Quantification of upslope and lateral inflow impacts on runoff discharge and soil loss in ephemeral gully systems under laboratory conditions

Journal of Hydrology 579 (2019) 124174

Contents lists available at ScienceDirect

Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol

Research papers

Quantification of upslope and lateral inflow impacts on runoff discharge and soil loss in ephemeral gully systems under laboratory conditions

T

⁎

Ximeng Xua,b, Fenli Zhenga,c, , Glenn V. Wilsond, Xunchang J. Zhange, Chao Qinf, Xu Hea a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Northwest A & F University, Yangling 712100, Shaanxi, PR China b Key Laboratory of Water Cycle and Related Land Surface Processes, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, PR China c Institute of Soil and Water Conservation, CAS & MWR, Yangling 712100, Shaanxi, PR China d USDA-ARS National Sedimentation Laboratory, Oxford 38655, MS, USA e USDA-ARS Grazinglands Research Laboratory, EI Reno 73036, OK, USA f State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, PR China

A R T I C LE I N FO

A B S T R A C T

This manuscript was handled by Marco Borga, Editor-in-Chief, with the assistance of Kelly Kibler, Associate Editor

Overland flow through an ephemeral gully (EG) system integrates flow from upslope areas and lateral side slopes. Lateral overland inflow creates tributaries to the main EG channel for runoff and sediment discharge which exacerbates the EG erosion and local soil degradation. However, research concerning tributary formation by overland flow, especially contributions of overland upslope and lateral inflows to EG erosion, are still lacking. Thus, simulated rainfall and inflow experiments under two erosive rainfall intensities (50 and 100 mm h−1) and two typical slope gradients (15° and 20°) were conducted under representative lateral and upslope overland inflow conditions using an 8-m long, 2-m wide and 0.6-m deep soil pan. The results showed that upslope and lateral inflow both contributed to the runoff connectivity of the EG channel and lateral rills in the EG system. For these simulated conditions, upslope inflow contributions to total runoff and soil loss were 62–78% and 65–81%, respectively, while lateral inflow only contributed around 10%. The contribution differences could be attributed to flow hydrodynamic characteristics in that shear stress and stream power in the EG channel were 4.9–8.6 times greater than those on the lateral slopes. Lateral inflow was important to lateral rill formation, which contributed to imbricated landform and lateral gradient formation process, and consequently promoted runoff and sediment connectivity of the EG system. Soil erosion induced by concentrated flow in channels and sheet flow in interill areas coarsened soil textures, soil particles lager than 0.02 mm increased 12.6% on average, which may contribute to reduction of local soil productivity. This work demonstrates the critical need of preventing both upslope and lateral drainage into EG channels.

Keywords: Ephemeral gully tributaries Flow hydrodynamics Concentrated flow Rill erosion Flow path Soil texture

1. Introduction Ephemeral gullies (EGs) are “small channels eroded by concentrated flow that can be easily filled by normal tillage only to be reformed again in the same location” (Soil Science Society of America, 2008; Zheng et al., 2017). This definition agrees with those for shallow gully erosion proposed by Zhu (1956) and mega-rill erosion proposed by Foster (1986). While the EG term still needs some clarification, EGs tend to have two opposite lateral side-slopes and an upper contributing area (Capra and La Spada, 2015). Surface runoff induces concentrated flow along the thalweg of rills or micro-depressions, which initiates headcuts and the formation of an EG main channel. With time, runoff on lateral

⁎

side-slopes concentrates in rills that feed the EG main channel, and these lateral rills develop into EG branches which can be fed by newly formed rills in previous interrill areas. EG main channel, EG branches and rills are organized with time into a fully developed EG network. Such an EG network increases the hydrologic connectivity of the landscape (Poesen et al., 2003), while also dissecting the landscape into an imbricated landform (Zhu, 1956; Gong et al., 2011). Capra et al. (2009) used EG system to represent “the whole of the main branch (EG channel) and the interconnected tributaries of an EG” to study rainfall characteristics impact on EG erosion. Thus, EG systems can be used as an integration of the entire network contributing to EG channels (Capra et al., 2011; Capra, 2013).

Corresponding author at: No. 26, Xi’nong Road, Institute of Soil and Water Conservation, Yangling, Shaanxi 712100, PR China. E-mail address: fl[email protected] (F. Zheng).

https://doi.org/10.1016/j.jhydrol.2019.124174 Received 30 April 2019; Received in revised form 28 August 2019; Accepted 23 September 2019 Available online 23 September 2019 0022-1694/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

that sediment became coarser after rills developed because of increases in flow depth and runoff energy. However, changes in the soil particle size distribution within the EG systems are still not clear. Former studies on EGs induced soil physical property changes are usually based on ground-truth surveys of fields combined with laboratory analysis of soil samples (Ollobarren et al., 2016; Xu et al., 2016). Few studies have been done to demonstrate erosion selectivity as an EG system develops in response to concentrated flow. Therefore, a series of laboratory rainfall and inflow experiments were conducted under different slope gradients, rainfall intensities, upslope and lateral inflow conditions to quantify how upslope and lateral inflows affect EG erosion. The specific objectives were: 1) to discriminate the contributions of upslope and lateral overland inflows to the runoff and soil loss rates in an EG system, 2) to understand lateral rill flow contributions to runoff connectivity of an EG system, 3) to quantify the flow hydrodynamic characteristics in EG system, and 4) to investigate the EG erosion impacts on soil particle size distribution changes.

Whatever the definition is, EG systems can be the dominant sediment source in agricultural watersheds. Previous studies conducted in U.S., Mediterranean region of Western Europe, Australia, Loess Plateau and the Mollisol region of China all showed that EGs contribute the majority of total soil loss and the contribution rate can even reach up to 100% (Casalí et al., 1999; Bennett et al., 2000a; Wijdenes et al., 2000; Poesen et al., 2003; Hancock and Evans, 2006; Cheng et al., 2007; Zhang et al., 2007; Porto et al., 2014; Capra and La Spada, 2015). Given the high soil loss, EG erosion leads to soil degradation and crop productivity loss (Liu et al., 2013). The reoccurrence over time of rainfall induced overland flow removing large amounts of soil from the EG system, and mechanical tillage operations filling the eroded channels with adjacent surface soil materials can greatly exacerbate the soil losses from an EG system (Nachtergaele et al., 2001; Gordon et al., 2008). These two cyclical events lead to the decline of both topsoil depth (Liu et al., 2013) and soil quality from lateral slopes adjacent to the EG (Xu et al., 2016; Ollobarren et al., 2016; Wilson et al., 2018). Ollobarren et al. (2016) observed the soil quality index (SQI) in an EG system in central Sicily and found the lowest SQI inside of the EG channel and in the nearby areas. Xu et al. (2016) compared the soil physicochemical properties in EG bottom and inter-gully areas and found the soil physical quality properties were predominately degraded in the first stage of EG erosion. In subsequent stages of EG erosion, soilchemical quality, i.e. nutrient losses, dominated the soil degradation. Hydrologic processes like rainfall (Capra et al., 2009; Han et al., 2017), overland flow and subsurface flow (Tebebu et al., 2010; Wilson, 2011; Wilson et al., 2015; Frankl et al., 2016; Zegeye et al., 2018) are the main drivers contributing to the formation and development of EGs. There are numerous subprocesses included, e.g. particle detachment by shear forces, headcutting (Bennett et al., 2000b; Hosseinalizadeh et al., 2019a,b), bed incision (Qin et al., 2019), gully widening (Wells et al., 2013; Qin et al., 2018a), pipe collapse (Bernatek-Jakiel and Poesen, 2018; Kariminejad et al., 2019), tension cracking and mass wasting of gully side-walls (Zegeye et al., 2018). In an EG system driven by surface flow that is mainly discussed in this paper, overland inflow can be divided into upslope inflow and lateral inflow. Upslope inflow is generated from the contributing area above the EG headcut below the watershed boundary line, Lateral inflow is generated from both sides of the channel by concentrated flow in rills, as well as sheet flow from interrill areas. Upslope inflow is related to the upslope drainage topography, soil properties and rainfall characteristics (Casalí et al., 1999; Cheng et al., 2007; Gutiérrez et al., 2009; Capra and La Spada, 2015; Zegeye et al., 2016; Xu et al., 2017). Upslope inflow can significantly increase the runoff velocity, shear stress and energy of flows through the EG channel and hence the EG erosion magnitude (Wu et al., 2004, 2019; Gong et al., 2011; Xu et al., 2017). Xu et al. (2017) conducted controlled laboratory experiments and found that runoff velocities were 23–79% larger with upslope inflow than without, and upslope inflow was more effective than rainfall at increasing runoff and soil erosion. Wu et al. (2019) used a series of field rainfall simulation experiments to investigate different inflow and sediment conditions impact on the EG erosion and found that flow velocities in EG channels were greater than those on the lateral slopes, and sediment concentration increase had a limited effect on the runoff velocities in the EG channel. Given the typical concave topography of EG systems in which the main channel lies in the bottom of a swale with two lateral side-slopes, interconnected tributaries of lateral inflow channels can also be the main source of runoff and sediment contributing to the EG erosion and local soil degradation when an EG system develops. However, studies that combine upslope and lateral inflows are essentially nonexistent and their interactions are not well understood or described especially on steep slope landforms like the Loess Plateau where EG systems are widely distributed. Soil particle size distribution can reflect the size selectivity of erosion, which has great significance on understanding soil erosion and degradation processes (Shi et al., 2012). Wang and Shi (2015) found

