sediment and solute transport on soil slope under simultaneous influence of rainfall impact and...

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HYDROLOGICAL PROCESSES Hydrol. Process. 24, 1446–1454 (2010) Published online 17 February 2010 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/hyp.7605 Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow Tailong Guo, 1,2 Quanjiu Wang, 2,3 * Dingqiang Li 1 and Laosheng Wu 4 1 Guangdong Institute of Eco-environment and Soil Science, Guangdong Key Laboratory of Comprehensive Control of Agro-environment, No. 808 Tianyuan Road, Tianhe District, Guangzhou 510650, Guangdong Province, China 2 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, No. 26 Xinong Road, YangLing, Shaanxi 712100, China 3 Xian University of Technology, No.5 South Jinhua, Xian, Shaanxi 710048, PR China 4 Department of Environmental Science, University of California, Riverside, CA 92521, USA Abstract: Soil erosion and nutrient losses with surface runoff in the loess plateau in China cause severe soil quality degradation and water pollution. It is driven by both rainfall impact and runoff flow that usually take place simultaneously during a rainfall event. However, the interactive effect of these two processes on soil erosion has received limited attention. The objectives of this study were to better understand the mechanism of soil erosion, solute transport in runoff, and hydraulic characteristics of flow under the simultaneous influence of rainfall and shallow clear-water flow scouring. Laboratory flume experiments with three rainfall intensities (0, 60, and 120 mm h 1 ) and four scouring inflow rates (10, 20, 30, and 40 l min 1 ) were conducted to evaluate their interactive effect on runoff. Results indicate that both rainfall intensity and scouring inflow rate play important roles on runoff formation, soil erosion, and solute transport in the surface runoff. A rainfall splash and water scouring interactive effect on the transport of sediment and solute in runoff were observed at the rainfall intensity of 60 mm h 1 and scouring inflow rates of 20 l min 1 . Cumulative sediment mass loss (Ms) was found to be a linear function of cumulative runoff volume (Wr) for each treatment. Solute transport was also affected by both rainfall intensity and scouring inflow rate, and the decrease in bromide concentration in the runoff with time fitted to a power function well. Reynolds number (Re ) was a key hydraulic parameter to determine erodability on loess slopes. The Darcy–Weisbach friction coefficients (f) decreased with the Reynolds numbers (Re ), and the average soil and water loss rate (M l ) increased with the Reynolds numbers (Re ) on loess slope for both scenarios with or without rainfall impact. Copyright 2010 John Wiley & Sons, Ltd. KEY WORDS loess soil; erosion; solute transport; scouring inflow rates; Reynolds number Received 11 September 2009; Accepted 15 December 2009 INTRODUCTION Soil erosion is reduced by rainfall includes detachment, transport, and deposition of soil particles due to raindrop splash and surface flow. Detachment and transport of soil particles are the function of the erosive forces of raindrop impact and shallow flowing water and their interaction. Recent soil erosion studies shed some light on understanding the interaction between various erosion processes (Zheng et al., 2000; Rouhipour et al., 2006). Earlier researchers noticed that raindrop impact on shallow water flow increases soil loss (Ellison, 1945; Ekren, 1950), which can produce more than twice the erosion than either rainfall or overland flow alone (Singer et al., 1981). Another group of researchers showed that both detachment and transportation of soil particles by raindrop impact are greatly affected by depth of flowing water (Mutchler and Hansen, 1970; Moss and Green, 1983). Transport capacity of inter-rill flow is greatly enhanced by impacting raindrops (Foster et al., 1984). * Correspondence to: Quanjiu Wang, State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, No. 26 Xinong Road, YangLing, Shaanxi 712100, China. E-mail: [email protected] Dissipation of raindrop energy by flow can render it less efficient in causing erosion (Ferreira and Singer, 1985). Soil detachment by water occurs primarily via the processes of splash from raindrop impact and scour by shallow flow from surface runoff (Nearing et al., 1991). Combining runoff flow and rainfall intensity produced a better indication of rainfall erosivity than the use of rainfall intensity alone (Truman and Bradford, 1995). Many researchers considered that the erosion processes are driven by the interaction of both rainfall and runoff flow, which implies that the deposited layer acts as a feedback to erosion processes (Guy et al., 1987; Proffitt and Rose, 1991; Hairsine and Rose, 1991, 1992a,b). The splash effect on soil erosion by rainfall under controlled laboratory conditions has been studied in detail (Al-Durrah and Bradford, 1982; Nearing and Bradford, 1985; Sharma et al., 1991), and the detachment effect by shallow clear-water flow has also received attention (Yong and Wenzel, 1971; Roels, 1984; Abrahams et al., 1986; Gilley et al., 1990; Nearing et al., 1991, 1997; Cochrane and Flanagan, 2001). However, the interactive effect of rainfall and shallow clear-water flow under laboratory conditions has received less attention. Copyright 2010 John Wiley & Sons, Ltd.

