Impact of cornstalk buffer strip on hillslope soil erosion and its hydrodynamic understanding

Catena 149 (2017) 417–425

Contents lists available at ScienceDirect

Catena journal homepage: www.elsevier.com/locate/catena

Impact of cornstalk buffer strip on hillslope soil erosion and its hydrodynamic understanding Ximeng Xu a, Fenli Zheng a,b,⁎, Chao Qin a, Hongyan Wu a, Glenn V. Wilson c a b c

Institute of Soil and Water Conservation, State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A & F University, Yangling 712100, Shaanxi, PR China Institute of Soil and Water Conservation, CAS & MWR, Yangling 712100, Shaanxi, PR China USDA-ARS National Sedimentation Laboratory, Oxford 38655, MS, USA

a r t i c l e

i n f o

Article history: Received 10 June 2016 Received in revised form 19 October 2016 Accepted 21 October 2016 Available online 9 November 2016 Keywords: Rainfall simulations Rill erosion Shear stress Stream power Runoff threshold The Loess Plateau

a b s t r a c t Soil erosion is still a serious concern on the Loess Plateau of China. Cornstalk buffer strips are not commonly utilized for erosion control on the Loess Plateau, and there is little hydrodynamic understanding of this soil erosion control practice. A simulated rainfall experiment was designed to investigate how a cornstalk buffer strip affected soil erosion and to enhance the hydrodynamic understanding of this method. Large loessial soil beds (10 m-long, and 3 m-wide) with slope gradient of 20° were subjected to three successive simulated rainfall events with intensities of 100 mm h−1 for each experimental run. The rainfall events were conducted by a down sprinkler rainfall simulator system. Two treatments (with and without a cornstalk buffer strip) were tested in the following four runs: 1) without cornstalk buffer strip, 2) with cornstalk buffer strip in the third rain event, 3) with cornstalk buffer strip in the second rain event, 4) with continuous cornstalk buffer strip in all three successive rainfall events. In treatments with buffer, a 1 m-width cornstalk buffer strip was applied. The results showed that, compared with the run without cornstalk buffer strip, the run with continuous cornstalk buffer strip in three successive rainfall events reduced sediment yield by 29.1% while the other two runs with cornstalk buffer strip in a single event only reduced sediment yield by 2.0%–9.1%, and early buffer run had a larger reduction in soil erosion than late buffer run. The runoff-sediment relationship coefficients revealed that cornstalk buffer decreased the sediment concentration and increased the runoff threshold required for soil erosion initiation. Moreover, the buffer strip increased sheet flow velocity in interrill areas, while it decreased concentrated flow velocity in rills. This promoted a shift of rill flow to subcritical laminar flow which reduced sediment yield. Cornstalk buffer strip also increased the critical hydrodynamic forces required for the initiation of soil erosion. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Mulches, e.g. straw, stalk, leaves, or plastic film, are often used to protect the soil surface from raindrop (splash) erosion and runoff detachment during the critical period of plant establishment (Smets et al., 2008). Many field and laboratory studies, focusing on the impact of mulches on water erosion, have been conducted in a wide range of environmental and topographic conditions (Kamara, 1986; Brown et al., 1989; Bradford and Huang, 1994; Döring et al., 2005; Mulumba and Lal, 2008; Wilson et al., 2008; Jordán et al., 2010; Shi et al., 2013; Montenegro et al., 2013; Prosdocimi et al., 2016a,b). Lal (1976) reported that organic mulches reduce soil loss by reducing raindrop impact, increasing surface storage and infiltration, decreasing runoff velocity, and improving soil qualities (soil structure, soil porosity and biological

⁎ Corresponding author at: No. 26, Xi'nong Road, Institute of Soil and Water Conservation, Yangling, Shaanxi 712100, PR China. E-mail addresses: [email protected] (X. Xu), fl[email protected], [email protected] (F. Zheng), [email protected] (C. Qin), [email protected] (H. Wu), [email protected] (G.V. Wilson).

http://dx.doi.org/10.1016/j.catena.2016.10.016 0341-8162/© 2016 Elsevier B.V. All rights reserved.

activity). Mulumba and Lal (2008) used a long term field plot to study the effects of mulching on soil physical properties and determined an optimum mulching rate for increasing soil porosity, available water capacity, soil moisture retention and aggregate stability which was meaningful for reducing soil erosion by water. Based on a 3-year experiment, Jordán et al. (2010) found that mulch helped to improve the soil physical properties and to reduce runoff coefficient and sediment yield, with an optimum rate of mulch application under semi-arid conditions in southern Spain set to 5 Mg ha−1 yr−1. By conducting an intermittent simulated rainfall, Montenegro et al. (2013) found that residue cover strongly affected infiltration, soil moisture, runoff and erosion. Prosdocimi et al. (2016a,b) conducted an experiment testing the effects of barely straw mulching on soil erosion on vineyards in eastern Spain and concluded that straw mulch was very effective in reducing soil particle detachment and surface runoff, and this benefit was achieved immediately after the application of the straw. Cornstalk, as one kind of typical organic mulch, can greatly reduce water erosion (Gilley et al., 1986; Wen et al., 2014). Cornstalk retention in fields is also important for promoting physical, chemical, and biological attributes of healthy soil in agricultural systems (Turmel et al.,

