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Stemflow contributions to soil erosion around the stem base under simulated maize-planted and rainfall conditions
Agricultural and Forest Meteorology 281 (2020) 107814
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Stemflow contributions to soil erosion around the stem base under simulated maize-planted and rainfall conditions
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Longshan Zhao , Qian Fang, Ye Yang, Hao Yang, Tonghang Yang, Hao Zheng College of Forestry, Guizhou University, Guiyang 550025, Guizhou Province, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Stemflow Stemflow partitioning Soil erosion Rill erosion
Stemflow is a primary pathway through which rainwater reaches the ground surface under crop cover. Substantial research suggests that stemflow amounts approach or exceed half the total precipitation on maize cropland; however, few studies have quantified the effect of stemflow on soil erosion during rainfall events. This study aimed to measure the effect of stemflow on soil erosion under controlled situations and determine the effects of stemflow on soil erosion. Maize stems were simulated using a 2-cm diameter PVC tube in 1.3 by 0.25 m steel boxes to introduce stemflows to the soil surface at a 10° slope. The rainfall intensities were 60, 90 and 120 mm/h; the stemflow amounts were 5, 10 and 15 g/s. The results showed that stemflow significantly increased soil erosion. As stemflow increased, the surface runoff and sediment rates sharply increased. Compared with a control slope (no stemflow), stemflow increased the surface runoff and sediment rates by more than three and twelve times, respectively. This result occurred because stemflow contributes to the formation of concentrated flows, which easily trigger rill erosion around the stem base. The sediment rate further increased with rill development. Soil erosion was small if stemflow did not occur during a rainfall event; otherwise, soil erosion was extensive due to stemflow-induced rill erosion. Our results provide new insights for the analysis of crop cover effects and soil erosion on cultivated lands. For maize-planted slopes, stemflow may also be indispensable when determining the soil erosion amount.
1. Introduction Maize crops are important food crops and are widely planted throughout the world (van Zelm et al., 2017; Zimmer et al., 2018). On sloping lands, planting maize is commonly considered an effective measure to control soil and water loss because it increases the vegetation cover (Mohammed and Gumbs, 1982; Kiboi et al., 2017; Luo et al., 2018). In recent decades, many studies have been conducted under laboratory and field conditions to determine the effects of planting maize on soil erosion mechanisms, especially the effects of the maize canopy on raindrop redistribution, interception and splash erosion on the soil surface (Lamm and Manges, 2000; Ma et al., 2015; Zheng et al., 2019). Previous studies on the characteristics of raindrops under maize canopies have concluded that the maize canopy mainly redistributes rainwater via throughfall and canopy interception (Armstrong and Mitchell, 1987; Parkin and Codling, 1990; Paltineanu and Starra, 2000; Liu et al., 2015). Canopy interception decreases the kinetic energy of raindrops (Wischmeier and Smith, 1978; Ma et al., 2015) and vastly reduces soil detachment and transportation caused by raindrop
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impacts, which are important soil erosion processes that largely determine the total amount of sediment in rainfall-induced erosion (Savat and Poesen, 1981; Mermut et al., 1997). The effect of the maize canopy on soil erosion is related to the growth of maize because the partitioning of rainwater into throughfall and canopy interception components varies at different maize growth stages (Liu et al., 2015; Zheng et al., 2019). Generally, with the growth of maize, the vegetation coverage and leaf area index gradually increase, and thus, the effect of a maize canopy on the prevention of rainfall-induced erosion becomes increasingly obvious (Ma et al., 2016; Luo et al., 2018). In other words, the soil area exposed to raindrop impacts decreases with increasing vegetation cover, leaf area index and interception efficiency. In addition, Zheng et al. (2014) suggested that the roots of maize contribute to a decrease in soil erodibility. As a result, the soil erosion that is directly induced by rainfall action is reduced at the soil surface. All of the effects discussed above can result in soil and water losses that are lower on maize-planted lands than on control or bare lands. However, some researchers have also noted different soil erosion results on maize-planted lands (Haynes, 1940; Quinn and Laflen, 1983; Ma et al., 2015). For example, Haynes (1940) and Ma (2011) noted that
Corresponding author. E-mail address: [email protected] (L. Zhao).
https://doi.org/10.1016/j.agrformet.2019.107814 Received 29 June 2019; Received in revised form 20 September 2019; Accepted 15 October 2019 0168-1923/ © 2019 Elsevier B.V. All rights reserved.