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Page 1: Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

HYDROLOGICAL PROCESSESHydrol. Process. 24, 1446–1454 (2010)Published online 17 February 2010 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.7605

Sediment and solute transport on soil slope undersimultaneous influence of rainfall impact and scouring flow

Tailong Guo,1,2 Quanjiu Wang,2,3* Dingqiang Li1 and Laosheng Wu4

1 Guangdong Institute of Eco-environment and Soil Science, Guangdong Key Laboratory of Comprehensive Control of Agro-environment, No. 808Tianyuan Road, Tianhe District, Guangzhou 510650, Guangdong Province, China

2 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy ofSciences, No. 26 Xinong Road, YangLing, Shaanxi 712100, China

3 Xian University of Technology, No.5 South Jinhua, Xian, Shaanxi 710048, PR China4 Department of Environmental Science, University of California, Riverside, CA 92521, USA

Abstract:

Soil erosion and nutrient losses with surface runoff in the loess plateau in China cause severe soil quality degradation andwater pollution. It is driven by both rainfall impact and runoff flow that usually take place simultaneously during a rainfallevent. However, the interactive effect of these two processes on soil erosion has received limited attention. The objectives ofthis study were to better understand the mechanism of soil erosion, solute transport in runoff, and hydraulic characteristicsof flow under the simultaneous influence of rainfall and shallow clear-water flow scouring. Laboratory flume experimentswith three rainfall intensities (0, 60, and 120 mm h�1) and four scouring inflow rates (10, 20, 30, and 40 l min�1) wereconducted to evaluate their interactive effect on runoff. Results indicate that both rainfall intensity and scouring inflow rateplay important roles on runoff formation, soil erosion, and solute transport in the surface runoff. A rainfall splash and waterscouring interactive effect on the transport of sediment and solute in runoff were observed at the rainfall intensity of 60 mm h�1

and scouring inflow rates of 20 l min�1. Cumulative sediment mass loss (Ms) was found to be a linear function of cumulativerunoff volume (Wr) for each treatment. Solute transport was also affected by both rainfall intensity and scouring inflow rate,and the decrease in bromide concentration in the runoff with time fitted to a power function well. Reynolds number (Re) wasa key hydraulic parameter to determine erodability on loess slopes. The Darcy–Weisbach friction coefficients (f) decreasedwith the Reynolds numbers (Re), and the average soil and water loss rate (Ml) increased with the Reynolds numbers (Re) onloess slope for both scenarios with or without rainfall impact. Copyright 2010 John Wiley & Sons, Ltd.

KEY WORDS loess soil; erosion; solute transport; scouring inflow rates; Reynolds number

Received 11 September 2009; Accepted 15 December 2009

INTRODUCTION

Soil erosion is reduced by rainfall includes detachment,transport, and deposition of soil particles due to raindropsplash and surface flow. Detachment and transport ofsoil particles are the function of the erosive forces ofraindrop impact and shallow flowing water and theirinteraction. Recent soil erosion studies shed some lighton understanding the interaction between various erosionprocesses (Zheng et al., 2000; Rouhipour et al., 2006).

Earlier researchers noticed that raindrop impact onshallow water flow increases soil loss (Ellison, 1945;Ekren, 1950), which can produce more than twice theerosion than either rainfall or overland flow alone (Singeret al., 1981). Another group of researchers showed thatboth detachment and transportation of soil particles byraindrop impact are greatly affected by depth of flowingwater (Mutchler and Hansen, 1970; Moss and Green,1983). Transport capacity of inter-rill flow is greatlyenhanced by impacting raindrops (Foster et al., 1984).

* Correspondence to: Quanjiu Wang, State Key Laboratory of SoilErosion and Dryland Farming on the Loess Plateau, Institute of Soiland Water Conservation, Chinese Academy of Sciences, No. 26 XinongRoad, YangLing, Shaanxi 712100, China. E-mail: [email protected]

Dissipation of raindrop energy by flow can render itless efficient in causing erosion (Ferreira and Singer,1985).

Soil detachment by water occurs primarily via theprocesses of splash from raindrop impact and scour byshallow flow from surface runoff (Nearing et al., 1991).Combining runoff flow and rainfall intensity produceda better indication of rainfall erosivity than the use ofrainfall intensity alone (Truman and Bradford, 1995).Many researchers considered that the erosion processesare driven by the interaction of both rainfall and runoffflow, which implies that the deposited layer acts as afeedback to erosion processes (Guy et al., 1987; Proffittand Rose, 1991; Hairsine and Rose, 1991, 1992a,b).