418

X. Xu et al. / Catena 149 (2017) 417–425

2015). The term buffer strip here implies to a strip of vegetation that acts as a filter for sediment and attached nutrients and pollutants (Barling and Moore, 1994; Hussein et al., 2007). Xu et al. (2015a) applied a cornstalk buffer strip using the whole plant cornstalk after harvest, which proved to be more efficient in erosion control than clipped short cornstalks applied in the simulated gully. It also served to shorten the slope length for runoff convergence and intercepted the sediment from upslope. Soil erosion by water is a process of detachment and transport of soil particles by rainfall and runoff. Flow detachment is often described by energy-based approaches and linked to the soil surface condition (Guo et al., 2013; Shen et al., 2016). Flow hydrodynamics are related to particle detachment and sediment transport. It is of great significance to evaluate the hydrodynamic characteristics of runoff on hillslopes with organic mulches. Gilley and Kottwitz (1995) measured the DarcyWeisbach roughness coefficients for surfaces with different kinds of crop residues under controlled laboratory conditions. They found that smaller diameter residue materials did influence hydraulic resistance when they substantially increased the total volume of resistance elements. Cassol et al. (2004) conducted a laboratory study to test the impact of crop residue on flow hydraulic conditions on sandy clay loam soil. They proposed that, due to the increase in the viscous forces from the physical interference of residue on runoff, soil surface residue cover caused an increase in water flow depth and hydraulic roughness which decreased the mean flow velocity, thus contributing to a reduction in interrill soil detachment and transport rate. Xu et al. (2015a,b) examined rill flow characteristics by conducting an indoor experiment with and without cornstalk buffer strip. Their results showed that rill flow velocity was decreased and rill flow energy was consumed after it went through the cornstalk buffer strip. Shear stress, unit stream power, and unit energy of cross section are basic hydrodynamic parameters used to evaluate soil detachment rates and characterize critical conditions required to initiate soil erosion (Nearing et al., 1997; An et al., 2012; Reichert and Norton, 2013; Zhang et al., 2015). Although hydrodynamic understanding of soil erosion has received more attention lately, the hydrodynamic characteristics associated with organic mulches are still unclear and need more quantification and understandings. The Loess Plateau is well known for its serious soil erosion caused by concentrated annual precipitation with intensive rainfall, steep slopes, less vegetation cover, and highly erodible silty soils (Cai, 2001). Although the Grain for Green Project (conversion steep slope farmlands to permanent vegetation cover), which is a national ecological project, has greatly increased the vegetation cover in this region, there are still large areas of farmlands needed for the food security on the Loess Plateau (Zhao et al., 2013; Chen et al., 2015). Corn (Zea mays L.) is one of the most common crops grown on the Loess Plateau in order to produce a large amount of grain necessary for local people. As one kind of byproduct, cornstalks are often utilized as biofuel or animal feed, which may contribute to air pollution (Li et al., 2002). Cornstalks are also pulverized and left on farmland to increase soil organic matter, but this method is not easy to implement on steep slopes of the Loess Plateau. So, it may be environmentally-friendly and sustainable to set up cornstalk buffer strips on loessial hillslopes for soil erosion control. Rainfall simulations have been recognized as an important method for water erosion research (Cerdà, 1998). Rainfall simulation is considered by several studies as a rapid and efficient method to study erosion, which can be better controlled than natural rainfalls (Cerdà, 1997; Iserloh et al., 2013a; Prosdocimi et al., 2016a,b). It has been widely used to assess the impact of several factors on soil erosion, such as slope, soil type, soil moisture, aggregate stability, surface structure, and vegetation cover on soil erosion processes (Arnaez et al., 2007; Blavet et al., 2009; Iserloh et al., 2012; Lassu et al., 2015; Marzen et al., 2015; Xiao et al., 2015). Field rainfall simulation experiments are often carried out with small portable rainfall simulators (Iserloh et al., 2012, 2013b; León et al., 2013; Rodrigo Comino et al., 2015, 2016a,b,c).

While laboratory-based rainfall simulations are often conducted with fixed rainfall simulator systems that can provide special requirements for laboratory environments that enable a large range of hydrologic, pedologic, and surface treatment conditions (de Lima and Singh, 2002; de Lima et al., 2003; Zhang et al., 2010; Shen et al., 2015; Li et al., 2016). The main goals of this research performed under laboratory conditions were to: a) quantify the reduction of soil erosion induced by a cornstalk buffer strip; b) determine the relationship between runoff and sediment yield on the hillslope; c) enhance the hydrodynamic understanding of cornstalk buffer effects on hillslope soil erosion processes. 2. Materials and methods 2.1. Experimental materials 2.1.1. Rainfall simulator system The experiments were carried out using a rainfall simulator under laboratory conditions in the rainfall simulation laboratory of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Yangling City, Shaanxi Province, China. A down sprinkler rainfall simulator system was used (He et al., 2014). This rainfall simulator system consists of three sets of nozzles in which the rainfall intensity can be set to the range from 30 to 350 mm h−1 by adjusting the nozzle size and water pressure. The nozzle type used in this study is SP (1.9 cm) type developed by the Institute of Soil and Water Conservation, Chinese Academy of Sciences & Ministry of Water Resources. The nozzles were installed 18 m above the ground as this height was enough for the majority raindrops to reach terminal velocity. Spatial distribution of rainfall and its intensity was measured using 10 rows and 3 columns of equally spaced rain gauges. The spatial uniformity of the simulated rainfall was controlled to above 90%. When rainfall intensity was set to 100 mm h−1, the simulated raindrop diameters were 0.2 to 3.1 mm with 85.7% (± 2.4%) of raindrop diameters less than 1.0 mm as calibrated by the stain method (Cerdà et al., 1997; Wang et al., 2015). Prior to the experiments, calibration of rainfall intensity was carried out in order that the tested rainfall intensity reached the requirement. 2.1.2. Soil bed A 10-m long, 3-m wide and 0.5-m deep soil pan with many drainage holes (2 cm aperture) at the bottom was used in the experiments. The soil pan can be inclined to the slope gradient from 0 to 30° with adjustment steps of 0.5°. A runoff collector was installed on the soil pan outlet and used to collect the sediment and runoff samples during the rainfall simulation. In this study, the soil pan was set to 20° which was the average slope gradient for rill erosion development on farmland of the Loess Plateau. The soil used in this study was loessial soil (fine-silty and mixed), classified as a Calcic Cambisols (USDA NRCS, 1999). Tested soils were collected from the top layer (20 cm) in the Ap horizon of a well-drained farmland site (tilled by hand) in Ansai County (36°45′N, 109°11′E), Yan'an City, Shaanxi Province, which is located in the hilly-gullied region of the Loess Plateau in northwest China. The soil texture was 28.3% sand (N 50 μm), 58.1% silt (50–2 μm), 13.6% clay content (b2 μm) determined by pipette method according to USDA soil classification system. Soil organic matter was 5.9 g kg−1 determined by the potassium dichromate oxidation-external heating method. The pH in water was 7.95, measured with a 1:2.5 solid-to-water ratio on a weight basis. Prior to the experiment, the soil was air-dried at 25 °C, and then, big clods were broken by hand into subangular-blocky clods less than 4 cm in size, but was not sieved and ground to keep the in-situ soil aggregation fabric. Soil water content was tested to calculate the soil amount needed for packing the soil pan. The lowest 10 cm of soil pan was filled with sand to allow free drainage of excess water. A highly permeable cloth was spread on the sand surface to separate the sand layer from the soil

X. Xu et al. / Catena 149 (2017) 417–425

layer. A 10-cm plow pan layer at a soil bulk density of 1.35 g cm−3 was packed above the sand layer, and a 20-cm tilled layer with a bulk density of 1.10 g cm−3 was packed above the plow pan layer. During the packing process, both the plow pan and tilled layer were packed in 5-cm increments, and each layer was raked lightly before the next layer was packed to ensure uniformity and continuity in soil structure. Manual tillage with a shovel (conventional tillage method on the Loess Plateau) was applied on the soil bed to a depth of approximately 20 cm along the contour line to simulate the natural tillage practice on croplands. To accelerate the surface channel development rate and simulate the natural slope condition on the Loess Plateau, a preformed rill was made on the mid-slope area of the soil bed (Fig. 1). According to the rill distribution and morphological characteristics on the loessial hillslope (Shen et al., 2015), a 3-m long, 25-cm wide and 15-cm deep rill was made from 5 to 8 m of the slope length of the soil bed, and soil materials in this rill were removed from the soil pan. After making the rill, the soil bed was self-settled for 48 h.