Agricultural and Forest Meteorology 281 (2020) 107814
L. Zhao, et al.
tube was sealed with silicon sealant to prevent the entry of water during rainfall. The stemflow was supplied by Markov bottles equipped with a flow control valve. These valves were checked before each experiment to ensure that a constant stemflow was added to each stem. During the experiments, the simulated stemflow uniformly flowed down to the soil surface along the surface of the PVC tube. The rainfall simulator was equipped with nine stainless steel downsprayers that produced raindrops with a mean diameter of 2.3 mm and a kinetic energy impact rate of 85% of natural rainfall at the corresponding intensities (Si et al., 2013). The height of the sprayer was 4 m, and the rainfall simulator could be set to simulate rainfall with intensities ranging from 20 to 150 mm/h by adjusting the water pressure in the supply pipe (Fig. 2). The detailed experimental procedures are described in the next subsection. The soil that was used was collected from the surface horizon on sloping land in Huaxi District (26°28′32″ N, 106°42′02″ E), Guizhou Province, China. The soil texture was 44.42% clay, 44.21% silt, and 11.37% sand. The natural surface soil had a bulk density ranging from 0.92–1.31 g/cm3 and soil porosity ranging from 50.46–65.21%. The organic matter, pH value, total nitrogen and total phosphorus of the soil were 5.13%, 7.6, 0.282 and 0.084 g/kg, respectively.
increased rill erosion occurred on cultivated land and indicated that the main reason may be related to the partitioning of rainfall into stemflow. During rainfall, the partitioning of rainfall into stemflow is generated because, except for a small portion of canopy interception that forms larger drops that then drip to the ground surface from the leaf edge, most of the canopy interception finally reaches the ground surface through the stem, i.e., in the form of stemflow (Lamm and Manges, 2000; Zheng et al., 2018). On maize-planted lands, previous studies have shown that the amount of stemflow increases with maize growth and that the maximum proportion of stemflow accounts for approximately 70% of the total rainfall amount (Parkin and Codling, 1990; Lamm and Manges, 2000; Liu et al., 2015). Existing publications have suggested that the presence of stemflow alters the soil surface water flux and thus causes heterogeneous infiltration of water into the soil (Zheng et al., 2019; Fan et al., 2015c). Based on two-dimensional HYDRUS models, Fan et al. (2015a) indicated the important effect of stemflow on the soil moisture in the vadose zone and groundwater recharge and their simulation results showed that infiltrating rainwater is distributed unevenly due to canopy interception and partitioning of rainfall into throughfall and stemflow. The above discussion shows that stemflow not only is the main component of rainfall but also has a great influence on surface hydrological processes. In recent decades, researchers have mainly focused on the reduction in splash erosion via the effects of crop canopies on the characteristics of falling raindrops, spatial variability of stemflow in differently managed plantations and the soil water dynamics around a stem base due to stemflow impacts (Liang et al., 2011; Levia and Germer, 2015; Fan et al., 2015b, 2015c). Meanwhile, several models have been presented to quantify the stemflow fluxes for different plant species (van Elewijck, 1989; Levia and Germer, 2015; Ma et al., 2016). However, little attention has been paid to relating stemflow to soil erosion. Furthermore, on sloping lands, rill erosion is usually attributed to concentrated flow erosion due to the soil surface roughness and heterogeneous microrelief (Helming et al., 1998; Valentin et al., 2005; Zhao et al., 2018a). Although the effects of plant stems have sometimes been mentioned and considered to be an important cause of soil erosion in some studies, these studies mainly focused on the effects of plant stems or plant residue on surface hydrodynamic forces (Levia and Germer, 2015; Liu et al., 2015; Luo et al., 2018). Certainly, few studies have tried to quantify and investigate the effects of stemflow on soil erosion processes under rainfall conditions. Therefore, it is currently unclear how stemflow affects the soil erosion process and how much soil and water loss is directly caused by stemflow. The objective of this study was to analyse the effect of stemflow on soil erosion under controlled situations and gain more comprehensive evidence that can be used to evaluate the soil and water conservation benefits of planting maize.
2.2. Experimental procedures 2.2.1. Soil preparation in the steel box The soils were air-dried at room temperature to approximately 7% water content and were then sieved through a 5 mm sieve to ensure homogeneity. The soil in each steel box was prepared as follows. First, the soil was packed into the steel box and uniformly spread by hand. Second, a wooden block was used to tamp across the soil surface to obtain a smooth surface in each steel box. The soil was packed into the steel box with a mean bulk density of 1.15 g/cm3. After the soil was prepared in the steel box, a 30 min pre-wetting rain at an intensity of 30 mm/h was immediately applied to the soil surfaces 24 h prior to the actual rainfall simulation experiment. The purpose of this pre-wetting rain was to recover soil cohesion after human disturbance and, more importantly, ensure that the initial soil moisture near the surface was at a similar level for all rainfall runs, hence allowing for a better comparison of the generated runoff and erosion associated with the different experimental treatments. In this study, the initial soil moisture near the surface (0–6 cm) was approximately 28.32% on average. 2.2.2. Rainfall simulation In previous studies, the stemflow was measured using a special collection unit that was attached to the lower part of a maize stem (Finney, 1984; Lamm and Manges, 2000; Ma, 2011; Zheng et al., 2019). These studies found that the maximum partitioning of rainfall into stemflow could account for half or more of the total rainfall amount and varied with increasing maize canopy and rainfall intensities. To enhance the applicability of the results, three rainfall intensities of 60, 90 and 120 mm/h and three stemflows of 5, 10 and 15 g/s were applied in this study. These values represent a wide range of the proportion of rainfall that is redistributed as stemflow (approximately 15%−90%) under three rainfall intensities. During rainfall, all runoff and sediment samples were collected after the runoff reached a steady state. Initially, to achieve this aim, the steel boxes were left in the rain for a specified amount of time (approximately 30 min after initiation of rainfall) to obtain a steady state of surface runoff under rainfall intensities of 60, 90 and 120 mm/h. After the runoff and sediment samples collected from the steel boxes reached an apparent steady state, which was defined as volume differences less than 2 mL for the runoff and sediment samples collected in three successive bottles (Darboux and Huang, 2005; Zhao et al., 2016), a series of runoff samples were collected in 1 L plastic bottles at 2 min intervals from the outlet of the steel box (no stemflow-Experiment A in Fig. 2).