2. Materials and methods 2.1. Experimental materials The same side sprinkler rainfall simulator system was used as Xu et al. (2017), which provided spatial uniformity of rainfall and terminal velocity. Raindrop diameters were calibrated by Laser Precipitation Monitor (mode 5.4110.10.200, Adolf Thies GmbH & Co. KG, Göttingen, Germany), varying from 0.2 to 3.8 mm. A slope adjustable soil pan was used to conduct the simulation experiments which is 8-m long, 2-m wide and 0.6-m deep. There are many drainage holes at the bottom with 2 cm aperture. At the upper end of the soil pan, an overflow tank was set to simulate the inflow from an upper contributing area in the field. Gravel was glued on the rigid floor connecting the tank and soil bed, which decreased the flow energy entering the soil bed and acted as baffle structure (Fig. 1a, 1b). Before each experiment, upslope inflow discharge was adjusted by intake valves and calibrated (Li et al., 2016). Lateral inflow to both sides of the soil pan was applied by flexible pipes with holes every 10-cm along the pipe. The inner diameter of the pipe was 12 mm, and the diameter of each hole was 2 mm. The lateral inflow pipe on each side of the soil pan supplied inflow under a constant head onto a waterproof plate on both sides of the soil pans. One end of the waterproof plate was attached to the sidewall and buried in the soil bed, while the other end was extended 20 cm away from the soil pan sidewall. The angle between the extended plate and horizontal plane was 20°, which was a typical angle of lateral slopes in EG systems on the Loess Plateau. These two plates were covered with permeable cloth to allow the lateral inflow to spread uniformly when it entered the soil bed (Fig. 1c). Lateral inflow discharge was calibrated before each test and lateral pipes were set up and removed easily within a short time during the experiments, which made it possible to shift lateral inflow conditions. In order to separately collect runoff and sediment samples from the EG channel and lateral slopes during each run, three triangular runoff and sediment collecting devices were installed at the lower end of soil pan (Fig. 1a). The triangular runoff collector in the middle was 40 cm high, which was coupled with a 30-cm deep end plate. The end plate was downward adjustable from the initial soil surface to 30 cm below the initial surface during the experiment as the EG developed. This was to avoid sediment deposition at the outlet of the soil pan as the EG channel developed. The width of the middle collector was 30 cm that is the typical EG width during experiments. Two 85-cm wide runoff collectors were installed on the two sides of the main channel. These collectors were fixed in place and installed at the initial soil bed surface.

2

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Fig. 1. Experimental setup of soil pan during (a) lateral overland inflow (Stage II) and (b) upslope inflow (Stage III), and (c) diagram of the lateral inflow setup.

intensities (50 and 100 mm h−1) were used in this study (Zhou and Wang, 1987). EGs are widely distributed on 15–35° hillslopes on the loess plateau, therefore two typical slope gradients (15° and 20°) were used in this study (Jiang et al., 1999). Three treatments were conducted to compare EG erosion under different slope gradients and rainfall intensities: treatment 1 was 15° slope and 50 mm h−1 rainfall intensity; treatment 2 was 20° slope and 50 mm h-1rainfall; and treatment 3 was 20° slope and 100 mm h−1 rainfall. Each treatment was replicated twice. Upslope and lateral inflow discharges were designed according to the EG system morphology observed on the Loess Plateau (Zhang et al., 1991; Cheng et al., 2007; Xu et al., 2017). Zhang et al. (1991) reported that the lateral distance between adjacent EGs was about 16 m on average, and the critical slope length for the initiation of EGs was about 40 m. Thus, for 8-m long soil bed, the lateral drainage area was approximately 128 m2 while upslope drainage area was 640 m2. Thus, the lateral and upslope inflow rates used were 8 and 40 L min−1, respectively, under typical rainfall intensity after applying the runoff coefficient observed in the research region (Wei et al., 2007). Upslope inflow discharge was about five times larger than lateral inflow rate because of the differences in drainage areas. To discriminate the contributions of upslope and lateral inflow to EG erosion, a series of experimental stages with and without upslope and lateral inflow were conducted (Table 1). A total of four stages were included in each run. The first stage (only rain, Stage I) was used to test the rainfall induced soil erosion. The second stage (rain + lateral

2.2. Soil bed preparation Soil materials used was loess soil (16.1% sand, 61.5% silt, 22.4% clay, Calcic Cambisols USDA Taxonomy), and the detailed soil characteristics and preprocessing procedures were the same as described in Xu et al. (2017). When packing the soil bed, a 5-cm sand layer was first settled at bottom for free drainage, then 25-cm plow pan and 20-cm tillage layer were packed in 5-cm increments. Detailed packing procedures can be found in Xu et al. (2017). An initial EG channel was built on soil bed surface according to the topographic characteristics of natural EGs (Gong et al., 2011). In this study, a curved plank was used to make this initial EG channel (Fig. 1), and the maximum depth of the pre-formed channel was about 12 cm. After forming the initial EG channel, manual tillage was applied to a depth of approximately 20 cm along the contour line using a shovel; this procedure was intended to simulate the natural cropland tillage practice in early April before the rainy season in the Loess Plateau. 2.3. Experimental design High intensity and short duration rainstorms frequently occur in the Loess Plateau during the summer causing severe soil loss, gully growth and related natural hazards (Wei et al., 2007; Han et al., 2017). Based on the erosive rainfall standard (I5 = 1.52 mm min−1; about 91.2 mm h−1) on the Loess Plateau, two representative rainfall 3

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Table 1 Experimental procedure. Experimental Stage

Treatments

Sediment sample number

Objective

I

Only rain

4

Rainfall induced erosion

II

Rain + Lateral inflow Only rain

4 2

Lateral inflow contribution to erosion Check erosion changes after lateral inflow

III

Rain + Upslope inflow Only rain

4 2

Upslope inflow contribution to erosion Check erosion changes after upslope inflow

IV

Rain + Upslope inflow + Lateral inflow Rain + Lateral inflow Only rain

4 4 2

Upslope + Lateral inflow contributions to erosion Lateral inflow contribution to erosion Check erosion changes after lateral inflow

would vary a little bit because of different slope degree and rainfall intensity, but all treatments were completed within 55–60 min, making treatments comparable. The number of samples in each stage is listed in Table 1. The runoff samples were weighed immediately, and then sediment settling and decanting were conducted. At last, all sediment samples were dried at 105 °C and weighed for further calculation. During experiments, a Leica Nova MS50 Total Station Laser Scanner (Leica Geosystems AG, Heerbrugg, Switzerland) was used to monitor the morphology changes as the EG system developed. The scanning resolution was 1 cm × 1 cm, which was high enough for analysis of EG morphology and tracking flow path changes. Acquired point cloud data were first denoised and unified in the Cyclone 6.0 software and then imported into ArcGIS 10.4 to form DEMs. Then, flow paths were extracted by hydrologic analysis module in Spatial Analysis tools. The processing steps for generating stream networks include identifying sinks, determining flow directions and calculating flow accumulations (Wu et al., 2018). Flow accumulation threshold was set as 150 to reflect all rills on the slope surface (Qin et al., 2018b). Overland flow velocities in EG channel and on lateral slopes were measured at four slope positions (1, 3, 5, 7 m of slope length) along the soil bed by KMnO4 dye tracer method with three replicates at each point. Flow depth was measured perpendicularly to the surface using a thin ruler and read to 0.1 mm precision.