The splash effect on soil erosion by rainfall undercontrolled laboratory conditions has been studied in detail(Al-Durrah and Bradford, 1982; Nearing and Bradford,1985; Sharma et al., 1991), and the detachment effectby shallow clear-water flow has also received attention(Yong and Wenzel, 1971; Roels, 1984; Abrahams et al.,1986; Gilley et al., 1990; Nearing et al., 1991, 1997;Cochrane and Flanagan, 2001). However, the interactiveeffect of rainfall and shallow clear-water flow underlaboratory conditions has received less attention.

Copyright 2010 John Wiley & Sons, Ltd.

Page 2: Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

SEDIMENT AND SOLUTE TRANSPORT ON SOIL SLOPE 1447

Some soil erosion experiments have been conductedwith rainfall simulator over the flumes (Hargrave andShaykewich, 1997; Flanagan et al., 2002; Arnaez et al.,2007), whereas others have been conducted with a scour-ing equipment at a constant rate at the top of flumes (Zhuet al., 1995; Nearing et al., 1997; Zhang et al., 2005). Butfew experiments provided such eroded flow with rainfallsimulator and water scouring simultaneously. With lim-ited data, it is difficult to distinguish the effect of rainfallfrom shallow flow on erosion and to fully understand thefundamental mechanisms of interaction between them.

Understanding the mechanisms of interaction betweenrainfall and surface water flow is necessary for the devel-opment of erosion and solute transport process models.The objectives of this study were to better understanderosion and solute transport processes in runoff simul-taneously affected by rainfall impact and shallow flowscouring under controlled laboratory conditions, to iden-tify possible mechanism and theoretical explanation fortheir interactions, and to provide experimental data fortesting mechanistic soil erosion model that considers bothrainfall impact and scouring simultaneously.

MATERIALS AND METHODS

Tested soil

The laboratory experiments were conducted in theState Key Laboratory of Soil Erosion and DrylandFarming on the Loess Plateau, Yangling, China. The soil

used for the experiments was a loessial loam collectedfrom no-farming land at YangLing, Shannxi Province,China. The bulk soil was gently crushed before passingthrough a 2-cm sieve, and the sieved soil was thoroughlymixed. The particle size distribution for the tested soilwas determined by wet sieving, and the soil physical andchemical properties were listed in Table I.

Rainfall simulation and water scouring experimentalsystem

The rainfall simulation and water scouring experimentswere conducted simultaneously under the laboratoryconditions. The experimental setup includes water supply(Part 1), water scouring supply (composed of a constanthead scouring set-up and water pump, Part 2a, b), rainfallsimulator (a down-sprinkle precipitation set-up, waterpipes, and a simulated rainfall controlling chamber, Part3), and an experimental soil flume (3Ð5 m long, 0Ð5 mwide, and 0Ð5 m deep. A steel catchment collector wasjointed at the base of the steel flume for collectingrunoff water into the plastic containers, Part 4), as shownin Figure 1. The initial inflow rates for every set ofexperiments were fixed at 10, 20, 30, and 40 lÐmin�1,respectively. Rainfall intensities were precisely controlledby changing the nozzle sizes and water pressure. Theheight of rainfall simulator from the ground is 23 m, andthe vertical distance between the mid point of soil bedand the ground is 1Ð35 m, the effective height of rainfallis 21Ð65 m. The simulated rainfall had an uniformity

Table I. Physical and chemical properties for the test soils

Particle size distribution (%) Organicmatter

(gÐkg�1)

Totalnitrogen(gÐkg�1)

Totalphosphorus

(gÐkg�1)

Totalpotassium(gÐkg�1)

PH

Soil <0Ð001 0Ð001–0Ð05 >0Ð05Loess soil 25Ð9 68Ð9 5Ð4 9Ð3 0Ð71 1Ð1 18Ð6 8Ð35

part3.Rainfall simulator

part2 a.Water scouring equipment

part2 b.Plastic container

part4 a.Steel flume

part2 b.Water pump

part1.Water tank

Figure 1. Sketch of experimental setup

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 24, 1446–1454 (2010)

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1448 T. GUO ET AL.

of above 90%, similar to natural rainfall in raindropdistribution and size. Calibrations of rainfall intensitieswere conducted at the beginning of each experiment. Theexperimental soil flume includes a steel flume and plasticcontainers. The slope of the flume can be adjusted thougha hydraulic jack. Many openings were made at the bottomof the steel flume to allow water to drain freely. Plasticcontainers were set at the bottom of the flume to catch therunoff. Each plastic container is 20 l in volume. About30–40 plastic containers were used in each treatment.