2. Materials and methods 2.1. Experimental setup and soils The experiments were carried out in steel boxes that were 1.3 m long, 0.25 m wide, and 0.15 m deep (Fig. 1). At the bottom of each steel box, ten holes with a diameter of 3 cm and were uniformly distributed at a 10 cm interval for draining the infiltrated water. At the downslope end of the steel box, a V-shaped drainage outlet was used to collect surface runoff and sediment samples during rainfall. The slope of the test box was 10°. A PVC tube was used to simulate a natural maize stem. In this study, only mature maize plants, which were made of PVC tubes that were 2 cm in diameter and 1 m in height, were simulated. For each steel box, a total of four stems were placed vertically and fixed to the bottom of the steel box at 30 cm intervals to allow for the introduction of stemflows to the soil surface. The 30 cm spacing interval is in accordance with the local maize planting practice. The hole at the top of each PVC 2
Agricultural and Forest Meteorology 281 (2020) 107814
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Fig. 1. Schematic diagram of the experimental setup used in this study.
Fig. 2. Illustration of the experimental processes and treatments conducted during rainfall.
the same rainfall procedure was used. For each run, the steel box was prepared with fresh soil. After rainfall, the sample bottles were weighed immediately, and approximately 5 mL of saturated alum was added to each bottle to flocculate the solid fraction. Then, the samples were kept at room temperature for approximately 24 h; then, the clear supernatant was poured off, and the sediments were decanted into iron basins and ovendried at 105 °C for approximately 12 h to eliminate water weight. The dry sediments were weighed, and the sediment mass was calculated by subtracting by the tare weight of the iron basin from the weight of the dry sediments. The runoff mass was calculated by subtracting the tare weight of the sample bottle and sediment mass from the weight of the sample bottle collected during the experiment. The runoff, sediment rate and sediment concentration under the different experimental stages were calculated from these data. The sediment concentration was
After sample collection, stemflow of 5 g/s, which was simulated by a Markov bottle, added to the steel box along the surface of the PVC tube. When the surface runoff collected from the downslope outlet of the steel box reached a steady state again, six runoff and sediment samples were collected (with stemflow-Experiment B in Fig. 2). Then, the stemflow was stopped, and after a few minutes, six runoff and sediment samples were collected again (no stemflow-Experiment C in Fig. 2). Then, stemflow of 10 g/s was fed into the steel box along the surface of the PVC tube. After an apparent steady state was reached at the outlet of the steel box, six additional samples were collected (with stemflowExperiment D in Fig. 2). Experiment E (no stemflow) and Experiment F (with stemflow of 15 g/s) were then carried out using the same procedures as Experiments C and D. These runoff and sediment samples represent the soil erosion characteristics of the different stages of Experiments A to F. For each rainfall intensity of 60, 90 and 120 mm/h,
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increased. The statistical analysis showed that the mean runoff rate was significantly different (p < 0.05) among the different stemflow rates. Specifically, the mean runoff rate was 7.34 g/s at the surface without stemflow (i.e., control in Fig. 4), while the runoff rates on the surfaces with stemflow ranged from 27.78 to 62.40 g/s (i.e., Experiments B, D, and F) (Table 1). Thus, at a stemflow of 5 to 15 g/s, the runoff loss increased by approximately 278% to 750% compared to the runoff loss of the control. When subtracting the amount of stemflow fed into the steel box from the collected runoff, the actual runoff rates in Experiments B, D, and F were still three times greater than those in Experiments A, C, and E, and furthermore, these increases in runoff increased with increasing rainfall intensities. This result further implies that the stemflow contributes to surface runoff generation and significantly increases the amount of runoff losses from maize-planted lands during a rainfall event.