2.1.3. Cornstalk buffer strip Corn (Zea mays L.) is one of the most popular crops and occupies a large area of farmland on the Loess Plateau. Whole plant cornstalks were found to be more efficient than clipped short cornstalks in controlling soil erosion (Xu et al., 2015a), so this method was applied in this study. The whole plant cornstalks were collected by using a sickle in Yangling (south fringe area of the Loess Plateau, where rainfall experiments were conducted) after harvest at August of 2014, and they were air dried before being applied on the soil bed.

419

2.2. Experimental design A total of 24 rainfall events (2 repetitions of 4 different experimental runs, each run consisting of 3 successive rainfall events) were conducted (Fig. 1). According to the standard of high intensity and short duration rainstorms in summer on the Loess Plateau (I5 = 1.52 mm min−1) (Zhou and Wang, 1987), the rainfall intensity of this study was set to 100 mm h−1 (1.67 mm min−1) for each event. Two treatments, with and without cornstalk buffer strip, were tested in four experimental runs to evaluate the soil erosion control effectiveness and timing effects of corn buffer strips (Fig. 1). All experimental runs involved three successive rainfall events and the duration of each rainfall event was 30 min. As a result, a total of 90 min rainfall duration and 150 mm precipitation were carried out on the soil bed for each experimental run. 24 h after the former rainfall event, the next rainfall event was conducted to allow water redistribution within the soil bed (Li and Shao, 2008). As shown in Fig. 1, the control run was conducted without cornstalk buffer strip during any of three rainfall events; the late buffer run was conducted with cornstalk buffer strip applied prior to the third rainfall event; the early buffer run was conducted with cornstalk buffer strip applied prior to the second and removed before the third event; the continuous buffer experimental run was implemented with cornstalk buffer strip applied prior to all three successive rainfall events. Cornstalks were laid on the soil surface as a 3 m long by 1 m wide strip perpendicular to and immediately above the initial rill head at about 4.3 m to 5.3 m of slope length for each event (Fig. 1). The thickness for cornstalk buffer strip was the diameter of one plant of cornstalk,

Fig. 1. Different runs applying cornstalk buffer strip mulching on the soil bed surface at different rainfall events: (1) no buffer, (2) buffer strip applied before third rain, (3) buffer strip applied before second rain, (4) continuous buffer strip before all three rains.

420

X. Xu et al. / Catena 149 (2017) 417–425

approximately 5 cm. Mulching density was controlled to be 1 t ha−1 for whole soil bed surface or in other words 10 t ha−1 for the cornstalk buffer strip mulching area. 2.3. Experimental procedure Before each experimental run, a pre-soak rain was applied at 30 mm h−1 for around 25 min to the soil surface at a 3° slope gradient until surface runoff occurred. The purpose of pre-soaking was to ensure comparative uniformity of the surface soil moisture and roughness condition. A plastic sheet covered the soil bed to prevent soil moisture evaporation and allow the soil water equilibrating with depth for 24 h after pre-soaking rain and prior to the experimental runs. During each rainfall event, once runoff occurred, sediment and runoff samples were taken succinctly at the soil pan outlet for the first 1 or 2 min and then adapted stepwise to 2-min intervals when the discharge reached steady state. During the rainfall events, surface runoff velocity (Vs) and flow depth were measured at five slope sections (1, 3, 5, 7 and 9 m of slope length) along the soil bed at 6-min intervals. Vs was measured by using KMnO4 dye tracer method. The time of tracer movement to a prescribed distance (0.5 or 1.0 m) was determined based on the color-front propagation using a stopwatch. Flow depth was measured perpendicularly to the surface using a thin ruler and read to 0.1 mm precision. As a cornstalk buffer strip was set at 4.3 m to 5.3 m of slope length in the rainfall events with a buffer, flow velocities at 5 m of slope length were measured from 5.3 m to 6.3 m of slope length. The runoff/sediment collection containers used in this experiment were 15-l buckets and the runoff was weighed with a platform scale. After sufficient time for sediment settling, samples were decanted, dried in an oven at 105 °C for 48 h until the weight did not change, and then weighed to calculate runoff and sediment yields. 2.4. Data analysis Vs was used to estimate the mean flow velocity (V) by the formula: V ¼ kVs

ð1Þ

where Vs is surface flow velocity (cm s−1); V is mean flow velocity (cm s−1); k is a coefficient that is 0.67 for laminar flow, 0.7 for transition flow, and 0.8 for turbulence flow (Li et al., 1996). The Reynolds number (Re) and the Froude number (Fr) were used to reflect the flow regime and calculated as follows: Re ¼

VR V Fr ¼ pffiffiffiffiffiffi ν gR

ð2Þ

where v is kinematic viscosity (cm2 s−1) determined at the test temperature (t) by ν = 0.01775 / (1 + 0.0337 t + 0.000221 t2); R is hydraulic radius (cm); g = 980 cm s−2. The Darcy-Weisbach friction coefficient (f) characterizing the flow retardation was calculated by Eq. (3) (Abrahams et al., 1986): f ¼

8gRJ V2

ð3Þ

where J is surface slope (m m−1) calculated as the tangent of the slope degree. Shear stress was calculated according to Eq. (4) (Foster et al., 1984): τ ¼ γRJ

ð4Þ

where τ is shear stress (Pa); γ is the gravity of water (N m−3). Yang (1973) presented the concept of unit stream power according to the equation of sediment transport. Unit stream power was

calculated by Eq. (5): P¼

dy dx dy ¼ ¼ VJ dt dt dx

ð5Þ

where P is unit stream power (m s−1). Unit energy of cross section (E) was the sum of water potential energy and kinetic energy and calculated by Eq. (6) (Zhang et al., 2015): E¼