Experimental processes

Two sets of experiments were carried out: one wasthe loess soil bed received scouring inflow water alone(without rainfall impact), and the other was the loesssoil bed received rainfall impact and scouring inflowwater simultaneously. Three rainfall intensities and fourflushing water inflow rates were used, which resulted in12 treatment combinations. Three rainfall intensities wereapplied: (1) shallow flow only (with simulated rainfallintensity is 0; (2) shallow flow with rainfall intensity of60 mmÐh�1; and (3) shallow flow with simulated rainfallof 120. The slope was fixed at 10° for all the experiments.

The treated soil sample was packed in the flumelayer with 5-cm increments with a final bulk densityof 1Ð35 gÐcm�3. After finishing the packing, a knownamount of water was sprayed with a hand-held sprayerto minimize the differences in antecedent soil moisturecontent among treatments. Initial soil moisture contentof tested soil was measured by an oven at 105 °C(about 8Ð3%) for 8 h before soil packing. Soil moisturecontent was adjusted to 9% (gravimetrically) for allthe treatments. After soil packing, the soil surface wascovered by a plastic film for 2 h for soil moistureredistribution in order for the soil to reach uniform watercontent in the profile. Potassium bromide (KBr) wasapplied at soil surface as a tracer for soil-dissolved solute.KBr was added to the water that was used to adjust thesoil water content to 9%. Solution of potassium bromide(KBr) was sprayed with a hand-held sprayer to the soilsurface at 80 g bromide per square metre before runningeach experiment.

All the treatments were run twice. During the exper-iment, all runoff water and sediment samples were col-lected in the plastic containers at the bottom end of theflume in 1–5 min. intervals. Flow velocities (length pertime) were measured once flow reached the outlet ini-tially. The dye method (Colouration of KMnO4 solution)was used in each experiment to measure flow velocities.The slope was divided into 7 sections for measuring theflow velocities along the slope length direction with 0Ð5-m interval. The mean surface flow velocity at the upperslope was the average of seven measured values. Therunoff water samples with sediments were settled sep-arated from the water, dried at 105 °C for 6–8 h. Thesediment concentration was expressed in dry sedimentmass per runoff volume, whereas the soil and water lossrate was defined as the total mass of runoff and sediment

per unit area per time. To determine bromide (solute)concentration, 200 ml runoff water was collected fromeach plastic container after the sediment was settled out.Bromide concentrations in runoff water and in sedimentsamples were measured by an ionometre (Model SX-3805, Shanghai Dapu Instrument Co., Ltd, China).

Hydraulic characteristics

The mean surface flow velocities at the upper slope(Vm) were used to estimate profile average flow veloc-ity (V) by the relation of V D aVm, where a was acoefficient. Early research showed that the coefficienta increased with the Reynolds number as flow changedfrom laminar to turbulent. He used the coefficient a of0Ð67, 0Ð70, and 0Ð80, for laminar flow, transition flow,and turbulent flow respectively (Abrahams et al., 1986).We assumed that the coefficient a also changes with flowregime, whereas the method for measuring the averageflow velocity is the same. A theoretical study showedthat the coefficient a is 0Ð67 (Li et al., 1996).

Although it is often a few millimetre deep, flowdepth is a very important factor affecting overland flow.Unfortunately, it is very difficult to be measured theflow depth on the slope surface because it is so dynamicalong the direction of slope. Assuming the slope flow isuniform, the average flow depth can be calculated from:

h D qw

V�1�

Where h is flow depth (cm), qw is unit discharge(cm2Ðs�1).

The Reynolds number (Re) and the Froude number(Fr) were calculated by Equations (2) and (3), respec-tively:

Re D Vh

��2�

Fr D V√

gh�3�

where v is kinematical viscosity (cm2Ð s�1), g is thegravity acceleration (cmÐs�2).

Various studies have investigated hydraulic character-istics of shallow overland flow without rainfall impact.Foster et al. (1984) and Abrahams et al. (1996) showedthat flow velocity increases as a function of slope gradi-ent for non-erodible rill. Govers (1992) and Nearing et al.(1997) showed that for an eroding rill, flow velocity isnot influenced by bed slope.

The Darcy–Weisbach equation is widely used todescribe overland flow characteristics. Under uniformflow conditions, the Darcy–Weisbach friction coefficient(f), is given as (Chow, 1959):

f D 8gRS

V2 �4�

where g is the gravity acceleration (mÐs�2), R ishydraulics radius (m), S is average slope, and V is flowvelocity (mÐs�1).