calculated as the ratio of dry sediment to runoff mass. The mean runoff and mean sediment rates for each treatment with and without stemflow were calculated based on the data of the three rainfall intensities. 2.3. Rill measurement To evaluate the effect of stemflow on rill development on the surface, the rill characteristics were also measured by hand using a ruler after each experimental stage (i.e., Experiments A-F). The rill characteristics, including rill length, width and depth, were recorded at cross-sections with a 5 cm interval from the downslope end to the top end of the steel box. 2.4. Data analysis Using runoff and sediment data obtained from the rainfall experiments, the stemflow contributions to runoff and sediment were determined as follows:
SCR =
R1 − R 0 × 100% R0
3.2. Sediment production The changes in sediment rate with different stemflows under different experimental conditions are shown in Fig. 5. Compared to the experimental conditions without stemflow (i.e., control), under the experimental conditions with stemflow, the sediment rate significantly increased. Furthermore, the statistical analysis showed that the mean sediment rates between the control and the experimental conditions with stemflow (i.e., the sediment rate in control vs. Experiments B, D and F) were significantly different (Fig. 6). These differences existed under three rainfall intensities (60, 90 and 120 mm/h). However, significant differences in sediment rates were not found among the different stemflow rates except for that between Experiments B and F at a rainfall intensity of 60 mm/h. This result occurred because the conditions of Experiments A-F were carried out as one successive rainfall event according to our experimental design. In the early stages of the experiment, such as in Experiment B, loose soil particles were widespread on the surface, so the sediment source was sufficient for transport by surface runoff, which is generated from rainfall alone. Therefore, even if the surface runoff is relatively small at a low stemflow, it can move large quantities of sediment at a maximum sediment carrying capacity. However, the opposite occurs at the later stages of rainfall, such as in Experiment F, because the soil particles spread over the soil surface were washed off by surface runoff in the early stage of the experiment. This characteristic is obvious at a high rainfall intensity. Although the sediment rate in Experiment B was larger than that in Experiments D and F, these data were all significantly greater than that in the control. Overall, soil erosion increased ten times after stemflow generation compared to that of the control (Table 1). This scenario suggests that our result is good for illustrating our study objective, which is that stemflow increases soil erosion. Under three rainfall intensity conditions, the dynamic changes in sediment concentrations under different experimental conditions (i.e., Experiments A to F) are illustrated in Fig. 7. The greatest sediment concentration was measured in Experiment B with rainfall intensities of 90 and 120 mm/h. This result occurs because exposed or/and unconsolidated soil particles on the surface are easily transported in the early stage of runoff generation. The greatest sediment concentration was measured in Experiment F with a rainfall intensity of 60 mm/h. This result occurred because, at a reduced rainfall intensity, the soil particles did not quickly wash off with runoff from the surface in the early stage of runoff generation, and when rill development later occurred on the surface, the soil particles were lost through rill flow and hence increased the sediment concentration. Overall, the changes in sediment concentration were significantly different between the experimental conditions with stemflow and the control.
(1)
where SCR represents the stemflow contribution to the generated runoff, %; R0 and R1 represent the runoff rates without stemflow (i.e., control) and with stemflow conditions, g/s, respectively. According to the experimental design, Experiments A, C and E are control measures for Experiments B, D and F, respectively, in this study. Therefore, the conditions without stemflow include Experiments A, C and E, and the conditions with stemflow include Experiments B, D and F. Similarly, the stemflow contribution to sediment generation (SCS) was calculated as follows:
SCS =
S1 − S0 × 100% S0
(2)
where SCS represents the SCS, %; S0 and S1 represent the sediment rates without stemflow (i.e., control) and with stemflow conditions, g/s, respectively. We used Microsoft Excel 10.0 for the data arrangement and plots. A one-way ANOVA was used to determine significant differences (significant at the p < 0.05 level) of sediment rate, runoff rate, sediment concentration, SCR, SCS, mean runoff rate and mean sediment rates between the different treatments. Statistical differences were determined using DPS 7.05, which is a data processing system presented by Tang and Zhang (2013). 3. Results 3.1. Runoff production The runoff generation process that was obtained after a steady state was reached is shown in Fig. 3. For all rainfall intensities, the surface runoff reached a steady state after approximately 30 min from the beginning of rainfall. The greater the rainfall intensity is, the greater the steady runoff rates. After the steady state of surface runoff was reached, the runoff rate was determined by only the rainfall intensity and stemflow. When stemflow was fed into the ground surface, the runoff rates significantly increased for all rainfall times. The runoff curves showed similar dynamic characteristics, increasing with increasing stemflow. The greater the stemflow, the greater the runoff rates. In addition, the runoff rates in Experiments A, C and E were very similar to each other (i.e., mean ± standard deviation of Experiments A, C and E were 7.34 ± 3.57, 6.96 ± 3.07 and 7.72 ± 4.40 g/s, respectively; p > 0.05), so they were combined when the stemflow contributions to runoff and sediment generation were calculated using Eqs. (1) and (2). The mean runoff rates of the different experimental conditions with stemflow and the control are shown in Fig. 4. Compared to the control, the mean runoff rates of the surfaces with stemflow significantly
3.3. Rill development Fig. 8 shows the changes in rill depth and width with increasing 4
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Fig. 3. Dynamic changes in the surface runoff with rainfall time under different stemflows and rainfall intensities (letters A to F represent different experimental treatments, as shown in Fig. 2).
developed directly to the third stem under 5 g/s stemflow (Fig. 8c). Subsequently, with the increase in runoff flow and continuous rain, headward erosion and rill wall collapse occurred continuously. The rill widened the most at the 15 g/s stemflow. Therefore, rill erosion was dominant, and lateral erosion was relatively weak at low rainfall intensities and stemflows. Accordingly, Fig. 9 shows the rill length characteristics under different stemflows at three rainfall intensities. At a rainfall intensity of 60 mm/h, the rill length of the slope under 15 g/s stemflow was longer than that of the slopes under 10 and 15 g/s stemflows. At rainfall intensities of 90 and 120 mm/h, the rill lengths showed similar characteristics among the three stemflows. That is, the rill lengths of the slope under 10 and 15 g/s stemflows were essentially equal, and both were significantly longer than that of the slope under 5 g/s stemflow.