aV 2 þh 2g

ð6Þ

where h is flow depth (cm); a is correction coefficient for kinetic energy. Comparisons of flow hydraulic and hydrodynamic characteristics at different positions were conducted using least significant difference (LSD) test, and the values were statistically significant at the 95% confidence. 3. Results and discussions 3.1. Runoff and sediment yield Cornstalk buffer strip delayed the runoff occurrence time and advanced the runoff ending time (Table 1). Compared with the control run, buffer strips delayed the runoff occurrence by 0.22 to 0.29 min and advanced the runoff ending time by 0.11 to 2.34 min. Air dried cornstalk buffer strips on the soil bed surface retarded the surface runoff. The upslope runoff was intercepted, restricting convergence to down slope runoff channels which delayed the runoff occurrence time and shortened the runoff ending time. Due to pre-soaking rain, surface runoff reached a steady rate quickly. The cornstalk buffer strip retarded runoff initiation and runoff rate, thereby promoting infiltration and changing the hillslope hydrological condition. Runoff volume for the continuous buffer run (Run 4: all three rainfall events with buffer) was decreased by 8.79 mm compared to the control run with no buffer strip. Runoff volume of other two experimental runs in which the buffer strip was applied for one of three rainfall events (Run 2: third rainfall events with buffer and Run 3: second rainfall events with buffer) were only decreased by 3.59 and 2.74 mm, respectively. Infiltration volume of continuous buffer run was increased by 77.4% compared with the control run, and the infiltration volumes of the other two experimental runs with a buffer were also increased by 23.9% to 31.2%. Cornstalk buffer strip obviously enhanced the infiltration volume but the effects on runoff volume appeared minor due to the magnitude of the runoff volume. The continuous buffer run (4) experienced a 29.1% sediment yield reduction compared with the control run, while the intermittent buffer runs (2 and 3) only had a 2.0% and 9.1% reduction in sediment yield, respectively. Moreover, the early buffer run (3) had a larger reduction in sediment yield than late buffer run (2). As a result, soil conservation measures such as cornstalk buffer strip should be applied on the farmland as soon as possible to get better results, which corresponds well with other studies (Bradford and Huang, 1994; Prosdocimi et al., 2016a,b). Excess infiltration runoff caused by high intensity rainfall and low soil surface infiltration capacity is the most common runoff regime on the Loess Plateau. This situation was also obtained in severe soil erosion on loessial hillslopes (Huang et al., 2003). Cornstalk buffer strip increased the infiltration rate and reduced the runoff amount, which could alleviate the excess infiltration runoff regime to a certain extent and reduce soil particle detachment and transport by runoff. In the area where soil was mulched with cornstalk, raindrop impact on the soil surface were eliminated which also contributes to the reduction of sediment yield (Marzen et al., 2015; Xiao et al., 2015). By shortening the slope length for runoff convergence and changing the hillslope hydrological condition, cornstalk buffer strip reduced the sediment yield

X. Xu et al. / Catena 149 (2017) 417–425

421

Table 1 Average total runoff, infiltration and sediment yield for each rainfall event.

Experimental

Rainfall

Runoff threshold

Runoff

Runoff

Runoff

Infiltrated

Sediment

run

event

for soil erosion

occurrence

ending time

volume

volume

yield (kgm–2)

initiation b/a

time (min)

(min)

(mm)

(mm)

(mm min–1) 1. Control run

2. Late buffer run

3. Early buffer run

4. Continuous buffer run

1

0.241

0.30(0.02)

33.98(0.34)

43.92(0.77)

6.08(0.77)

13.27(0.88)

2

0.206

0.23(0.01)

35.97(0.40)

46.60(0.95)

3.40(0.95)

17.55(1.56)

3

0.193

0.30(0.02)

36.58(0.12)

48.04(0.98)

1.96(0.98)

14.59(1.28)

Total

138.56(1.75)

11.44(1.75)

45.41(1.31)

1

0.282

0.30(0.03)

34.15(0.27)

43.30(1.02)

6.70(1.02)

13.60(0.61)

2

0.185

0.28(0.00)

35.64(0.59)

45.36(0.72)

4.64(0.72)

18.40(1.14)

3

0.196

0.58(0.05)

36.47(0.18)

46.31(0.46)

3.69(0.46)

12.51(0.75)

Total

134.97(2.04)

15.03(2.04)

44.51(1.34)

1

0.254

0.30(0.00)

34.12(0.24)

43.80(0.94)

6.20(0.94)

13.63(0.70)

2

0.266

0.50(0.02)

35.47(0.32)

45.31(0.62)

4.69(0.62)

16.70(1.12)

3

0.128

0.30(0.02)

35.17(0.57)

46.71(0.73)

3.29(0.73)

10.95(0.52)

Total

135.82(1.57)

14.18(1.57)

41.28(1.22)

1

0.310

0.53(0.03)

32.62(0.41)

39.30(1.04)

10.70(1.04)

9.46(1.21)

2

0.241

0.52(0.00)

33.63(0.20)

45.14(0.98)

4.68(0.98)

12.94(0.28)

3

0.260

0.52(0.01)

35.21(0.08)

45.27(0.38)

4.73(0.38)

9.77(0.62)

Total

129.71(1.96)

20.29(1.96)

32.17(1.64)

Note: rows within gray background were the rainfall event with cornstalk buffer strip

and controlled soil erosion on loessial hillslope. Similar erosion control results were also found using other materials and application methods (Gilley et al., 1986; Jordán et al., 2010; Montenegro et al., 2013). 3.2. Relationship between runoff and sediment A linear relationship between runoff and sediment was described by studies on the loessial hillslope (Zheng et al., 2008, 2012). Fig. 2 shows the relationships between the sediment rate and the runoff rate in different rainfall events in this study which were described by: SR ¼ aRR−b

ð7Þ

where SR is the sediment rate (g min− 1 m− 2); RR is the runoff rate (mm min−1 m−2); a and b are coefficients. The coefficient a was able to reflect the relationship between sediment rate and runoff rate, whose physical meaning could be described as the ability of runoff to detach and transport sediment. The values of a illustrate the soil erosion intensity induced by different runoff rates and soil surface management. At the same time, the ratio of b to a (b/a) was able to illustrate the runoff rate required to initiate the soil loss, its value can be recognized as the runoff rate threshold for soil erosion initiation (Table 1). For the control run without a buffer strip, the a value in the second rainfall event was much larger than that in the first and third rainfall event (Fig. 2). This can be explained by the changing of water erosion patterns and the rill development stages (Berger et al., 2010; Shen et al., 2015). Splash and sheet erosion were the dominate erosion processes before rills formed in the first rain, so the sediment yield was relatively small. Headcut retreat of the original constructed rill along with sidewall collapse and deep-cut erosion accelerated the rill development processes during the second rain, which resulted in the greatest

sediment yield among the three rainfall events. During the third rain, rills eroded down to the plow pan which restricted further rill development and as a result the sediment yield in the third rain was smaller than that in second rain. Compared with the control run without buffer, obvious decreases of a values and increases in the runoff rate threshold (b/a values) could be seen in continuous buffer run (Table 1). The values of a in the three respective successive rainfall events with cornstalk buffer strip was 10.2%, 18.6% and 24.8% lower than those without cornstalk buffer strip. The threshold values in three respective rainfall events with buffer were 29.2%, 14.3%, and 36.8% higher than those without buffer. It can be concluded that more runoff was required to initiate soil erosion after cornstalk buffer strip was applied. As a result, the effectiveness of cornstalk buffer strip on reducing soil erosion can be illustrated by these two coefficients. A shift to a higher threshold by applying a buffer strip not only results in later initiation of runoff but also less sediment yield. The buffer strip was most affective at increasing the threshold when applied early. 3.3. Mulching impacts on flow hydraulic parameters and hydrodynamic mechanisms 3.3.1. Flow velocity along the slope Average flow velocity during the rainfall events increased when the slope length was greater (Fig. 3). At 1 m of slope length positions, sheet flow velocities varied from 9.64 to 13.94 cm s−1 with a standard deviation of 2.30 cm s− 1, while rill flow velocities varied from 16.83 to 24.63 cm s− 1 with a standard deviation of 3.31 cm s− 1. Relatively small standard deviations showed the good repeatability of experiment, this could be also proved by the flow velocities variations at 3 m of slope length. At 5 m of slope length where cornstalk buffer strips were applied, average sheet flow velocity of interrill areas in rainfall events