inflow, Stage II) was to acquire the lateral inflow impact, followed by a check of rainfall induced soil erosion at this stage. In the third stage (rain + upslope inflow, Stage III), upslope inflow was added to test its impact while eliminating the lateral inflow component. This was followed again by a check on rainfall induced erosion for this stage. At the fourth stage (rain + upslope inflow + lateral inflow, Stage IV), upslope and lateral inflow were both applied to test their combined impacts on soil loss, then soil erosion rates under lateral inflow applied while eliminating upslope inflow condition and rain-only condition were checked. When calculating inflow contribution, rain induced runoff and soil loss in each stage (Stage II, III, IV) was first calculated by interpolating the values in nearby rain-only conditions as:

Rri =

Ri − 1 + Ri S + Si Sri = i − 1 2 2

(1)

where Rri, and Sri are the rain induced runoff and soil loss at Stage i, i = II, III, IV; Ri, and Si are the runoff and soil loss rate of only rain treatment at Stage i, i = II, III, IV. For Stage II (lateral inflow) and III (upslope inflow), the contribution of inflow from each individual source was calculated as the combined (rainfall plus inflow) measurement minus the contribution of rain only part. For Stage IV when upslope and lateral inflow were both applied with rainfall, contribution of each inflow component was calculated as:

RCl =

Rr + l − RrIV Rr + l

RCu + l =

·100% SCl =

Rr + u + l − RrIV Rr + u + l

RCu = RCu + l − RCl

Sr + l − SrIV Sr + l

·100% SCu + l =

2.5. Flow hydrodynamic characteristics

·100%

Sr + u + l − SrIV Sr + u + l

SCu = SCu + l − SCl

Depth averaged flow velocity (V) was calculated based on the surface velocity (Vs) measured during the experiment as:

·100% (2)

V = k Vs

where RCl, RCu+l, RCu, SCl, SCu+l, SCu are the runoff and soil loss contributions from the lateral inflow, the sum of upslope and lateral inflow, and the upslope inflow, respectively. Rr+l, Rr+u+l, Sr+l, Sr+u+l are runoff and soil loss rate of rain + lateral inflow and rain + upslope inflow + lateral inflow treatments at Stage IV.

(3)

where k is the coefficient that is 0.67 for laminar flow and 0.8 for turbulent flow (Horton et al., 1934; Emmett, 1970; Li et al., 1996). Generally, lateral overland flow is dominated by laminar flow while flow in EG channel is turbulent flow. Thus, mean flow velocities at different positions were calculated using different coefficients. Shear stress reflected runoff energies to deliver the mass in runoff and was calculated as:

2.4. Experimental procedure A flowchart of experiments is shown in Fig. 2. Pre-soak rain with 30 mm h−1 rainfall intensity was conducted to decrease spatial variations of surface soil moisture and roughness conditions (Xu et al., 2017). Before each experiment, rainfall intensity and inflow rates were calibrated to target values. Spatial uniformity of rainfall intensity was over 90% that met the requirements. The soil pan was then adjusted to the tested slope and exposed to the prescribed rainfall intensity and inflow rate. Upslope and lateral inflows were applied or removed by following the treatment conditions listed in Table 1 for the different stages. Once runoff reached an equilibrium rate (runoff rate variation smaller than 10% average value) in different stages, which usually took about 1–2 min, runoff and sediment samples were collected continuously in 15-L buckets at the outlets every 1–2 min. The duration from adjusting boundary hydrological condition to equilibrium stage

τ = γRJ

(4)

where τ is shear stress (Pa), γ is the specific weight of water (N m−3), R is hydraulic radius calculated by flow depth (m), J is surface slope (m m−1) calculated as the sine of the slope degree. As the bed slope and channel slope would be different at various spatial positions through the experiment, DEMs acquired from scanned point clouds at corresponding moments of the experiment were used to get the average slope degree values of the measured flow paths. Stream power, representing the expenditure of the power of water per unit area, was calculated as (Nearing et al., 1991):

ω=τV where ω is stream power (N m−1 s−1). 4

(5)

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Fig. 2. The flowchart for conducting the experiment.

2.6. Soil particle size distribution analysis

3. Results

In order to compare soil particle size distributions before and after EG erosion, soil samples were collected both in EG channel and on lateral slope surfaces at 1 m increments from 1 to 7 m slope length to evaluate soil degradation by soil erosion. Soil samples on the lateral slope surfaces were collected 0.5 m away from the EG channel. A pipette method was used in this study for particle size analysis. Before testing, soil samples were pre-processed to remove organic matter and disperse clays. Soil samples were first air dried and passed through a 2-mm sieve, then, 6% H2O2 and 0.2 mol L−1 HCl were used to eliminate organic matter and carbonate, respectively. 0.05 mol L−1 HCl and distilled water were used to wash out Ca2+ and Cl-, then 0.5 mol L−1 NaOH was added and the solution allowed to stand overnight after stirring (Eshel et al., 2004). At last, the solution was dispersed by using ultrasonic mixing (160 W, 10～15 min). After the preprocessing, 10 g soil samples were weighed for pipette analysis.

3.1. EG development processes Fig. 3 shows a typical EG system developing and the morphological changes associated with the experimental stages. With rainfall, upslope inflow and lateral inflow applied at different stages, the ephemeral gully and rills developed over time. The 100 mm h−1 and 20° treatment is presented as an example of the overland flow impacts on ephemeral gully development. At the beginning of Stage I when only rainfall was applied (Fig. 3a), sheet flow from lateral slopes gradually concentrated into the pre-formed EG channel from both sides according to the preformed topography; as such, sheet erosion was dominant at this stage. During Stage II, lateral inflow combined with rainfall induced concentrated flow along the lateral slopes as well as in EG channel. Convergent flow within the EG channel initiated headcutting of the EG main channel (Fig. 3b). Lateral slopes of the EG system during Stage II gradually formed fixed flow paths and rill erosion started to dominate the soil erosion process. In Stage III, upslope inflow into the EG main channel resulted in disconnected headcuts (Fig. 3c) and then became connected to form a well-defined EG channel (Fig. 3d). In Stage IV, the addition of lateral inflow to rainfall and upslope inflow quickly resulted 5

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Fig. 3. Typical ephemeral gully system developed at each stage of development for the 20° and 100 mm h−1 treatment: (a) Stage I, sheet flow erosion by rain only, (b) Stage II, rills and EG main channel erosion by rain plus lateral inflow, (c) and (d) Stage III, main EG channel erosion by rainfall plus upslope inflow, (e) and (f) Stage IV, main EG channel deepening and widening as well as the development of lateral rills and EG tributaries due to rain plus upslope and lateral inflow.

and 2, respectively. This was the result of excess infiltration runoff induced by the larger rainfall intensity (Wei et al., 2007; Xu et al., 2017). When lateral inflow was applied at Stage II (Table 2), runoff rates were significantly increased compared with Stage I rainfall only. The runoff rate increased by 298%, 353%, and 214% for treatments 1, 2, and 3 (50 mm h−1 rainfall intensity for the 15° and 20°slopes and 100 mm h−1 rainfall intensity for the 20° slope), respectively. The subsequent runoff rates for rainfall alone were all larger than those at Stage I by 144%, 178%, and 166%, respectively. This can be attributed to the saturation of soil and development of rills providing flow path connectivity on lateral slopes. When upslope inflow was added to this rainfall condition (Stage III), the runoff rates increased by more than an order of magnitude (1163% and 1274%) for treatments 1 and 2, respectively, but only by 292% for treatment 3. Runoff rates under the three treatments varied from 64.0 to 68.9 L min−1, from 83.2 to 88.4 L min−1 and from 99.4 to 104.4 L min−1, respectively. While the EG and lateral rills were substantially more developed at this stage compared to Stage II, the rainfall only prior to Stage III was essentially the same as following Stage III. Similarly, for Stage IV (rain + upslope inflow + lateral inflow), the runoff rates increased about the same as in Stage III (1445%, 1354%, and 282% for treatments 1, 2, and 3, respectively). Under these three treatments, runoff rates varied from 77.2 to 83.5 L min−1, from 89.3 to 95.3 L min−1 and from 107.6 to 112.9 L min−1, respectively. When upslope inflow was removed, the runoff rates obviously decreased (Fig. 4). However, runoff rates for rainfall plus lateral inflow at Stage IV were 215%, 203%, and 72% larger for the three treatments, respectively, than for this same condition at Stage II, varying from 16.3 to 20.1 L min−1, from 18.7 to 22.7 L min−1 and from 40.7 to 43.2 L min−1, respectively.