slope length at different stemflows under three rainfall intensities. Because rill characteristics were measured at the cross-sections at 5-cm intervals, each point represents a record of rill depth and width. Missing points indicate that there was no rill erosion at that position on the soil surface, and this finding also reflects that rill erosion was discontinuous on the slope. The widths and depths of rills differed at the different slope positions. Under a rainfall intensity of 60 mm/h and stemflows of 5 and 10 g/ s, the rills on the surface were mainly discontinuous around the stem base. When the stemflow increased to 15 g/s, the discontinuous rills between the first stem and the second stem (i.e., stem #1 and #2 in Fig. 8a) connected. As a result, more sediment was lost through the rill, leading to the expansion of the rill, and the rill began to deepen (Fig. 8a). This result means that lateral erosion had started to occur (i.e., rill wall collapse). At the same time, rill erosion began to occur around the fourth stem base (i.e., stem #4 in Fig. 8a). At a rainfall intensity of 90 mm/h, rill erosion occurred continuously with increasing stemflow, and the head of the rill reached the fourth stem at 10 and 15 g/s stemflows (Fig. 8b). As the stemflow increased, the rill became wider (Fig. 8e) because both sides of the rill wall collapsed with increasing stemflow. However, the change in the depth of the rill was not obvious. In addition, the rill width fluctuated over time because the rill walls collapsed due to the abundant sediment that was not transported from the surface and was temporarily deposited in the rills. When the rainfall intensity was 120 mm/h, the rill
4. Discussion Our results show that stemflow significantly increased soil erosion, which may be related to rainwater transformation mechanisms above and below the canopy. During rainfall, rainwater is transformed into throughfall and canopy interception, and this process leads to a reduction in the impact of rainfall detachment on soils because the canopy interception process decreases the raindrop energy that reaches the ground surface, which decreases soil detachment and transportation by raindrop action under the maize canopy (Quinn et al., 1983; Fig. 4. Mean runoff rates of different stemflow conditions (Experiments B, D and F represent stemflow rates of 5, 10, and 15 g/s) and different rainfall intensities (60, 90 and 120 mm/ h) compared to the control (no stemflow); the bars represent the standard deviations (N = =3); different letters indicate significantly different means based on the least significant difference (LSD) method (at p < 0.05) for each rainfall intensity.
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Table 1 Average runoff rate, sediment rate, sediment concentration and maize stemflow contributions to runoff and sediment generation during rainfall. Experiments
Stemflow rate (g/s)
Runoff rate (g/s)
Sediment concentration (g/kg)
Sediment rate (g/s)
SCR (%)
SCS (%)
A, C, E B D F
0 5 10 15
7.34 ± 3.61a 27.78 ± 4.56b 48.71 ± 4.11c 62.40 ± 3.64d
4.28 ± 2.42a 21.10 ± 9.64b 10.55 ± 0.56c 8.69 ± 3.39c
0.19 2.96 2.43 2.54
/ 278.56 ± 5.26a 563.55 ± 9.94b 750.17 ± 7.65c
/ 1503.08 ± 121.47a 1214.52 ± 76.84a 1275.64 ± 95.52a
± ± ± ±
0.18a 1.74b 0.33b 1.01b
Data (mean ± standard deviation) are the average values of three rainfall intensities;. SCR and SCS represent the maize stemflow contribution to runoff and sediment generation, respectively; SCR = =(runoff rate with stemflow- runoff rate without stemflow)/runoff rate without stemflow × 100%; SCR= =(sediment rate with stemflow- sediment rate without stemflow)/sediment rate without stemflow × 100%. The means within a column followed by a dissimilar letter are significantly different at the p = =0.05 level using the least significant difference (LSD) method (N = =3).
Fig. 5. Dynamic changes in the sediment with rainfall time under different stemflows and rainfall intensities (letters A to F represent the different experimental treatments as shown in Fig. 2).
erosion capacity of raindrops is reduced, then the total amount of soil erosion on the slope due to surface runoff will decrease accordingly. In terms of rainfall-induced erosion, the crop canopy certainly has positive impacts on sediment reduction by reducing the total rainwater that directly falls on the soil surface. As a result, soil erosion decreased on the maize-planted slope during rainfall. This result has been used in the revised universal soil loss equation (RUSLE) (Wischmeier and Smith, 1978) and explains why a low sediment rate was observed during rainfall when stemflow was not generated (Kinnell, 2005). However, from the point of view of rainwater conversion, the
Ma et al., 2015). An et al. (2012) showed that sediment concentrations were greatly reduced by 48.13% to 96.50% after raindrop impacts were eliminated. The total reduction in soil erosion is associated with changes in soil detachment and transportation due to raindrop impacts, which are important subprocesses that largely determine the total amount of sediment during the rainfall erosion process (Savat and Poesen, 1981; Mermut et al., 1997). These authors further explained that raindrop impacts are an important factor that increases the hydrodynamic forces of runoff, which are closely related to the detachment and transportation capacity of surface runoff. That is, if the
Fig. 6. Mean sediment rates of different stemflow conditions (Experiments B, D and F represent stemflow rates of 5, 10, and 15 g/s) and different rainfall intensities (60, 90 and 120 mm/ h) compared to the control (no stemflow); the bars represent standard deviations (N = =3); different letters indicate significantly different means based on the least significant difference (LSD) method (at p < 0.05) for each rainfall intensity.