422

X. Xu et al. / Catena 149 (2017) 417–425

Fig. 2. Runoff-sediment linear relationships for each rainfall event. Note that runs with a buffer strip are in gray color.

with a buffer were 5.62 cm s− 1 greater than those in rainfall events without a buffer; while the average rill flow velocity in rainfall events with a buffer were 6.54 cm s−1 smaller than in rainfall events without a buffer. At the 7 m of slope length, flow velocity showed the same trend as that at the 5 m of slope length, i.e. cornstalk buffer strip dispersed the upslope runoff which transformed concentrated flow in rills to interrill sheet flow. As the slope length increased, sheet flow velocity and rill flow velocity at 9 m of slope length in rainfall events without buffer were both larger than those in rainfall events with buffer.

Fig. 3. Average sheet flow and rill flow velocity along the slope. Note that flow velocities in covered treatment at 5 m of slope length were measured from 5.3 m to 6.3 m of slope length.

Rill flow dominates sediment delivery on steep slopes (Peng, et al., 2015), and rill areas receive the runoff and sediment delivered from interrill areas (Knapen et al., 2007). As a result, rill flow merits more attention in soil erosion control. Similar result was also obtained by Wen et al. (2014) in the Mollisol region of northeastern China, showing that rill flow velocities with a buffer were smaller than those without a buffer, which reduced soil erosion.

3.3.2. Hydraulic and hydrodynamic characteristics Flow hydraulic and hydrodynamic characteristics below cornstalk buffer strip mulching position (5.5 to 6 m of slope length) were calculated and displayed in Table 2. Compared with the rainfall events without buffer, sheet flow velocity significantly increased but rill flow velocity decreased in rainfall events with a buffer. According to the values of Re and Fr, it can be concluded that both sheet flow and rill flow in this slope section represented transition flow conditions; sheet flow was rapid while rill flow was streaming flow. Furthermore, f of sheet flow decreased 9.7% while f of rill flow increased 12.4% after cornstalk buffer strips were applied. Other hydrodynamic characteristics, e.g., τ, P and E, corresponded well to flow conditions, their values for sheet flow increased 27.3%, 15.9% and 28.0%, respectively after cornstalk buffer strips were applied; while on the other hand, their values for rill flow decreased 25.1%, 13.5% and 25.5%, respectively, due to the influence of the buffer strips. Within rills, cornstalk buffer strips numerically increased the flow resistance, and reduced the unit stream power, and significantly reduced the shear stress and unit energy of cross section. These factors combined to reduce the soil erosion. Compared with the sheet flow, concentrated flow definitely contributes more to slope erosion (Knapen et al., 2007; Al-Hamdan et al., 2012), so the total erosion amount reduced even though the sheet flow velocity was significantly increased by the buffer strip.

X. Xu et al. / Catena 149 (2017) 417–425

423

Table 2 Average total flow hydraulic and hydrodynamic characteristics below the cornstalk buffer mulching position (5.3 to 6.3 m of slope length). Flow regime

Surface treatment

Average flow velocity V (cm s−1)

Reynolds number Re

Froude number Fr

Darcy–Weisbach friction f

Average shear stress τ (Pa)

Unit stream power w (m s−1)

Unit energy of cross section E (cm)

Sheet flow

Without buffer With buffer Without buffer With buffer

24.31(3.26)b

1121.0(364.6)c

1.21(0.19)a

2.17(0.84)bc

15.31(4.27)c

0.088(0.012)ab

0.75(0.16)c

27.90(1.34)a 24.51(3.50)ab

1648.0(633.7)bc 1.24(0.16)a 3820.8 (756. 6)a 0.60(0.09)b

1.96(0.51)c 7.32(2.18)ab

19.49(6.67)c 51.98(5.77)a

0.102(0.005)a 0.089(0.013)ab

0.96(0.23)c 2.04(0.24)a

21.20(5.00)b

2486.8(789.6)b

8.23(4.61)a

38.91(6.60)b

0.077(0.018)b

1.52(0.37)b

Rill flow

0.61(0.13)b

Note: different letters (a, b, c) indicate that hydraulic and hydrodynamic characteristics within a column are significantly different at 0.05 level.

3.3.3. Flow regime zoning According to the open channel flow theory, Re = 500 and 5000 are the thresholds that separate laminar flow, transition flow and turbulent flow, while Fr = 1 separates subcritical from supercritical flow. In the log-log plot of runoff velocity and hydraulic radius, three boundary lines (Re = 500, Re = 5000, and Fr = 1) were drawn to separate sheet flow (Fig. 4a) and rill flow (Fig. 4b) into six flow regimes (Zhang

et al., 2014; Xu et al., 2015b). In treatments without cornstalk buffer, sheet flow never showed turbulent flow regime while concentrated flow in rills reached turbulent conditions. After cornstalk buffer strip was applied, sheet flow shifted from subcritical to supercritical flow and from laminar to transition and/or turbulent flow, and some data with buffer reached supercritical-turbulent flow zone. In contrasts, rill flow switched from supercritical-turbulent flow to subcritical-transition

Fig. 4. Flow regime zoning of sheet flow (a) and concentrated flow (b).