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 24, 1446–1454 (2010)

Page 4: Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

SEDIMENT AND SOLUTE TRANSPORT ON SOIL SLOPE 1449

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(Scouring inflow rate is 40L/min)

No rainfall impactWith 60mm/h rainfall impactWith 120mm/h rainfall impact

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Figure 2. Change in runoff rate with time in two sets of experiments with three rainfall intensities and four scouring inflow rates

RESULTS AND DISCUSSION

Runoff

The curves for measured runoff rate as a functionof time under different rainfall intensities and scouringinflow rates are shown in Figure 2. When the scouringinflow rate was at low level (at the scouring inflow rateof 10, 20, 30 lÐmin�1), the effect of rainfall impact onrunoff rate was obvious. But while the scouring inflowrate was at high level (at the scouring inflow rate of40 lÐmin�1), the effect of rainfall impact on runoff ratebecame low (Figure 2). This may be attributed to the factthat higher scouring inflow rate increased runoff, whileat higher scouring rate, the rainfall impact induced soilsurface sealing decreased. At high scouring inflow rate,the difference in runoff rate between experiments withand without rainfall impact decreased.

Figure 3 shows that the cumulative runoff volumeincreases nearly linearly with time under the labora-tory conditions. As expected, cumulative runoff vol-ume increased as rainfall intensity increased from 0 to120 mmÐh�1 under different scouring inflow rates rangingfrom 10 to 40 lÐmin�1. The slope of these straight linesgradually merged to the same value for all the treatments,with or without rainfall impact, as the scouring inflow rateincreased from 10 to 40 lÐmin�1 (Figure 3). The resultsshown in Figures 2 and 3 indicate that the runoff rate andvolume resulted from the scouring inflow alone are lessthan the runoff rate and runoff volume from the combi-nation of scouring inflow water and rainfall impact. Thusboth the rainfall intensity and the scouring inflow rateplay important roles in runoff from slopes.

Sediments

Figure 4 shows the changes in sediment concentrationwith time from the runoff experiments. The influence ofrainfall impact was not apparent when the scouring inflowrate was 10, 20, and 30 lÐmin�1, respectively. Change insediment concentrations indicates that the runoff processdid not reach steady-state conditions, either for with orwithout rainfall impact. However, when the scouringinflow rate was at 40 lÐmin�1, increasing rainfall intensitysubstantially decreased the sediment concentration inrunoff, and the fluctuation of sediment concentration wasreduced for both conditions of with and without rainfallimpact.

Relationship between sediments and runoff

Relationship between sediment mass and runoff vol-ume under different rainfall intensities and scouringinflow rates is shown in Figure 5. The cumulative sed-iment mass (Ms) is a function of the cumulative runoffvolume (Wr) for each treatment and their relationshipscan be well described by the linear equations (Figure 5).This pattern is similar to those observed in field bareplots where runoff had a significantly positive correlationwith soil loss (Huang and Bradford, 1993; Flanagan et al.,2002; Benik et al., 2003). Relationship between sedimentmass and runoff volume has commonly been regardedas a linear function (Huang and Bradford, 1993; Arnaezet al., 2007). Other studies show that sediment mass isweakly correlated with runoff volume, but strongly cor-related with rainfall intensity (Norton, 2000).

Soil solute transport with runoff

The process of dissolved chemical transfer from soilto runoff and transport to the plot outlet is very complex.

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 24, 1446–1454 (2010)

Page 5: Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

1450 T. GUO ET AL.

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(Scouring inflow rate is 40L/min)

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Figure 3. Change in cumulative runoff volume with time in two sets of experiments with three rainfall intensities and four flushing water inflow rates

(Scouring inflow rate is 10L/min)

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Figure 4. Change in sediment concentration with time in two sets of experiments with three rainfall intensities and four flushing water inflow rates

Dissolved chemical transport in runoff is influenced bywater solubility of the chemical as well as by rainfallintensity (Shigaki et al., 2007). Bromide (Br�) is a non-reactive tracer that has been widely used to study waterflows. The decrease of bromide concentrations in runoffwith time under different rainfall intensities and scouringinflow rates is shown in Figure 6. These curves have two

characteristics. Firstly, a stage of bromide concentrationfall down sharply for all treatments from the beginningto 10 min. Secondly, a stage of bromide concentrationstabilizes after 10 min later (Figure 6).