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Fig. 7. Dynamic changes in the sediment concentration with rainfall time under different stemflows and rainfall intensities (letters A to F represent the different experimental treatments as shown in Fig. 2).
Fig. 8. Rill depth and width in the direction of slope length under different stemflow conditions (5, 10 and 15 g/s). 7
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Fig. 9. Rill length under different stemflow conditions; the bars represent standard deviations (N = =3); different letters indicate significantly different means based on the least significant difference (LSD) method (at p < 0.05) for each rainfall intensity.
development in the upslope area is equivalent to headward erosion, which has been widely studied in previous publications (Gómez et al., 2003; Wells et al., 2009); the process of rill development in the downslope area is due to downcutting and scouring actions from concentrated flow. Different from the slope without stemflow, the observed flow lines were formed between the spaces of stems after stemflow was input to the slope with stemflow. As a result, sediment transport was mostly concentrated in these flow lines, and more importantly, concentrated flow increased with the addition of rainfall runoff; thus, detachment and the transport capacity of concentrated flow increased (Rauws and Govers, 1988; Slattery and Bryan, 1992; Polyakov and Nearing, 2003). Finally, flow lines gradually developed to rills in the downslope area. An obvious characteristic is that the initial rills did not connect, but in addition to the increases in stemflow and rainfall duration, headward erosion accelerated; the rills that were previously separated gradually became connected, forming a connected flow path; hence, the length of the rill increased. Meanwhile, the depth and width also increased with the increase in stemflow-induced lateral erosion. The development rates of rills showed different characteristics between high and relatively low stemflows. The greater the stemflow, the longer the rill length. This result is confirmed by the results reported by Sun et al. (2013) because high stemflow is equal to high surface flow, which promotes the generation of rills on the soil surface. In this study, no significant differences in rill length were measured between 10 and 15 g/s stemflows under 90 and 120 mm/h rainfall intensities. This result occurred because our data were obtained from a slope with a small area (1.3 m length). Under high stemflow and rainfall intensity conditions, rills quickly developed from the first stem to the fourth stem. After that, the rill width and depth mostly changed. Therefore, the depth and width of rills exhibited apparent fluctuations along the slope direction (Fig. 8). Based on our experimental observations, we concluded that a potential rill erosion area exists around the stem base due to stemflow. The main mechanism by which stemflow contributes to rill development can be illustrated as an idealized drawing in Fig. 10. During a rainfall event, canopy interception flows down to the ground along stems, thus forming stemflow. Then, a portion of the stemflow infiltrates the soil along crop roots and becomes soil water; meanwhile, another portion of stemflow becomes surface flow and flows downslope. The stemflow that becomes surface flow contributes to the total water loss from the surface and increases the detachment and transportation capacity of surface runoff; finally, rill development occurs on the soil surface around the stem base with prolonged rainfall. As a result, a potential rill erosion area was formed on the soil surface around the stem base in this study. The mechanism through which maize
impact of the crop canopy on soil erosion was different before and after stemflow generation. When the flow from canopy interception travels down through the stem, stemflow is generated and contributes to surface runoff from the slope. At this point, the transportation of soil by surface flow increases on the slope, and this process represents a driving condition for soil erosion intensification. Furthermore, rill erosion is also a key reason for the increased soil erosion on maize-planted slopes due to stemflow. During rainfall, under increased crop canopy cover, the amount of canopy interception storage gradually increases with the decrease in the amount of throughfall. Therefore, a large amount of rainwater finally reaches the ground via the stem. A laboratory experiment reported by Ma (2011) suggested that the largest proportion of the rainwater volume to the total rainfall reached approximately 70%. When stemflow reaches the soil surface, it contributes to the formation of concentrated flows around the stem base but not overland flows. Such concentrated flows result in increased soil detachment in the area around the stem base, contributing to rill occurrence, ultimately intensifying soil erosion on sloping farmland. Therefore, the sediment rates observed during rainfall in Experiments B, D and F with stemflow were greater than those in Experiments A, C and E without stemflow (i.e., control in Fig. 6). This result implies that stemflow can significantly change the surface hydraulic characteristics and hence impact the soil erosion process. In addition, the high variations in the sediment rate observed with different amounts of stemflow may be related to the rainfall erosion processes. Many studies have suggested that the sediment detachment process varies before and after rill development (Góvers 1991; Bradford and Huang, 1994; Zhao et al., 2018a). After rill erosion occurred, the rill development process continued to advance as stemflow increased. In the early stage of rill development, the surface soil was extensively denuded, and the slope sediment yield increased dramatically. However, when rill development was complete, rill erosion was relatively weakened because surface water could flow from the slope along the rill in a short time, and sediment loss along the slope was relatively reduced, which was the main reason for the above changes in sediment rate and sediment concentration (Gómez et al., 2003; Zhao et al., 2018b). Rill erosion occurs on the soil surface after stemflow appears because concentrated flow is generated around the stem base due to stemflow. Under steady state runoff conditions, the flow velocity and flow amount increase on the surface near a stem, increasing soil detachment and runoff transportation (Merten et al., 2001; Zhang et al., 2003). In addition, from our observations of rill development in the experimental processes, rill erosion occurred on the soil surface after a short stemflow time, and a rill incision first began to occur around the stem base and then extended downslope and upslope. The process of rill 8
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Fig. 10. Schematic diagram of rill erosion due to the impact of stemflow.