424

X. Xu et al. / Catena 149 (2017) 417–425

Fig. 5. The relationship between sediment rate and shear stress, unit stream power, as well as unit energy of cross section.

flow when the cornstalk buffer applied. According to Zhang et al. (2014), flow regime is decided by a large range of factors in which surface mulching is one of the most important and often leads towards subcritical and laminar flow conditions which contributed to reducing the sediment yield. In our study, buffers led rill flow from turbulent to transition conditions without reaching fully laminar conditions. 3.3.4. Hydrodynamic understanding of sediment yield Based on previous studies focusing on calculating the critical hydrodynamic parameters to initiate raindrop erosion and rill erosion (An et al., 2012; Shen et al., 2016), the critical values, such as shear stress, unit stream power, and unit energy of cross section, for initiating soil erosion could be obtained by establishing relationships between sediment yield and excess shear, stream power, and unit energy properties. As illustrated in Fig. 5, sediment yield increased linearly with the increase of shear stress, unit stream power, and unit energy of cross section. In rainfall events without buffer, the linear relationship could be described as follows:   Sr ¼ 0:417ðτ–1:207Þ R2 ¼ 0:91; n ¼ 14   Sr ¼ 248:24ðP–0:0291Þ R2 ¼ 0:80; n ¼ 14   Sr ¼ 16:46ðE–0:161Þ R2 ¼ 0:82; n ¼ 14

ð8Þ

In rainfall events with buffer, the linear relationship could be expressed as:   Sr ¼ 0:396ðτ–5:916Þ R2 ¼ 0:88; n ¼ 10   Sr ¼ 176:17ðP–0:0346Þ R2 ¼ 0:84; n ¼ 10   Sr ¼ 11:04ðE–0:222Þ R2 ¼ 0:86; n ¼ 10

initiation of soil erosion in order to enhance the understanding of their hydrodynamic impacts. The results showed that 1) cornstalk buffer strips delayed runoff occurrence time and advanced the runoff ending time, it also reduced the total runoff amount and increased the infiltration volume. Continuous buffer tests had a reduction of 29.1% in total sediment yield compared with the control run without a buffer strip. Early application of a buffer had a larger reduction in sediment yield than late buffer application; 2) a linear relationship was fitted between runoff and sediment. According to the coefficients of runoff-sediment relationship, it could be concluded that cornstalk buffer strip decreased the sediment concentration and increased the runoff rate threshold for soil erosion initiation; 3) while the buffer strip increased the sheet flow velocity in interrill areas, it decreased the rill flow velocity and promoted a shift in rills from supercritical turbulent towards subcritical laminar flow conditions, which is meaningful for reducing soil erosion. Cornstalk buffer strip can increase the flow resistance, reduce shear stress, unit stream power, unit energy of cross section, thereby reducing soil erosion; 4) compared with non-buffer events, the critical shear stress increased from 1.207 to 5.916 Pa, the critical unit stream power increased from 0.0291 to 0.0346 m s−1, and the critical unit energy of cross section increased from 0.161 to 0.222 cm, respectively in events with buffer strip. As a result, cornstalk buffer strip enhanced the critical hydrodynamic forces required for the initiation of soil erosion and reduced the hydrodynamic forces operating on the soil surface which contributed to the reduction of hillslope sediment yield. Acknowledgments

ð9Þ

where Sr is sediment rate (kg min−1); τ is shear stress (Pa); P is unit stream power (m s−1); E is unit energy of cross section (cm). The intercept, i.e., Sr = 0, represents the critical hydrodynamic force required to initiate soil loss. For rainfall events without buffer, critical shear stress, critical unit stream power, and critical unit energy of cross section were 1.207 Pa, 0.0291 m s−1, and 0.161 cm, respectively. In rainfall events with buffer, the corresponding values calculated were 5.916 Pa, 0.0346 m s−1, and 0.222 cm, respectively. The results indicated that cornstalk buffer strip caused the critical shear stress, the critical unit stream power, and the critical stream power for soil erosion initiation to increase by 390.1%, 18.9%, and 37.9%, respectively. Buffer strip could be used on hillslopes to reduce the sediment yield by enhancing the critical hydrodynamic forces required to initiate soil erosion. 4. Conclusions Rainfall simulation experiments under high intensity erosive rainfall events (100 mm h−1) and high inclination (20°) were conducted to investigate the impact of cornstalk buffer strips on the processes of

This study was supported by the National Natural Science Foundation of China (No. 41271299), Special-Funds of Scientific Research Programs of State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau (A314021403-C2), and China Scholarship Council funds. We thank the editor and reviewers for their suggestions, which greatly improved our paper. References Abrahams, A.D., Parsons, A.J., Luk, S.H., 1986. Resistance to overland flow on desert hillslopes. J. Hydrol. 88, 343–363. Al-Hamdan, O.Z., Pierson, F.B., Nearing, M.A., Stone, J.J., Williams, C.J., Moffet, C.A., Kormos, P.R., Boll, J., Weltz, M.A., 2012. Characteristics of concentrated flow hydraulics for rangeland ecosystems: implications for hydrologic modeling. Earth Surf. Process. Landf. 37 (2), 157–168. An, J., Zheng, F.L., Lu, J., Li, G.F., 2012. Investigating the role of raindrop impact on hydrodynamic mechanism of soil erosion under simulated rainfall conditions. Soil Sci. 177 (8), 517–526. Arnaez, J., Lasanta, T., Ruiz-Flaño, P., Ortigosa, L., 2007. Factors affecting runoff and erosion under simulated rainfall in Mediterranean vineyards. Soil Tillage Res. 93, 324–334. Barling, R.D., Moore, I.D., 1994. Role of buffer strips in management of waterway pollution: a review. Environ. Manag. 18 (4), 543–558. Berger, C., Schulze, M., Rieke-Zapp, D., Schlunegger, F., 2010. Rill development and soil erosion: a laboratory study of slope and rainfall intensity. Earth Surf. Process. Landf. 35 (12), 1456–1467. Blavet, D., De Noni, G., Le Bissonnais, Y., Leonard, M., Maillo, L., Laurent, J.Y., Asseline, J., Leprun, J.C., Arshad, M.A., Roose, E., 2009. Effect of land use and management on the early stages of soil water erosion in French Mediterranean vineyards. Soil Tillage Res. 106, 124–136.

