol. 199, 115–128.

Huang, C.H., 1998. Sediment regimes under different slope and surface hydrologicconditions. Soil Sci. Soc. Am. J. 62, 423–430.

Kinnell, P.I.A., 1990. The mechanics of raindrop induced flow transport. Aust. J. SoilRes. 28, 497–516.

Kinnell, P.I.A., 2000. The effect of slope length on sediment concentrationsassociated with side-slope erosion. Soil Sci. Soc. Am. J. 64, 1004–1008.

Kinnell, P.I.A., 2005. Raindrop impact induced erosion processes and prediction: areview. Hydrol. Process. 19, 2815–2844.

Laflen, J.M., Lane, L.J., Foster, G.R., 1991. WEPP: a new generation of erosionprediction technology. J. Soil Water Conserv. 46, 34–38.

Legout, C., Leguédois, S., Le Bissonnais, Y., 2005. Aggregate breakdown dynamicsunder rainfall compared with aggregate stability measurements. Eur. J. Soil Sci.56, 225–237.

Le Bissonnais, Y., 1996. Aggregate stability and assessment of soil crustability anderodibility: I. Theory and methodology. Eur. J. Soil Sci. 47, 425–437.

Loch, R.J., Donnollan, T.E., 1983. Field rainfall simulator studies on two clay soils ofthe Darling downs, Queensland. II. Aggregate breakdown, sediment propertiesand soil erodibility. Aust. J. Soil Res. 21, 47–58.

Malam Issa, O., Le Bissonnais, Y., Planchon, O., Favis-Mortlock, D., Silvera, N.,Wainwright, J., 2006. Soil detachment and transport on field- and laboratory-scale interill areas: erosion processes and the size-selectivity of erodedsediment. Earth Surf. Proc. Land. 31, 929–939.

Meyer, L.D., Line, D.E., Harmon, W.C., 1992. Size characteristics of sediment fromagricultural soils. J. Soil Water Conserv. 47, 107–111.

Meyer, L.D., Wischmeier, W.H., 1969. Mathematical simulation of the process of soilerosion by water. Trans. ASAE 12 (754–758), 762.

Mitchell, J.K., Mostaghimi, S., Pound, M., 1983. Primary particle and aggregate sizedistribution of eroded soil from sequenced rainfall events. Trans. ASAE 26,1773–1777.

130 Z.H. Shi et al. / Journal of Hydrology 454–455 (2012) 123–130

Morgan, R.P.C., Quinton, J.N., Smith, R.E., Govers, G., Poesen, J.W.A., Auerswald, K.,Chisci, G., Torri, D., Styczen, M.E., 1998. The European soil erosion model(EUROSEM): a process-based approach for predicting sediment transport fromfields and small catchments. Earth Surf. Proc. Land. 23, 527–544.

Moss, A.J., Walker, P.H., Hutka, J., 1979. Raindrop-stimulated transportation inshallow water flows: an experimental study. Sediment Geol. 22, 165–184.

Palis, R.G., Okwach, G., Rose, C.W., Saffigna, P.G., 1990. Soil erosion processes andnutrient loss. I. The interpretation of enrichment ratio and nitrogen loss inrunoff sediment. Aust. J. Soil Res. 28, 623–639.

Parsons, A.J., Abrahams, A.D., Luk, S.H., 1991. Size characteristics of sediment ininterrill overland flow on a semiarid hillslope, southern Arizona. Earth Surf.Proc. Land. 16, 143–152.

Proffitt, A.P.B., Rose, C.W., 1991. Soil erosion processes: II. Settling velocitycharacteristics of eroded sediment. Aust. J. Soil Res. 29, 685–695.

Rose, C.W., Yu, B., Ghadiri, H., Asadi, H., Parlange, J.Y., Hogarth, W.L., Hussein, J.,2007. Dynamic erosion of soil in steady sheet flow. J. Hydrol. 333, 449–458.

Shi, Z.H., Yan, F.L., Li, L., Li, Z.X., Cai, C.F., 2010. Interrill erosion from disturbed andundisturbed samples in relation to topsoil aggregate stability in red soils fromsubtropical China. Catena 81, 240–248.

Teixeira, P.C., Misra, R.K., 1997. Erosion and sediment characteristics of cultivatedforest soils as affected by the mechanical stability of aggregates. Catena 30,119–134.

Wan, Y., El-Swaify, S.A., 1998. Characterizing interrill sediment size by partitioningsplash and wash processes. Soil Sci. Soc. Am. J. 62, 430–437.

Young, R.A., 1980. Characteristics of eroded sediment. Trans. ASAE 23 (1139–1142),1146.

Young, R.A., Onstad, C.A., 1978. Characterisation of rill and interrill eroded soil.Trans. ASAE 21, 1126–1130.

Zhang, Y., Zheng, X.L., Zhan, X.H., 2011. Impacts of young trees on hydrauliccharacteristics of overland flow and sediment particle size in Loess Plateau.Chin. Bull. Soil Water Conserv. 31, 7–15 (in Chinese).