Solute concentration change with time in runoff wasoften described by exponential functions (Ahuja et al.,1982; Ahuja and Lehman, 1983). Some researchers used

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 24, 1446–1454 (2010)

Page 6: Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

SEDIMENT AND SOLUTE TRANSPORT ON SOIL SLOPE 1451

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s (k

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No rainfall impactWith 60mm/h rainfall impactWith 120mm/h rainfall impactLinearity (No rainfall impact)Linearity (With 60mm/h rainfall impact)Linearity (With 120mm/h rainfall impact)

Figure 5. Relationship between cumulative sediment mass (Ms) andcumulative runoff volume (Wr) for the first 10 min. Ms is representativefor cumulative sediment yield, kg; Wr is representative for cumulative

runoff volume, m3

the limited film model to describe the chemical trans-port process (Parr et al., 1987; Wallach et al., 1988;Havis et al., 1992; Wallach and Sharbtai, 1992a,b). Oth-ers described the decay of dissolved chemical concen-tration in runoff with time as a power function (Wanget al., 1998). Our results show that they can be fittedto either a power function or an exponential function(R2 D 0Ð23–0Ð92), but better to a power function (R2 D0Ð60–0Ð99). Under the same conditions (the same scour-ing inflow rate and rainfall intensity), the R2 of powerfunction fitting is always greater than that of exponentialfunction fitting. The fitting was improved by includingthe scouring inflow rate.

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Figure 7. Change in cumulative bromide mass and cumulative sedimentyield as cumulative runoff volume increases

Relationship between solute in runoff and sediment

The dissolved chemical loss is closely related with thesediment loss (Catt et al., 1994; Hansen and Nielsen,1995; Hargrave and Shaykewich, 1997; Teixeira andMisra, 2005). The cumulative bromide mass in runoffis well correlated with cumulative sediment mass beforethe cumulative runoff volume reached at certain value(Figure 7). For example, for Treatment C6, this cumula-tive runoff volume was 0Ð231 m3. But after this runoffvolume, the relationship between the two reversed.Cumulative bromide mass in runoff was not well relatedwith cumulative sediment mass at anytime (Figure 7).Chemicals in runoff can be transferred in solution formby mixing of rainwater with soil solution, dissolution ofthe chemical partly present in solid form, and desorptionof the chemical from eroded sediment.

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g/L

)

No rainfall impactWith 60mm/h rainfall impactWith 120mm/h rainfall impact

(Scouring inflow rate is 20L/min)

0.00

50.00

100.00

150.00

200.00

0 5 10 15 20 25

Time (min)

Bro

mid

e co

ncen

trat

ion

inru

noff

(m

g/L

)B

rom

ide

conc

entr

atio

n in

runo

ff (

mg/

L)

No rainfall impact

With 60mm/h rainfall impact

With 120mm/h rainfall impact

(Scouring inflow rate is 30L/min)

0.00

50.00

100.00

200.00

0 5 10 15 20 25

Time (min)

Bro

mid

e co

ncen

trat

ion

inru

noff

(m

g/L

)

No rainfall impactWith 60mm/h rainfall impactWith 120mm/h rainfall impact

(Scouring inflow rate is 40L/min)

0.00

50.00

100.00

150.00

200.00

250.00

300.00

350.00

400.00

0 5 10 15 20

Time (min)

No rainfall impactWith 60mm/h rainfall impactWith 120mm/h rainfall impact

Figure 6. Change in bromide concentration in runoff with scouring inflow rates and rainfall intensities

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 24, 1446–1454 (2010)

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1452 T. GUO ET AL.

Table II. The Reynolds numbers (Re) and the Froude numbers(Fr) for with or without the effect of simulated rainfall on loess

slope

TreatmentsNo.

Unit widthof inflow

rate(m2Ðs�1)

Average flowvelocity(mÐs�1)

Averageflow depth

(mm)

Re Fr

T1 0Ð00033 0Ð30 0Ð83 239 3Ð40C1 0Ð00033 0Ð24 1Ð23 325 3Ð83C5 0Ð00033 0Ð39 0Ð85 414 3Ð58T2 0Ð00067 0Ð37 1Ð56 441 2Ð43C2 0Ð00067 0Ð28 2Ð24 566 3Ð81C6 0Ð00067 0Ð45 1Ð47 723 3Ð13T3 0Ð0010 0Ð40 2Ð28 905 2Ð31C3 0Ð0010 0Ð41 2Ð34 1006 2Ð58C7 0Ð0010 0Ð51 1Ð99 1017 2Ð91T4 0Ð0013 0Ð36 3Ð34 1140 1Ð84C4 0Ð0013 0Ð41 3Ð03 1188 2Ð44C8 0Ð0013 0Ð46 2Ð85 1221 2Ð34