X. Xu et al. / Catena 149 (2017) 417–425 Bradford, J.M., Huang, C.-h., 1994. Interrill soil erosion as affected by tillage and residue cover. Soil Tillage Res. 31, 353–361. Brown, L.C., Foster, G.R., Beasley, D.B., 1989. Rill erosion as affected by incorporated crop residue and seasonal consolidation. Trans. ASAE 32 (6), 1967–1978. Cai, Q.G., 2001. Soil erosion and management on the Loess Plateau. J. Geogr. Sci. 11 (1), 53–70. Cassol, E.A., Cantalice, J.R.B., Reichert, J.M., Mondardo, A., 2004. Interrill surface runoff and soil detachment on a sandy clay loam soil with residue cover. Pesq. Agrop. Brasileira 39 (7), 685–690. Cerdà, A., 1997. Soil erosion after land abandonment in a semiarid environment of southeastern Spain. Arid Soil Res. Rehabil. 11, 163–176. Cerdà, A., 1998. Effect of climate on surface flow along a climatological gradient in Israel: a field rainfall simulation approach. J. Arid Environ. 38, 145–159. Cerdà, A., Ibáñez, S., Calvo, A., 1997. Design and operation of a small and portable rainfall simulator for rugged terrain. Soil Technol. 11, 163–170. Chen, Y., Wang, K., Lin, Y., Shi, W., Song, Y., He, X., 2015. Balancing green and grain trade. Nat. Geosci. 8 (10), 739–741. de Lima, J.L.M.P., Singh, V.P., 2002. The influence of the pattern of moving rainstorms on overland flow. Adv. Water Resour. 25, 817–828. de Lima, J.L.M.P., Singh, V.P., de Lima, M.I.P., 2003. The influence of storm movement on water erosion: storm direction and velocity effects. Catena 52, 39–56. Döring, T.F., Brandt, M., Heβ, J., Finckh, M.R., Saucke, H., 2005. Effects of straw mulch on soil nitrate dynamics, weeds, yield and soil erosion in organically grown potatoes. Field Crop Res. 94, 238–249. Foster, G.R., Huggins, L.F., Meyer, L.D., 1984. A laboratory study of rill hydraulics: II. Shear stress relationships. Trans. ASAE 27 (3), 797–0804. Gilley, J.E., Kottwitz, E.R., 1995. Darcy-Weisbach roughness coefficients for surfaces with residue and gravel cover. Trans. ASAE 38 (2), 539–544. Gilley, J.E., Finkner, S.C., Spomer, R.G., Mielke, L.N., 1986. Runoff and erosion as affected by corn residue: part I. Total Losses. Trans. ASAE 29 (1), 157–160. Guo, T., Wang, Q., Li, D., Zhuang, J., Wu, L., 2013. Flow hydraulic characteristic effect on sediment and solute transport on slope erosion. Catena 107, 145–153. He, J., Li, X., Jia, L., Gong, H., Cai, Q., 2014. Experimental study of rill evolution processes and relationships between runoff and erosion on clay loam and loess. Soil Sci. Soc. Am. J. 78 (5), 1716–1725. Huang, M., Zhang, L., Gallichand, J., 2003. Runoff responses to afforestation in a watershed of the Loess Plateau, China. Hydrol. Process. 17 (13), 2599–2609. Hussein, J., Yu, B., Ghadiri, H., Rose, C., 2007. Prediction of surface flow hydrology and sediment retention upslope of a vetiver buffer strip. J. Hydrol. 338 (3), 261–272. Iserloh, T., Fister, W., Seeger, M., Willger, H., Ries, J.B., 2012. A small portable rainfall simulator for reproducible experiments on soil erosion. Soil Tillage Res. 124, 131–137. Iserloh, T., Ries, J.B., Arnáez, J., Boix-Fayos, C., Butzen, V., Cerdà, A., Echeverría, M.T., Fernández-Gálvez, J., Fister, W., Geißler, C., Gómez, J.A., Gómez-Macpherson, H., Kuhn, N.J., Lázaro, R., León, F.J., Martínez-Mena, M., Martínez-Murillo, J.F., Marzen, M., Mingorance, M.D., Ortigosa, L., Peters, P., Regüés, D., Ruiz-Sinoga, J.D., Scholten, T., Seeger, M., Solé-Benet, A., Wengel, R., Wirtz, S., 2013a. European small portable rainfall simulators: a comparison of rainfall characteristics. Catena 110, 100–112. Iserloh, T., Ries, J.B., Cerdà, A., Echeverría, M.T., Fister, W., Geiβler, C., Kuhn, N.J., León, F.J., Peters, P., Schindewolf, M., Schmidt, J., Scholten, T., Seeger, M., 2013b. Comparative measurements with seven rainfall simulators on uniform bare fallow land. Z. Für Geomorphol. Suppl. 57, 11–26. Jordán, A., Zavala, L.M., Gil, J., 2010. Effects of mulching on soil physical properties and runoff under semi-arid conditions in southern Spain. Catena 81 (1), 77–85. Kamara, C.S., 1986. Mulch-tillage effects and soil loss properties on an ultisol in the humid tropics. Soil Tillage Res. 8, 131–144. Knapen, A., Poesen, J., Govers, G., Gyssels, G., Nachtergaele, J., 2007. Resistance of soils to concentrated flow erosion: a review. Earth-Sci. Rev. 80 (1), 75–109. Lal, R., 1976. Soil Erosion Problems on an Alfisol in Western Nigeria and Their Control: Mulching Effect on Runoff and Soil Loss. I.I.T.A. Monograph No. 1. International Institute for Tropical Agriculture, Ibadan, Nigeria. Lassu, T., Seeger, M., Peters, P., Keesstra, S.D., 2015. The Wageningen rainfall simulator: set-up and calibration of an indoor nozzle-type rainfall simulator for soil erosion studies. Land Degrad. Dev. 26, 604–612. León, J., Echeverría, M.T., Badía, D., Martí, C., Álvarez, C.J., 2013. Effectiveness of wood chips cover at reducing erosion in two contrasted burnt soils. Z. Für Geomorphol. Suppl. 57, 27–37. Li, Y., Shao, M., 2008. Infiltration characteristics of soil water on loess slope land under intermittent and repetitive rainfall conditions. Chin. J. Appl. Ecol. 19 (7), 1511–1516 (in Chinese with English abstract). Li, G., Abrahams, A.D., Atkinson, J.F., 1996. Correction factors in the determination of mean velocity of overland flow. Earth Surf. Process. Landf. 21 (6), 509–515. Li, J., Wang, L.X., Shao, M.A., Fan, T.L., 2002. Simulation of maize potential productivity in the Loess Plateau region of China. Acta Agron. Sin. 28 (4), 555–560 (in Chinese with English abstract). Li, G., Zheng, F., Lu, J., Xu, X., Hu, W., Han, Y., 2016. Inflow rate impact on hillslope erosion processes and flow hydrodynamics. Soil Sci. Soc. Am. J. 80 (3), 711–719. Marzen, M., Iserloh, T., Casper, M.C., Ries, J.B., 2015. Quantification of particle detachment by rain splash and wind-driven rain splash. Catena 127, 135–141. Montenegro, A.A.A., Abrantes, J.R.C.B., De Lima, J.L.M.P., Singh, V.P., Santos, T.E.M., 2013. Impact of mulching on soil and water dynamics under intermittent simulated rainfall. Catena 109, 139–149. Mulumba, L.N., Lal, R., 2008. Mulching effects on selected soil physical properties. Soil Tillage Res. 98 (1), 106–111.