in connectivity of lateral rills with the main channel (Fig. 3e). These rill tributaries to the main channel occurred on both lateral side-slopes and resulted in rapid widening of the EG main channel. As Stage IV progressed, the EG channel depth increased and rills on lateral slopes became EG tributaries that themselves had rill branches thereby forming a complete EG system (Fig. 3f). It was the synergistic effects of rainfall, upslope inflow and lateral inflow that led to the progressive rill and ephemeral gully erosion that produced the EG system and excessive soil loss. 3.2. Runoff and soil loss responses to different inflow conditions 3.2.1. Runoff responses Runoff processes responded to the inflow conditions as expected (Fig. 4). As rainfall continued in Stage I, the runoff rate increased with time. When lateral inflow was added to rainfall in Stage II, the runoff rate jumped up particularly for the higher rainfall and slope condition. When rainfall was combined with upslope inflow in Stage III, the runoff rate jumped up even higher. Despite the formation of rills and a wellconnected EG main channel, the runoff dropped back down to the previous rainfall only runoff rate. When all three sources were contributing in Stage IV, the runoff rate jumped up to slightly higher than Stage III due to the addition of lateral inflow but then dropped down significantly in Stage IV when upslope inflow ceased. For the 50 mm h−1 rainfall intensity, runoff rates in the 20° treatment (treatment 2) were slightly larger (4%) than those in 15° treatment (treatment 1) when only rainfall was applied with means of 2.16 and 2.25 L min−1 (Table 2), respectively. However, when rainfall intensity increased to 100 mm h−1 (treatment 3), the runoff rate averaged 9.79 L min−1 and was 2.2–3.8 times larger than those in treatments 1 6

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Fig. 4. Runoff variations for the three treatments as a function of time and hydrologic conditions imposed. Error bars show the standard deviations of two replicates.

impact on soil loss was greater than that on runoff alone. For Stage I, when slope for the 50 mm h−1 rainfall intensity was increased from 15° to 20°, the soil loss rate increased by 55% (Table 2). However, when rainfall intensity for 20° slope was increased from 50 mm h−1 to 100 mm h−1, the soil loss rate increased by 500% varying from 1.5 to 4.8 kg min−1. The addition of lateral inflow in Stage II increased soil loss rates by 966%, 715%, and 293% over rainfall alone for treatments 1, 2, and 3, respectively. Soil loss in Stage II was dominated

3.2.2. Soil loss responses Soil loss rates exhibited similar patterns as the runoff responses (Figs. 4 and 5). At the beginning when only rain was applied, soil loss rate was low and slightly increased with time. As for runoff, soil loss rate jumped when lateral flow was included, then jumped even higher when upslope inflow was added and dropped back down between each stage when rainfall alone was applied. The main differences between runoff and soil loss responses was the influencing degree, the inflow

Table 2 Average runoff and soil loss rates, standard deviations in bracket, and contributions of upslope and lateral inflow for treatments 1 (15° slope and 50 mm h−1 rainfall intensity), treatment 2 (20° slope and 50 mm h−1 rainfall intensity), and treatment 3 (20° slope and 100 mm h−1 rainfall intensity). Experimental Stage

Treatments

Runoff rate (L min−1)

Soil loss rate(kg min−1)

1

2

3

1

2

3

I

Only rain

2.16 (0.79)

2.25 (0.48)

9.79 (6.40)

0.38 (0.19)

0.59 (0.23)

3.54 (2.53)

II

Rain + Lateral inflow

8.60 (3.14) 5.28 (3.32) 3.72 56.74

10.20 (2.68) 6.26 (2.72) 4.26 58.28

30.71 (4.86) 26.05 (6.98)

4.81 (1.89) 3.02 (1.50) 1.81 62.47

13.91 (4.08) 11.66 (4.12)

17.92 41.65

4.05 (1.54) 2.36 (0.52) 1.37 66.17

66.67 (27.07) 5.16 (3.69) 5.22 92.17

85.99 (3.03) 6.36 (4.09) 6.31 92.66

102.11 (6.83) 29.03 (6.31) 27.54 73.03

35.68 (12.43) 2.09 (2.00) 2.23 93.76

40.58 (2.67) 3.09 (2.65) 3.06 92.47

46.98 (4.01) 13.46 (3.91) 12.56 73.27

79.75 (21.69) 17.88 (3.77) 9.94 (3.33) 7.55 90.53 12.95 77.58

92.45 (7.11) 20.54 (6.45)

110.93 (2.68) 41.78 (6.73) 31.93 (5.92) 30.48 72.52 10.19 62.34

42.60 (12.88) 8.70 (1.73) 6.64 (2.07) 4.37 89.75 10.18 79.58

48.29 (3.81) 9.17 (2.85) 6.74 (3.08) 4.92 89.82 8.81 81.01

56.43 (3.20) 19.95 (4.86) 13.96 (3.78) 13.71 75.70 11.06 64.65

Only rain Rain induced part when lateral inflow applied* Lateral inflow contribution (%) III

Rain + Upslope inflow Only rain Rain induced part when upslope inflow applied* Upslope inflow contribution (%)

IV

Rain + Upslope inflow + Lateral inflow Rain + Lateral inflow Only rain Rain induced part when both inflow applied* Upslope + Lateral inflow contributions (%) Lateral inflow contribution (%) Upslope inflow contribution (%)

9.86 (2.00) 8.11 91.23 13.45 77.78

7.60 45.36

* Note: rain induced part was the average value of only rain conditions in former stage and current stage, it was then used as reference to calculate the lateral and upslope inflow contributions. 7

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Fig. 5. Soil loss variations for the three treatments as a function of time and hydrologic conditions imposed. Error bars show the standard deviations of two replicates.

induced runoff and soil loss. When rainfall intensity increased from 50 mm h−1 to 100 mm h−1, upslope inflow contributions to runoff and soil loss decreased to 73.0% and 73.3%, which is to be expected since upslope inflow rate was not adjusted and held constant while the rainfall intensity increased. Under the rain + upslope inflow + lateral inflow condition, upslope inflow plus lateral inflow total contributions to runoff and soil loss were around 90.8% and 89.8%, respectively, in the two 50 mm h−1 treatments (1 and 2) and about 73.6% in the 100 mm h−1 treatment (3). Rainfall accounted for the remaining 10% runoff and soil loss. Upslope and lateral inflow had different contributions to total runoff rate and soil loss separately, lateral inflow contributions to total runoff and soil loss were from 10.2% to 13.5% and from 8.8% to 11.1%, respectively, while upslope inflow contributions to total runoff and soil loss were 62.3–77.8% and 64.7–81.0%. This result showed that upslope inflow induced a greater contribution to runoff and soil loss than lateral inflow which only contributed around 10%. The total area being simulated was 784 m2 in which rainfall was applied to only 2%, whereas lateral and upslope inflow were simulated for 16.3 and 81.6%, respectively. Thus, lateral inflow contributions were less than the area represented whereas rainfall contributions were greater than the area represented.

by rill erosion on the lateral slopes for treatments 1 and 2 and by ephemeral gully head advance and rill development for treatment 3 in response to higher rainfall intensity. After lateral inflow was terminated and runoff was generated by rainfall alone, soil loss rates decreased substantially but were still 521%, 412%, and 229% greater for treatments 1, 2, and 3, respectively, than those in Stage I for rainfall only. Thus, the effects of soil wetting up and rill occurrence were more dramatic on soil loss than runoff. In Stage III (rain + upslope inflow), soil loss rates for the three treatments increased by 1412%, 1244%, and 303% over the previous rainfall alone. This is a greater increase than observed for runoff. When upslope inflow was subsequently terminated, soil loss rates went back down to essentially the same level as before upslope inflow following Stage II despite the better flow path connectivity. For Stage IV (rain + upslope inflow + lateral inflow), soil loss rates for the three treatments were 41.7–44.2 kg min−1, 45.4–49.7 kg min−1, and 53.7–59.2 kg min−1, respectively. This is higher than all other hydrologic conditions and is a larger increase from Stage III to IV (1938%, 1463% and 319%, for treatments 1, 2, 3, respectively) than observed for runoff. When upslope inflow and lateral inflow were eliminated, soil loss rates in treatments 1 and 2 were more than double what they were after Stage II and III but only slightly higher for treatment 3.