Flow hydraulic characteristics

Based on the foregoing Equations (1), (2) and (3), thevalues of flow velocity, average flow depth, Reynoldsnumbers (Re), and Froude numbers (Fr) were calcu-lated and listed in Table II. Flow velocity increases asthe scouring inflow rate increases, and they range from0Ð24 to 0Ð51 mÐs�1 (Table II). There are some differencesin flow velocity between experiments with or withoutrainfall impact. When the unit width inflow rate was rela-tively small (0Ð001 and 0Ð0013 m2Ð s�1), the flow velocityincreased linearly as the rainfall intensity increased. Butas the unit width inflow rate was relatively high (0Ð00033and 0Ð00067 m2Ðs�1), such linear relationship betweenflow velocity and rainfall intensity disappeared. Since thewidth of flume was the same for all experiments, the aver-age flow depth increased as the flow velocity from 0Ð83 to3Ð34 mm as flow rate changed from low to high. Underthe same simulated rainfall intensity, the Froude num-bers decreased as the scouring inflow rate increased, andthey ranged from 1Ð84 to 3Ð83 (Table II). The Reynoldsnumbers among different scouring inflow rates under thesame simulated rainfall intensity ranged from 239 to 1221(Table II). According to the criterion of open channelflow, overland flows on the slope changed from laminarflow to turbulence flow as flow rates increased in ourexperiment.

The Darcy–Weisbach friction coefficients (f) in differ-ent overland flows are closely related to the correspond-ing Reynolds numbers (Re). Savat (1980) used sand andloess soils to study hydraulic resistance due to grain fric-tion in a laboratory flume. His result shows that hydraulicgrain roughness decreases as Re increases. Rauws andGovers (1988) showed that for a non-erodible bed withartificial roughness elements glued to a flat bed, the rela-tionship between Re and f varies as a function of slope.Gilley et al. (1990) measured the flow velocities in rillsfor 10 soils under a constant slope in field studies, andthey showed that f is the negative exponential functionof Re for each soil. Gilley et al. (1992) and Prosser and

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 200 400 600 800 1000 1200 1400

Reynold numbers (Re)

Dar

cy-w

eisb

ach

coef

fici

ent

(f)

No rainfall impact

With 60mm/h rainfall impact

With 120mm/h rainfall impact

Exponential (No rainfallimpact)

Exponential (With 60mm/hrainfall impact)

Exponential (With 120mm/hrainfall impact)

Figure 8. Relationship between the Darcy–Weisbach friction coefficient(f) and the Reynolds number (Re) of the flow on the loess slope

Dietrich (1995) showed that f increases as Re increasesunder the condition of flow depth less than the size ofthe physical roughness elements. Nearing et al. (1997)used sand and loess to study f change with slope usingconstant hydraulic radius, and he found that f decreasesas Re increases. However, few studies investigated therelationship between f and Re under the laboratory con-dition with the interactive effect of scouring inflow waterand rainfall.

Based on Equations (1) and (4), the Darcy–Weisbachfriction coefficient (f) with or without the effect ofsimulated rainfall was calculated. The Darcy–Weisbachfriction coefficient (f) decreases as the Reynolds num-ber (Re) increases (Figure 8). Darcy–Weisbach fric-tion coefficients are a negative exponential functionof Reynolds numbers with or without rainfall impact(Figure 8). The relationships between f and Re aref D 3Ð44 e�0Ð0026Re (R2 D 0Ð98) for no rainfall impact;f D 2Ð19 e�0Ð0026Re (R2 D 0Ð99) for 60 mm h�1 rain-fall impact; and f D 1Ð43 e�0Ð0026Re (R2 D 0Ð95) for120 mm h�1 rainfall impact at the p < 0Ð01 significancelevel. The fitted constants decrease from 3Ð44 to 1Ð43as rainfall intensities increase from 0 to 120 mm h�1,whereas the constant of exponent of the fitted functionsdoes not change: all of the three treatments have the samevalue of �0Ð0026. Our results are similar to other findings(Savat, 1980; Gilley et al., 1990; Nearing et al., 1997),although our experiments were conducted under differ-ent laboratory conditions and soil with different hydrauliccharacteristics of simulated rainfall and scouring inflow.