425

Nearing, M.A., Norton, L.D., Bulgakov, B.A., Larionov, G.A., West, L.T., Dontsova, K.M., 1997. Hydraulics and erosion in eroding rills. Water Resour. Res. 33 (4), 865–876. Peng, W., Zhang, Z., Zhang, K., 2015. Hydrodynamic characteristics of rill flow on steep slopes. Hydrol. Process. 29 (17), 3677–3686. Prosdocimi, M., Cerdà, A., Tarolli, P., 2016a. Soil water erosion on Mediterranean vineyards: a review. Catena 141, 1–21. Prosdocimi, M., Jordán, A., Tarolli, P., Keesstra, S., Novara, A., Cerdà, A., 2016b. The immediate effectiveness of barley straw mulch in reducing soil erodibility and surface runoff generation in Mediterranean vineyards. Sci. Total Environ. 547, 323–330. Reichert, J.M., Norton, L.D., 2013. Rill and interrill erodibility and sediment characteristics of clayey Australian Vertosols and a Ferrosol. Soil Res. 51, 1–9. Rodrigo Comino, J., Brings, C., Lassu, T., Iserloh, T., Senciales, J., Martínez Murillo, J., Ruiz Sinoga, J., Seeger, M., Ries, J., 2015. Rainfall and human activity impacts on soil losses and rill erosion in vineyards (Ruwer Valley, Germany). Solid Earth. 6, 823–837. Rodrigo Comino, J., Iserloh, T., Lassu, T., Cerdà, A., Keesstra, S.D., Prosdocimi, M., Brings, C., Marzen, M., Ramos, M.C., Senciales, J.M., Ruiz Sinoga, J.D., Seeger, M., Ries, J.B., 2016a. Quantitative comparison of initial soil erosion processes and runoff generation in Spanish and German vineyards. Sci. Total Environ. 565, 1165–1174. Rodrigo Comino, J., Iserloh, T., Morvan, X., Malam Issa, O., Naisse, C., Keesstra, S.D., Cerdà, A., Prosdocimi, M., Arnáez, J., Lasanta, T., Ramos, M.C., Marqués, M.J., Ruiz Colmenero, M., Bienes, R., Ruiz Sinoga, J.D., Seeger, M., Ries, J.B., 2016b. Soil erosion processes in European vineyards: a qualitative comparison of rainfall simulation measurements in Germany, Spain and France. Hydrology 3, 6. Rodrigo Comino, J., Ruiz Sinoga, J.D., Senciales González, J.M., Guerra-Merchán, A., Seeger, M., Ries, J.B., 2016c. High variability of soil erosion and hydrological processes in Mediterranean hillslope vineyards (Montes de Málaga, Spain). Catena 145, 274–284. Shen, H.O., Zheng, F.L., Wen, L.L., Lu, J., Jiang, Y.L., 2015. An experimental study of rill erosion and morphology. Geomorphology 231, 193–201. Shen, H.O., Zheng, F.L., Wen, L.L., Han, Y., Hu, W., 2016. Impacts of rainfall intensity and slope gradient on rill erosion processes at loessial hillslope. Soil Tillage Res. 155, 429–436. Shi, Z.H., Yue, B.J., Wang, L., Fang, N.F., Wang, D., Wu, F.Z., 2013. Effects of mulch cover rate on interrill erosion processes and the size selectivity of eroded sediment on steep slopes[J]. Soil Sci. Soc. Am. J. 77 (1), 257–267. Smets, T., Poesen, J., Knapen, A., 2008. Spatial scale effects on the effectiveness of organic mulches in reducing soil erosion by water. Earth-Sci. Rev. 89 (1), 1–12. Turmel, M.S., Speratti, A., Baudron, F., Verhulst, N., Govaerts, B., 2015. Crop residue management and soil health: a systems analysis. Agric. Syst. 134, 6–16. USDA NRCS, 1999. Soil Taxonomy: A Basic System of Soil Classification for Making and Interpreting Soil Surveys. Agric. Handbook 436. 2nd edition. U.S. Government Printing Office, Washington DC. Wang, H., Gao, J.E., Zhang, M.J., Li, X.H., Zhang, S.L., Jia, L.Z., 2015. Effects of rainfall intensity on groundwater recharge based on simulated rainfall experiments and a groundwater flow model. Catena 127, 80–91. Wen, L.L., Zheng, F.L., Shen, H.O., Gao, Y., 2014. Effects of corn straw mulch buffer in the gully head on gully erosion of sloping cropland in the black soil region of Northeast China. J. Sediment. Res. 6, 73–80 (in Chinese with English abstract). Wilson, G.V., McGregor, K.C., Boykin, D., 2008. Residue impacts on runoff and soil erosion for different corn plant populations. Soil Tillage Res. 99 (2), 300–307. Xiao, L., Hu, Y., Greenwood, P., Kuhn, N.J., 2015. A combined raindrop aggregate destruction test-settling tube (RADT-ST) approach to identify the settling velocity of sediment. Hydrology 2, 176. Xu, X.M., Zheng, F.L., Qin, C., Wu, H.Y., 2015a. Erosion control effects of cornstalk mulching on loess hillslope with gully. Trans. CSAM 46 (8), 130–137 (in Chinese with English abstract). Xu, X.M., Zheng, F.L., Wu, H.Y., Qin, C., 2015b. Impacts of cornstalk mulching buffer strip on rill erosion and its hydrodynamic character. Trans. CSAE 31 (24), 111–119 (in Chinese with English abstract). Yang, Z.T., 1973. Incipient motion and sediment transport. Trans. ASAE 99 (10), 9198–9314. Zhang, G.H., Shen, R.C., Luo, R.T., Cao, Y., Zhang, X.C., 2010. Effects of sediment load on hydraulics of overland flow on steep slopes. Earth Surf. Process. Landf. 35 (15), 1811–1819. Zhang, K.D., Wang, G.Q., Sun, X.M., Wang, J.J., 2014. Hydraulic characteristic of overland flow under different vegetation coverage. Adv. Water Sci. 25 (6), 825–834 (in Chinese with English abstract). Zhang, L.T., Gao, Z.L., Yang, S.W., Li, Y.H., Tian, H.W., 2015. Dynamic processes of soil erosion by runoff on engineered landforms derived from expressway construction: a case study of typical steep spoil heap. Catena 128, 108–121. Zhao, G., Mu, X., Wen, Z., Wang, F., Gao, P., 2013. Soil erosion, conservation, and eco-environment changes in the loess plateau of China. Land Degrad. Dev. 24 (5), 499–510. Zheng, M.G., Cai, Q.G., Cheng, Q.J., 2008. Modelling the runoff-sediment yield relationship using a proportional function in hilly areas of the Loess Plateau, North China. Geomorphology 93 (3), 288–301. Zheng, M.G., Yang, J.S., Qi, D.L., Sun, L.Y., Cai, Q.G., 2012. Flow-sediment relationship as functions of spatial and temporal scales in hilly areas of the Chinese Loess Plateau. Catena 98, 29–40. Zhou, P.H., Wang, Z.L., 1987. Soil erosion rainfall standard in the Loess Plateau. Bull. Soil Water Conserv. 7 (1), 38–44 (in Chinese with English Abstract).