width and depth of rills with prolonged rainfall duration. As a result, stemflow increases the amount of loosened sediment on a slope more than that on a slope without stemflow. Therefore, whether stemflow is produced represents a key factor that influences the soil erosion process during rainfall events. In other words, if stemflow is generated, soil erosion will increase because of rill erosion; otherwise, soil erosion is small. In natural situations, stemflow is generated from the redistribution of crop canopy rainfall; thus, the relative proportions of rainwater to stemflow differ with the growth of plants. Thus, the contribution of stemflow to soil erosion should be taken into account in soil erosion predication to provide a more reliable assessment of sloping lands. For example, in the case of extensive crops, such as maize, the estimation of water erosion risk from simple data such as vegetation cover may lead to inaccurate results. In addition, the effect of stemflow on soil erosion was assessed for only mature plants and proved to be significant in this study. To achieve a better understanding of how stemflow affects soil erosion, our results need to be further extended for a wide range of experimental situations. In future studies, it may be important to discuss the effect of stemflow on soil water changes, soil surface hydrological dynamics and soil particle detachment and transportation in natural field conditions.
stemflow influences soil and water loss from the ground surface is by impacting the rainfall characteristics through rainfall redistribution, which leads to alterations in rainfall erosivity and a change in the dominant erosion type from sheet erosion to rill erosion under the maize canopy. Our results suggest that stemflow contributes to soil erosion on cultivated lands. This result provides another perspective for researchers to understand the mechanisms of soil erosion on cultivated lands with crop cover. Although our results reveal the role of stemflow in soil erosion, limitations exist because the experiments were conducted under laboratory situations and the maize stems were substituted with PVC tubes. This simplification resulted in differences in the generation of stemflow between our experiment and natural situations in the field. In our experiment, rill erosion occurred once the stemflow was added to the soil surface. However, rill erosion is very complex in natural situations because there are many factors other than stemflow that can affect soil erosion, such as soil surface roughness, rainfall duration, and soil properties. Most importantly, in natural situations, plant residue cover usually exists in maize lands (Wilson et al., 2008; Gerrits et al., 2010). Therefore, the effect of stemflow on soil erosion is also affected by surface cover. In our study, all experiments were conducted on the soil surface without considering surface cover. Therefore, high sediment rates were measured in all experimental stages with stemflow; however, future studies should determine how much higher these rates are than the actual situation. At this point, our data are not complete enough to fully interpret the processes and mechanisms for the effect of maize stemflow on surface runoff and sediment production on agricultural lands. Our results and experimental setup will help us design new erosion experiments and improve stemflow-induced erosion research to further understand the effects of maize crops on the process of erosion on agricultural lands.
Declaration of Competing Interests 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. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (41867014, 41601293), the Science and Technology Project of Guizhou Province (QKHJC[2016]1027, QKH [2016]ZC2835, QKHJC[2017]1041), and the Excellent Young Science and Technology Talents Program of Guizhou Province (QKHPTRC[2019]5671).
5. Conclusions This study showed an example of how stemflow contributes to surface soil erosion. An experimental setup including a soil box and stemflow simulation device was built and run. The results from our laboratory studies show that stemflow has a significant effect on soil erosion processes and soil erosion amounts. The contribution of stemflow to concentrated flow can trigger the occurrence of rill erosion around the stem base and further promote the expansion of the length,
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Stemflow contributions to soil erosion around the stem base under simulated maize-planted and rainfall conditions
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Longshan Zhao , Qian Fang, Ye Yang, Hao Yang, Tonghang Yang, Hao Zheng College of Forestry, Guizhou University, Guiyang 550025, Guizhou Province, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Stemflow Stemflow partitioning Soil erosion Rill erosion
Stemflow is a primary pathway through which rainwater reaches the ground surface under crop cover. Substantial research suggests that stemflow amounts approach or exceed half the total precipitation on maize cropland; however, few studies have quantified the effect of stemflow on soil erosion during rainfall events. This study aimed to measure the effect of stemflow on soil erosion under controlled situations and determine the effects of stemflow on soil erosion. Maize stems were simulated using a 2-cm diameter PVC tube in 1.3 by 0.25 m steel boxes to introduce stemflows to the soil surface at a 10° slope. The rainfall intensities were 60, 90 and 120 mm/h; the stemflow amounts were 5, 10 and 15 g/s. The results showed that stemflow significantly increased soil erosion. As stemflow increased, the surface runoff and sediment rates sharply increased. Compared with a control slope (no stemflow), stemflow increased the surface runoff and sediment rates by more than three and twelve times, respectively. This result occurred because stemflow contributes to the formation of concentrated flows, which easily trigger rill erosion around the stem base. The sediment rate further increased with rill development. Soil erosion was small if stemflow did not occur during a rainfall event; otherwise, soil erosion was extensive due to stemflow-induced rill erosion. Our results provide new insights for the analysis of crop cover effects and soil erosion on cultivated lands. For maize-planted slopes, stemflow may also be indispensable when determining the soil erosion amount.