4. Discussion 3.3. Discrimination of upslope and lateral inflow contributions to EG erosion

4.1. Interaction between EG morphology and flow path changes

The contributions of upslope and lateral inflow were discriminated for all treatments and hydrologic conditions. Under the rain + lateral inflow condition, lateral inflow contributions to slope surface runoff and soil loss were influenced by rainfall intensity. Lateral inflow contributions to runoff and soil loss varied from 56.7% to 58.3% and from 62.5% to 66.2%, respectively, for treatments 1 and 2. Thus, when slope gradient increased, the lateral inflow contributions did not change much. When rainfall intensity increased from 50 to 100 mm h−1, lateral inflow contributions to runoff and soil loss decreased to 41.7% and 45.4%, respectively. Under the rain + upslope inflow condition, upslope inflow contributions to runoff and soil loss varied from 92.2% to 92.7% and from 92.5% to 93.8%, respectively, for treatments 1 and 2. Thus, an increase of slope gradient from 15° to 20° had no effect on upslope inflow

Fig. 6 shows rill and EG channel flow paths changes at different EG development stages. Topography changes can lead to EG flow path variations, while flow path directions can also influence the EG erosion processes. Thus, interactions between EG morphology and flow path changes still need clarification during the EG development process. At the beginning stage, the original land surface appeared smooth but subtle dendritic flow paths existed on the surface. These micro-topography changes fostered convergence of sheet flow and initiation of rill erosion which can lead to alterations in flow paths (Fig. 6a and 6b). Once headcutting developed connectivity within the EG main channel, it became fixed in place and promoted the connectivity to the channel (Poesen et al., 2003). Lateral inflow added to the formation of relatively fixed rill flow paths on both sides of the main EG channel, which tended to decrease the total flow path length and branching of rills (Fig. 6b). 8

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Fig. 6. DEMs from LiDAR with flow paths of rills and ephemeral gully channels of the EG system.

linkages to the main EG channel. For the EG system that developed in this study, nearly all runoff and sediment was transported through the EG main channel as more than 95% of the total runoff and sediment was collected by the middle 30-cm wide collector (Fig. 1a). Lateral inflow was not only important in the formation of lateral rills that provide landscape connectivity, but also contributes to the imbricated landform formation process (Gong et al., 2011), which has generally been overlooked in previous research. More works need to be done on the lateral inflow contributions to clarify how lateral inflow induces land surface morphologic changes and runoff connectivity enhancement.

By the end of the experiment, a permanent EG main channel formed as disconnected headcuts were linked together. The major lateral rill flow paths guided runoff and sediment to this main EG channel; total length and branches of flow paths were both decreased due to channel annexation (Fig. 6c). Upslope inflow transported the sediment through the EG channel while lateral inflow transported runoff and sediment through the lateral rills (developed into EG tributaries later) and EG channel. It is clear in Fig. 6 that the EG system provides enhanced linkages from upland and lateral slopes for transferring runoff and sediment (Poesen et al, 2003). Thus, EG flow paths changes can be used to explain the upslope and lateral inflow impacts on soil erosion in the EG system.

4.3. Flow hydrodynamic characteristics in EG channel and on lateral slopes 4.2. Lateral inflow contributions Spatial distributions of flow hydrodynamic characteristics both in EG channel and on lateral slopes were displayed under the rain + upslope inflow + lateral inflow condition in the 15°and 50 mm h−1 treatment, which is the condition most similar to the field hydrological condition for overland flow from both directions (Fig. 7). In the EG channel, flow velocities increased as the slope length increased and the largest flow velocity was 0.38 m s−1 obtained at 7 m of slope length, flow shear stress and stream power fluctuated along the EG channel, which peaked at 5 m of slope length as 10.4 N m−2 and 3.4 N m−1 S−1. On lateral slopes, the flow velocity did not linearly vary along the slope, which could be attributed to the dominance of sheet flow before rill networks developed on the lateral side-slopes. After rill became connected on the slope, surface runoff by rainfall and lateral inflow was able to merge and form tributaries to the main EG channel. Compared with the flow hydrodynamic characteristics in the EG channel, flow shear stress and stream power on lateral slopes were extremely smaller at all slope positions, representing great differences in erosive force and energy of runoff (Zhang et al., 2014). The spatially averaged shear stress and stream power in the EG channel were 4.9–8.6 times higher than those on lateral slopes. Similarly, upslope and lateral inflow contributions to total soil loss were about 70% and 10% respectively (Table 2), which was consistent with the result of flow hydrodynamic differences. The results suggest that, in a surface flow dominated EG system, the difference between contributions of upslope

Xu et al. (2017) demonstrated how upslope inflow played an important role in EG erosion. This study confirmed the significance of upslope inflow and showed that it can over-ride the contribution of lateral inflow. Upslope inflow contributions to runoff and soil loss were around 80% when rainfall intensity was at 50 mm h−1 and more than 60% of total runoff and soil loss when rainfall intensity increased to 100 mm h−1. This result verified that upslope inflow is the main controlling factor for EG erosion in surface flow dominated EG systems, while in other conditions, subsurface flow, side-bank erosion, and within-gully shear stress might be the main contributor to the soil loss (Zegeye et al., 2018). The question could be asked if the lateral inflow only contributes around 10% to total runoff and soil loss, is it still valuable for understanding EG erosion? The answer seems to be certain that lateral inflow is still important for runoff and sediment connectivity of the EG system with interrill areas. The extensive imbrication of the interrill areas by lateral rills connected to the main channel is likely one reason why the rainfall contribution exceeded its proportional area. In an EG system, upslope inflow mainly contributes to the growth of the main EG channel, but both lateral and upslope inflow play critical roles in generating rill networks and providing connectivity of interrill areas for runoff and sediment delivery to the main channel. Rill networks on lateral slopes collect sheet flows from interrill areas and provide 9

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Wang et al., 2014). During the rainfall and inflow stage, fine particles were much easier to be transported while coarse particles were relatively harder to be transported. The deposition or lack of suspension of sands induced a coarsening of soil left on the slope surface and in channels. Differences in PSD between the EG main channel and the lateral slope soils were minor (Table 3). This can be attributed to the EG system providing a highly-connected erosion system through which surface sediment was transported without being transport limited. To illustrate the spatially distributed erosion impacts on the soil PSD, the percentages of the various size particles changed as a function of slope length in response to the different treatments as shown in Fig. 8. In treatment 1 (15° and 50 mm h−1), coarse silt and fine sand particle contents were a little larger in the EG channel, while clay and fine silt contents were correspondingly smaller in the EG channel compared with lateral slopes (Fig. 8a and 8b). On the lateral slopes, coarse silt and fine sand contents showed fluctuations along the slope length and were larger than the contents in the original soil. This could be attributed to the coarser particles being harder to detach and transport to the outlet. Conversely, fine silt and clay contents at different slope positions were all under 20%, illustrating that more fine silt and clay particles were transported to the outlet during the experiment. Compared with the fluctuation range of soil particle contents on the lateral slopes, the fluctuation range in the EG channel was larger, which was related to the serious erosion that included mass failures of sidewalls and random temporary deposition. In treatment 2 (20° and 50 mm h−1), soil particle contents also showed a fluctuation trend along the slope length. On lateral slopes, fine sand and coarse silt contents showed a complementary relationship as both contents were larger than those in original soil. In contrasts, clay and fine silt contents were smaller than those before experiments (Fig. 8c). In the EG channel, the fluctuation range was wider than on lateral slopes (Fig. 8d). Compared with fluctuation range of soil particle contents in 15° slope treatment, the fluctuation in the 20° slope treatments showed a wider range. Slope gradient increment increased the detachment randomness of soil particles which might be a reason for higher fluctuation. In treatment 3 (20° and 100 mm h−1 treatment), soil particle contents on the lateral slopes exhibited small fluctuations from 1 m to 6 m of slope length. At the 7 m of slope length, coarse silt content dramatically increased while fine sand content decreased (Fig. 8e). This could be related to raindrop splash dominated at this location because of less surface runoff at the downslope area. Compared with lateral slopes, coarse particle content decreased while fine particles increased (Fig. 8f). This is the result of larger runoff energy flushing sediment readily through the EG channel compared with rills on lateral slopes (Foster, 1986; Wang and Shi, 2015).