Relationship between erosion and hydraulics

Change in flow hydraulics influences the soil ero-sion process on the slopes (Zheng et al., 2004). Theseflow hydraulic parameters include shear velocity (Gov-ers, 1985), flow hydraulic shear stress (Nearing et al.,1991; Huang and Bradford, 1993; Prosser and Dietrich,1995; Zhu et al., 1995), stream power (Proffitt and Rose,1991), and unit stream power (Nearing et al., 1997).These parameters are important for building process-based erosion model and helping understand the flow

Copyright 2010 John Wiley & Sons, Ltd. Hydrol. Process. 24, 1446–1454 (2010)

Page 8: Sediment and solute transport on soil slope under simultaneous influence of rainfall impact and scouring flow

SEDIMENT AND SOLUTE TRANSPORT ON SOIL SLOPE 1453

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0 200 400 600 800 1000 1200 1400

Reynolds numbers (Re)

Ave

rage

soi

l an

d w

ater

los

s ra

te (

kg.m

−2.m

in−1

)

No rainfall impact

With 60mm/h rainfall impact

With 120mm/h rainfall impact

Figure 9. Relationship between the average soil loss rate (Ml) and theReynolds number (Re)

detachment of erosion process by the energy-basedapproach.

The Reynolds number (Re) is one of critical hydraulicparameters for characterizing the flow regimes. It repre-sents a ratio of kinetic to viscous force of the flow andindicates the level of flow turbulence. Our results showedthat the Reynolds number (Re) increased as rainfall rateincreased (Table II), which are a result of increasingflow turbulence on the slopes, and consequently, highersoil erosion. The average soil and water loss rate (Ml)increased linearly with the Reynolds numbers on theslope for both with or without rainfall impact (Figure 9).The relationship between Ml and Re can be expressed asan exponential function: Ml D 0Ð031Re0Ð94 (R2 D 0Ð88)under no rainfall impact; Ml D 0Ð024Re0Ð98 (R2 D 0Ð90)for 60 mm h�1 rainfall impact; and Ml D 0Ð005Re1Ð21

(R2 D 0Ð96) for 120 mm h�1 rainfall impact. All therelationships are significant at p < 0Ð01 level. Rainfallcan make the curve between Ml and Re more precip-itously. The amplitudes of the fitted exponential func-tions decrease as rainfall intensity increases, whereas theconstants in the exponents increase as rainfall intensityincreases.

CONCLUSION

Soil erosion is driven by both rainfall impact and runoffflow (Guy et al., 1987; Proffitt and Rose, 1991; Hairsineand Rose, 1991, 1992a,b). But few experiments testederosions under interactive effect of rainfall impact andwater scouring. In our experiments, although both soilsediment and solute mass in runoff increased as runoffvolume increased under the interactive effect of rainfalland scouring water, the degree of the effect was differentbetween the two when the cumulative runoff volumereached 0Ð231 m3 (Treatment C6: rainfall intensity is60 mm h�1, and inflow rate is 20 l min�1). Namely,the cumulative bromide mass in runoff correlated wellwith the cumulative sediment mass before the cumulativerunoff reached at the value 0Ð231 m3, but the relationshipbetween the two reversed when the cumulative runoffbeyond this value.

Results indicate that rainfall intensity and scouringinflow rate play important roles on runoff and sedimenttransport including changes of runoff rate, runoff volume,and sediment concentration. Cumulative sediment mass(Ms) was a function of cumulative runoff volume (Wr)for each treatment and their relationships can be welldescribed by the linear functions. Solute transport in theeroded flow was also influenced by rainfall intensity andscouring inflow rate. The decay of bromide concentration(as a tracer in soil surface layer) in runoff with time canbe better fitted to a power function.

The Reynolds number (Re) is a key hydraulic param-eter to characterize such eroded flow on loess slope. TheDarcy–Weisbach friction coefficient decreases with theReynolds number, and the average soil and water lossrate (Ml) increases with the Reynolds number on the loessslope with or without the impact of rainfall.

In a real scenario, erosion driven by the interactionof rainfall and overland flow is influenced by otherimportant factors, such as slope steepness, soil watercontent, position (upland and lowland), vegetation inslope land, etc. In this research, we only discussed theslope conditions in a laboratory setup. Further studiesfor interactive effect of erosion due to splash rainfall andscouring water are needed to investigate the roles of otherfactors under more realistic conditions.

ACKNOWLEDGEMENTS

This work was partially supported by the CAS/SAFEAInternational Partnership Program for Creative ResearchTeams-Process simulation of soil and water of a water-shed, the Special Foundation for Science and Tech-nology Project of Guangdong Province (Project No:2008A080800028), the Youth Foundation of The Guang-dong Province Academy of Sciences (Project No:qnjj200810), and the Director Foundation of State KeyLaboratory of Soil Erosion and Dryland Farming onLoess Plateau (Project No: 10501-252). We thank for theassistance of Institute of Soil and Water Conservation ofChinese Academy of Sciences. Wenjuan Bai is acknowl-edged for her indispensable assistance in the laboratory.We are also grateful to Chang-bin Wang and Jun Zhaofor their assistance.

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