Fig. 7. Flow hydrodynamic characteristics at different slope positions under Rain + Upslope inflow + Lateral inflow condition for the 20° and 100 mm h−1 treatment. Error bars show the standard deviations.

and lateral inflows was induced by the differences in flow hydrodynamic characteristics in the EG channel and on lateral slopes. 4.4. Soil particle size distribution changes in EG channel and on lateral slopes The significance of the rills and EG tributaries of the lateral sideslopes is seen in changes in the particle size distribution (PSD) in the EG channel and on lateral slopes from before and after experiments. Xu et al. (2006) showed how soil quality changes due to changes in particle size distribution in response to management. They reasoned that this was due to erosion being a selective process with respect to PSD in that some areas will be enriched in sand sizes and others in clay size particles depending upon their landscape position (Rhoton et al., 1979). Compared with the soil textures before the tests, the clay (< 0.002 mm) and fine silt (0.002–0.02 mm) contents following the treatments imposed all decreased (Table 3). In contrasts, the coarse silt (0.02–0.05 mm) and fine sand (> 0.25 mm) contents increased. Coarse sand contents were all less than 1%, varying from 0.04% to 0.37%. Total contents of clay and fine silt (< 0.02 mm) after the erosion experiment decreased from 42.5% to 31.8–36.0%, across all erosion treatments and positions. While total contents of coarse silt and fine sand (0.02–0.25 mm) after the experiments increased from 57.4% to 64.0–68.1%, across all erosion treatments and positions. In the EG system, raindrop splash and surface runoff detachment are highly associated with size selectivity of eroded sediment (Shi et al., 2012, 2013;

4.5. Limitations of this experimental study This study quantified how upslope and lateral inflows influence soil loss in a typical ephemeral gully system on the Loess Plateau. This study addresses the current research gap related to the upslope and lateral inflow impacts on flow hydrodynamics and soil degradation in an EG system, especially lateral inflow contributions to the runoff connectivity of the EG system through the formation of lateral rills. But the inflow applied in this study was clear water for both lateral and upslope directions. In field conditions, overland flow from both upslope drainage area and lateral slopes will be sediment-laden flow due to its detachment and consequently transport downstream. Compared with clear water inflow, upslope and lateral inflow with different sediment concentrations would cause less soil loss in the ephemeral gully system tested, because of transport capacity and detachment capacity limitations (Ellison, 1948; Zheng et al., 2000; Liu et al., 2019). Future studies should concentrate on the impacts of upslope and lateral sedimentladen flows on ephemeral gully erosion (Wu et al., 2019). 10

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

Table 3 Soil particle size distribution changes in different treatments. Average values of different spatial positions and standard deviations in bracket. Treatments

Clay (< 0.002 mm)

Fine silt (0.002–0.02 mm)

Coarse silt (0.02–0.05 mm)

Fine sand (0.05–0.25)

Coarse sand (> 0.25 mm)

Original soil materials 50 mm h−1, 15° in EG channel

22.380 16.652 (0.686) 16.792 (0.985) 17.573 (0.702) 18.018 (1.068) 17.971 (0.744) 17.933 (0.810) 17.399 17.581

20.167 15.177 (1.575) 17.337 (1.282) 15.665 (0.735) 16.103 (0.536) 18.005 (1.694) 16.660 (0.851) 16.282 16.700

41.384 45.489 (1.180) 44.024 (2.741) 43.364 (3.409) 45.358 (2.199) 41.412 (2.458) 47.826 (3.352) 43.422 45.736

16.040 22.638 (1.866) 21.777 (2.184) 23.062 (3.048) 20.195 (3.105) 22.569 (1.747) 17.536 (3.811) 22.756 19.836

0.028 0.044 (0.027) 0.069 (0.046) 0.372 (0.624) 0.325 (0.627) 0.042 (0.042) 0.044 (0.013) 0.153 0.146

50 mm h−1, 15° on lateral slope 50 mm h−1, 20° in EG channel 50 mm h−1, 20° on lateral slope 100 mm h−1, 20° in EG channel 100 mm h−1, 20° on lateral slope EG channel average Lateral slope average

Fig. 8. Soil particle size distribution changes as a function of slope length on lateral slopes and in the EG channel in different treatments.

upslope and lateral inflows in this study were based upon typical morphologic features, i.e. contributing areas, of the loess plateau. Thus, the results are situation-dependent and spatially variable, but the understanding gained of inflow contributions to the hydrologic connectivity and soil degradation of the EG system in this study is widely applicable. Soil degradation by coarsening the soil with EG system development can contribute to loss of productivity as observed in the black soil region of China (Liu et al., 2013) and other areas like in Italy (Ollobarren et al., 2016).

This study demonstrated the EG erosion processes influenced by overland flow. However, subsurface flow from both upslope and lateral directions could also produce instabilities leading to further bank erosion and headcut development (Wilson, 2011; Zegeye et al., 2018). Future laboratory experiments or field investigation on quantifying these processes are also needed. The specific size of the drainage areas above the EG and on the lateral slopes are highly variable and will lead to different inflow rates, and subsequently different contribution results. While such results are situation-dependent and spatially variable, 11

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

5. Conclusions

Research Inventory. National Academy Press, Washington DC, pp. 90–118. Frankl, A., Deckers, J., Moulaert, L., Van Damme, A., Haile, M., Poesen, J., Nyssen, J., 2016. Integrated solutions for combating gully erosion in areas prone to soil piping: innovations from the drylands of Northern Ethiopia. Land Degrad. Dev. 27, 1797–1804. https://doi.org/10.1002/ldr.2301. Gong, J., Jia, Y., Zhou, Z., Wang, Y., Wang, W., Peng, H., 2011. An experimental study on dynamic processes of ephemeral gully erosion in loess landscapes. Geomorphology 125, 203–213. https://doi.org/10.1016/j.geomorph.2010.09.016. Gordon, L.M., Bennett, S.J., Alonso, C.V., Bingner, R.L., 2008. Modeling long-term soil losses on agricultural fields due to ephemeral gully erosion. J. Soil Water Conserv. 63 (4), 173–181. https://doi.org/10.2489/jswc.63.4.173. Gutiérrez, Á.G., Schnabel, S., Contador, F.L., 2009. Gully erosion, land use and topographical thresholds during the last 60 years in a small rangeland catchment in SW Spain. Land Degrad. Dev. 20, 535–550. https://doi.org/10.1002/ldr.931. Han, Y., Zheng, F., Xu, X., 2017. Effects of rainfall regime and its character indices on soil loss at loessial hillslope with ephemeral gully. J. Mount. Sci. 14 (3), 527–538. https://doi.org/10.1007/s11629-016-3934-2. Hancock, G.R., Evans, K.G., 2006. Gully position, characteristics and geomorphic thresholds in an undisturbed catchment in northern Australia. Hydrol. Process. 20, 2935–2951. https://doi.org/10.1002/hyp.6085. Horton, R.E., Leach, H.R., Van Vliet, R., 1934. Laminar sheet-flow. Eos, Trans. AGU 15 (2), 393–404. Hosseinalizadeh, M., Kariminejad, N., Chen, W., Pourghasemi, H.R., Alinejad, M., Behbahani, A.M., Tiefenbacher, J.P., 2019a. Gully headcut susceptibility modeling using functional trees, naïve Bayes tree, and random forest models. Geoderma 342, 1–11. https://doi.org/10.1016/j.geoderma.2019.01.050. Hosseinalizadeh, M., Kariminejad, N., Chen, W., Pourghasemi, H.R., Alinejad, M., Behbahani, A.M., Tiefenbacher, J.P., 2019b. Spatial modelling of gully headcuts using UAV data and four best-first decision classifier ensembles (BFTree, Bag-BFTree, RS-BFTree, and RF-BFTree). Geomorphology 329, 184–193. https://doi.org/10. 1016/j.geomorph.2019.01.006. Jiang, Y., Wang, Z., Hu, G., Hao, X., 1999. Distribution features of shallow gully. Res. Soil Water Conserv. 6 (2), 181–184 (in Chinese). Kariminejad, N., Hosseinalizadeh, M., Pourghasemi, H.R., Bernatek-Jakiel, A., Alinejad, M., 2019. GIS-based susceptibility assessment of the occurrence of gully headcuts and pipe collapses in a semi-arid environment: Golestan Province, NE Iran. Land Degrad. Dev. https://doi.org/10.1002/ldr.3397. Li, G., Abrahams, A.D., Atkinson, J.F., 1996. Correction factors in the determination of mean velocity of overland flow. Earth Surf. Proc. Land. 21 (6), 509–515. https://doi. org/10.1002/(SICI)1096-9837(199606)21:6<509::AID-ESP613>3.0.CO;2-Z. 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. https://doi.org/10.2136/sssaj2016.02.0025. Liu, J., Zhou, Z., Zhang, X.J., 2019. Impacts of sediment load and size on rill detachment under low flow discharges. J. Hydrol. 570, 719–725. https://doi.org/10.1016/j. jhydrol.2019.01.033. Liu, H., Zhang, T., Liu, B., Liu, G., Wilson, G.V., 2013. Effects of gully erosion and gully filling on soil depth and crop production in the black soil region, northeast China. Environ. Earth Sci. 68 (6), 1723–1732. https://doi.org/10.1007/s12665-012-1863-0. Nachtergaele, J., Poesen, J., Steegen, A., Takken, I., Beuselinck, L., Vandekerckhove, L., Govers, G., 2001. The value of a physically based model versus an empirical approach in the prediction of ephemeral gully erosion for loess-derived soils. Geomorphology 40, 237–252. https://doi.org/10.1016/S0169-555X(01)00046-0. Nearing, M.A., Bradford, J.M., Parker, S.C., 1991. Soil detachment by shallow flow at low slopes. Soil Sci. Soc. Am. J. 55, 339–344. https://doi.org/10.2136/sssaj1991. 03615995005500020006x. Ollobarren, P., Capra, A., Gelsomino, A., La Spada, C., 2016. Effects of ephemeral gully erosion on soil degradation in a cultivated area in Sicily (Italy). Catena 145, 334–345. https://doi.org/10.1016/j.catena.2016.06.031. Poesen, J., Nachtergaele, J., Verstraeten, G., Valentin, C., 2003. Gully erosion and environmental change: importance and research needs. Catena 50, 91–133. https://doi. org/10.1016/S0341-8162(02)00143-1. Porto, P., Walling, D.E., Capra, A., 2014. Using 137Cs and 210Pbex measurements and conventional surveys to investigate the relative contributions of interrill/rill and gully erosion to soil loss from a small cultivated catchment in Sicily. Soil Tillage Res. 135, 18–27. https://doi.org/10.1016/j.still.2013.08.013. Qin, C., Zheng, F., Wells, R.R., Xu, X., Wang, B., Zhong, K., 2018a. A laboratory study of channel sidewall expansion in upland concentrated flows. Soil Tillage Res. 178, 22–31. https://doi.org/10.1016/j.still.2017.12.008. Qin, C., Zheng, F., Xu, X., Wu, H., Shen, H., 2018b. A laboratory study of rill network development and morphological characteristics on loessial hillslope. J. Soil. Sediment. 18, 1679–1690. https://doi.org/10.1007/s11368-017-1878-y. Qin, C., Zheng, F., Wilson, G.V., Zhang, X.J., Xu, X., 2019. Apportioning contributions of individual rill erosion processes and their interactions on loessial hillslopes. Catena 181, 104099. https://doi.org/10.1016/j.catena.2019.104099. Rhoton, F.E., Smeck, N.E., Wilding, L.P., 1979. Preferential clay mineral erosion from watersheds in the Maumee River Basin. J. Environ. Qual. 8, 547–550. https://doi. org/10.2134/jeq1979.00472425000800040021x. Shi, Z., Fang, N., Wu, F., Wang, L., Yue, B., Wu, G., 2012. Soil erosion processes and sediment sorting associated with transport mechanisms on steep slopes. J. Hydrol. 454, 123–130. https://doi.org/10.1016/j.jhydrol.2012.06.004. Shi, Z., Yue, B., Wang, L., Fang, N., Wang, D., Wu, F., 2013. Effects of mulch cover rate on interrill erosion processes and the size selectivity of eroded sediment on steep slopes. Soil Sci. Soc. Am. J. 77, 257–267. https://doi.org/10.2136/sssaj2012.0273. Soil Science Society of America. 2008. Glossary of Soil Science Terms, Madison, WI. Tebebu, T.Y., Abiy, A.Z., Zegeye, A.D., Dahlke, H.E., Easton, Z.M., Tilahun, S.A., Collick,

Simulated rainfall and inflow experiments that concentrated on quantifying upslope and lateral inflows impact on EG erosion were conducted. The following results were observed: 1) upslope inflow was the dominant (70–80%) contributor to total runoff and soil loss, while contributions of lateral inflow to total runoff and soil loss were only around 10%, and rainfall contributed the other 10–20%; 2) lateral inflow was not only important in the formation of lateral rills contributing to runoff connectivity, but also contributed to the development of an imbricated landform pattern and the lateral gradient of the hillslope; 3) the spatial averaged shear stress and stream power in the EG main channel were 4.9–8.6 times those on lateral slopes, which explained the contribution differences of upslope and lateral inflows; 4) surface concentrated flow and sheet flow in the EG system shifted the soil particle size distribution to be coarser, as soil particles lager than 0.02 mm increased by 12.6% averagely. As most EG systems are related to farmland and pastures, the EG erosion is a vital influencing factor in farmland and pasture soil degradation processes, which needs further research. This work demonstrates the critical need of preventing both upslope and lateral drainage contributions to EG channels to protect valuable soil resources from degradation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This study was supported by the National Key R&D Program of China (Grand NO. 2016YFE0202900), National Natural Science Foundation of China (Grant NO. 41571263) and the External Cooperation Program of Chinese Academy of Sciences (Grant NO. 161461KYSB20170013). References Bennett, S.J., Casalí, J., Robinson, K.M., Kadavy, K.C., 2000a. Characteristics of actively eroding ephemeral gullies in an experimental channel. Trans. ASAE 43, 641–649. https://doi.org/10.13031/2013.2745. Bennett, S.J., Alonso, C.V., Prasad, S.N., Römkens, M.J.M., 2000b. Experiments on headcut growth and migration in concentrated flows typical of upland areas. Water Resour. Res. 36, 1911–1922. https://doi.org/10.1029/2000WR900067. Bernatek-Jakiel, A., Poesen, J., 2018. Subsurface erosion by soil piping: significance and research needs. Earth Sci. Rev. 185, 1107–1128. https://doi.org/10.1016/j.earscirev. 2018.08.006. Capra, A., La Spada, C., 2015. Medium-term evolution of some ephemeral gullies in Sicily (Italy). Soil Tillage Res. 154, 34–43. https://doi.org/10.1016/j.still.2015.07.001. Capra, A., 2013. Ephemeral gully and gully erosion in cultivated land: a review. In: Lannon, E.C. (Ed.), Drainage Basins and Catchment Management: Classification, Modelling and Environmental Assessment. Nova Science Publishers, Hauppauge, NY. Capra, A., Di Stefano, C., Ferro, V., Scicolone, B., 2011. Morphological characteristics of ephemeral gullies in Sicily, South Italy. Landform Anal. 17, 27–32. Capra, A., Porto, P., Scicolone, B., 2009. Relationships between rainfall characteristics and ephemeral gully erosion in a cultivated catchment in Sicily (Italy). Soil Tillage Res. 105, 77–87. https://doi.org/10.1016/j.still.2009.05.009. Casalí, J., López, J.J., Giráldez, J.V., 1999. Ephemeral gully erosion in southern Navarra (Spain). Catena 36, 65–84. https://doi.org/10.1016/S0341-8162(99)00013-2. Cheng, H., Zou, X., Wu, Y., Zhang, C., Zheng, Q., Jiang, Z., 2007. Morphology parameters of ephemeral gully in characteristics hillslopes on the Loess Plateau of China. Soil Tillage Res. 94, 4–14. https://doi.org/10.1016/j.still.2006.06.007. Ellison, W.D., 1948. Soil detachment by water in erosion processes. Trans. AGU 29 (4), 499–502. https://doi.org/10.1029/TR029i004p00499. Emmett, W.W., 1970. The Hydraulics of Overland Flow on Hillslopes. US Government Printing Office 662A: A-1-A-68. Eshel, G., Levy, G.J., Mingelgrin, U., Singer, M.J., 2004. Critical evaluation of the use of laser diffraction for particle-size distribution analysis. Soil Sci. Soc. Am. J. 68 (3), 736–743. https://doi.org/10.2136/sssaj2004.7360. Foster, G.R., 1986. Understanding ephemeral gully erosion. In: National Research Council, Board on Agriculture, Soil Conservation (Eds.), Assessing the National

12

Journal of Hydrology 579 (2019) 124174

X. Xu, et al.

572, 517–527. https://doi.org/10.1016/j.jhydrol.2019.03.037. Xu, M., Zhao, Y., Liu, G., Wilson, G.V., 2006. Identification of soil quality factors and indicators for the loess plateau of china. Soil Sci. 171 (5), 400–413. https://doi.org/ 10.1097/01.ss.0000209359.55322.aa. Xu, M., Li, Q., Wilson, G.V., 2016. Degradation of soil physicochemical quality by ephemeral gully erosion on sloping cropland of the hilly Loess Plateau, China. Soil Tillage Res. 155, 9–18. https://doi.org/10.1016/j.still.2015.07.012. Xu, X., Zheng, F., Wilson, G.V., Wu, M., 2017. Upslope inflow, hillslope gradient and rainfall intensity impacts on ephemeral gully erosion. Land Degrad. Dev. 28 (8), 2623–2635. https://doi.org/10.1002/ldr.2825. Zegeye, A.D., Langendoen, E.J., Stoof, C.R., Tilahun, S.A., Dagnew, D.C., Zimale, F.A., Guzman, C.D., Yitaferu, B., Steenhuis, T.S., 2016. Morphological dynamics of gully systems in the subhumid Ethiopian Highlands: the Debre Mawi watershed. Soil 2 (3), 443–458. https://doi.org/10.5194/soil-2-443-2016. Zegeye, A.D., Langendoen, E.J., Guzman, C.D., Dagnew, D.C., Tilahun, S.A., Steenhuis, T.S., 2018. Gullies, a critical link in landscape soil loss: A case study in the sub-humid highlands of Ethiopia. Land Degrad. Dev. https://doi.org/10.1002/ldr.2875. Zhang, K., Tang, K., Wang, B., 1991. A study on characteristic value of shallow gully erosion on slope farmland in the Loess, Plateau. J. Soil Water Conserv. 5 (2), 8–13 (in Chinese). Zhang, Q., Dong, Y., Li, F., Zhang, A., Lei, T., 2014. Quantifying detachment rate of eroding rill or ephemeral gully for WEPP with flume experiments. J. Hydrol. 519, 2012–2019. https://doi.org/10.1016/j.jhydrol.2014.09.040. Zhang, Y., Wu, Y., Liu, B., Zheng, Q., Yin, J., 2007. Characteristics and factors controlling the development of ephemeral gullies in cultivated catchments of black soil region, Northeast China. Soil Tillage Res. 96, 28–41. https://doi.org/10.1016/j.still.2007.02. 010. Zheng, F., Huang, C.H., Norton, L.D., 2000. Vertical hydraulic gradient and run-on water and sediment effects on erosion processes and sediment regimes. Soil Sci. Soc. Am. J. 64 (1), 4–11. https://doi.org/10.2136/sssaj2000.6414. Zheng, F., Huang, C.H., Xu, X., 2017. Gulley Erosion. Encyclopedia of Soil Science, Third ed. CRC Press, pp. 1059–1063. https://doi.org/10.1081/E-ESS3-120053854. 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). Zhu, X., 1956. Classification on the soil erosion in the Loess region. Acta Pedol. Sin. 2, 99–115 (in Chinese with English abstract).

A.S., Kidnau, S., Moges, S., Dadgari, F., Steenhuis, T.S., 2010. Surface and subsurface flow effect on permanent gully formation and upland erosion near Lake Tana in the northern highlands of Ethiopia. Hydrol. Earth Syst. Sci. 14, 2207–2217. https://doi. org/10.5194/hess-14-2207-2010. Wang, L., Shi, Z., 2015. Size selectivity of eroded sediment associated with soil texture on steep slopes. Soil Sci. Soc. Am. J. 79 (3), 917–929. https://doi.org/10.2136/ sssaj2014.10.0415. Wang, L., Shi, Z., Wang, J., Fang, N., Wu, G., Zhang, H., 2014. Rainfall kinetic energy controlling erosion processes and sediment sorting on steep hillslopes: a case study of clay loam soil from the Loess Plateau. China. J. Hydrol. 512, 168–176. https://doi. org/10.1016/j.jhydrol.2014.02.066. Wei, W., Chen, L., Fu, B., Huang, Z., Wu, D., Gui, L., 2007. The effect of land uses and rainfall regimes on runoff and soil erosion in the semi-arid loess hilly area, China. J. Hydrol. 335, 247–258. https://doi.org/10.1016/j.jhydrol.2006.11.016. Wells, R.R., Momm, H.G., Rigby, J.R., Bennett, S.J., Bingner, R.L., Dabney, S.M., 2013. An empirical investigation of gully widening rates in upland concentrated flows. Catena 101, 114–121. https://doi.org/10.1016/j.catena.2012.10.004. Wijdenes, D.J.O., Poesen, J., Vandekerckhove, L., Ghesquiere, M., 2000. Spatial distribution of gully head activity and sediment supply along an ephemeral channel in a Mediterranean environment. Catena 39, 147–167. https://doi.org/10.1016/S03418162(99)00092-2. Wilson, G.V., 2011. Understanding soil-pipe flow and its role in ephemeral gully erosion. Hydrol. Process. 25, 2354–2364. https://doi.org/10.1002/hyp.7998. Wilson, G.V., Rigby, J.R., Dabney, S.M., 2015. Soil pipe collapses in a loess pasture of Goodwin Creek Watershed, Mississippi: role of soil properties and past land use. Earth Surf. Proc. Land. 40, 1448–1463. https://doi.org/10.1002/esp.3727. Wilson, G.V., Wells, R., Kuhnle, R., Fox, G., Nieber, J., 2018. Sediment detachment and transport processes associated with internal erosion of soil pipes. Earth Surf. Proc. Land. 43, 45–63. https://doi.org/10.1002/esp.4147. Wu, H., Xu, X., Zheng, F., Qin, C., He, X., 2018. Gully morphological characteristics in the loess hilly-gully region based on 3D laser scanning technique. Earth Surf. Proc. Land. 43, 1701–1710. https://doi.org/10.1002/esp.4332. Wu, M., Zheng, F., Huang, B., 2004. Experimental study on upslope runoff effects on ephemeral gully erosion processes at loessial hillslope. Res. Soil Water Conserv. 11 (4), 74–76 (in Chinese with English abstract). Wu, T., Pan, C., Li, C., Luo, M., Wang, X., 2019. A field investigation on ephemeral gully erosion processes under different upslope inflow and sediment conditions. J. Hydrol.

13