1. Introduction Maize crops are important food crops and are widely planted throughout the world (van Zelm et al., 2017; Zimmer et al., 2018). On sloping lands, planting maize is commonly considered an effective measure to control soil and water loss because it increases the vegetation cover (Mohammed and Gumbs, 1982; Kiboi et al., 2017; Luo et al., 2018). In recent decades, many studies have been conducted under laboratory and field conditions to determine the effects of planting maize on soil erosion mechanisms, especially the effects of the maize canopy on raindrop redistribution, interception and splash erosion on the soil surface (Lamm and Manges, 2000; Ma et al., 2015; Zheng et al., 2019). Previous studies on the characteristics of raindrops under maize canopies have concluded that the maize canopy mainly redistributes rainwater via throughfall and canopy interception (Armstrong and Mitchell, 1987; Parkin and Codling, 1990; Paltineanu and Starra, 2000; Liu et al., 2015). Canopy interception decreases the kinetic energy of raindrops (Wischmeier and Smith, 1978; Ma et al., 2015) and vastly reduces soil detachment and transportation caused by raindrop
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impacts, which are important soil erosion processes that largely determine the total amount of sediment in rainfall-induced erosion (Savat and Poesen, 1981; Mermut et al., 1997). The effect of the maize canopy on soil erosion is related to the growth of maize because the partitioning of rainwater into throughfall and canopy interception components varies at different maize growth stages (Liu et al., 2015; Zheng et al., 2019). Generally, with the growth of maize, the vegetation coverage and leaf area index gradually increase, and thus, the effect of a maize canopy on the prevention of rainfall-induced erosion becomes increasingly obvious (Ma et al., 2016; Luo et al., 2018). In other words, the soil area exposed to raindrop impacts decreases with increasing vegetation cover, leaf area index and interception efficiency. In addition, Zheng et al. (2014) suggested that the roots of maize contribute to a decrease in soil erodibility. As a result, the soil erosion that is directly induced by rainfall action is reduced at the soil surface. All of the effects discussed above can result in soil and water losses that are lower on maize-planted lands than on control or bare lands. However, some researchers have also noted different soil erosion results on maize-planted lands (Haynes, 1940; Quinn and Laflen, 1983; Ma et al., 2015). For example, Haynes (1940) and Ma (2011) noted that
Corresponding author. E-mail address: [email protected] (L. Zhao).
https://doi.org/10.1016/j.agrformet.2019.107814 Received 29 June 2019; Received in revised form 20 September 2019; Accepted 15 October 2019 0168-1923/ © 2019 Elsevier B.V. All rights reserved.
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tube was sealed with silicon sealant to prevent the entry of water during rainfall. The stemflow was supplied by Markov bottles equipped with a flow control valve. These valves were checked before each experiment to ensure that a constant stemflow was added to each stem. During the experiments, the simulated stemflow uniformly flowed down to the soil surface along the surface of the PVC tube. The rainfall simulator was equipped with nine stainless steel downsprayers that produced raindrops with a mean diameter of 2.3 mm and a kinetic energy impact rate of 85% of natural rainfall at the corresponding intensities (Si et al., 2013). The height of the sprayer was 4 m, and the rainfall simulator could be set to simulate rainfall with intensities ranging from 20 to 150 mm/h by adjusting the water pressure in the supply pipe (Fig. 2). The detailed experimental procedures are described in the next subsection. The soil that was used was collected from the surface horizon on sloping land in Huaxi District (26°28′32″ N, 106°42′02″ E), Guizhou Province, China. The soil texture was 44.42% clay, 44.21% silt, and 11.37% sand. The natural surface soil had a bulk density ranging from 0.92–1.31 g/cm3 and soil porosity ranging from 50.46–65.21%. The organic matter, pH value, total nitrogen and total phosphorus of the soil were 5.13%, 7.6, 0.282 and 0.084 g/kg, respectively.
increased rill erosion occurred on cultivated land and indicated that the main reason may be related to the partitioning of rainfall into stemflow. During rainfall, the partitioning of rainfall into stemflow is generated because, except for a small portion of canopy interception that forms larger drops that then drip to the ground surface from the leaf edge, most of the canopy interception finally reaches the ground surface through the stem, i.e., in the form of stemflow (Lamm and Manges, 2000; Zheng et al., 2018). On maize-planted lands, previous studies have shown that the amount of stemflow increases with maize growth and that the maximum proportion of stemflow accounts for approximately 70% of the total rainfall amount (Parkin and Codling, 1990; Lamm and Manges, 2000; Liu et al., 2015). Existing publications have suggested that the presence of stemflow alters the soil surface water flux and thus causes heterogeneous infiltration of water into the soil (Zheng et al., 2019; Fan et al., 2015c). Based on two-dimensional HYDRUS models, Fan et al. (2015a) indicated the important effect of stemflow on the soil moisture in the vadose zone and groundwater recharge and their simulation results showed that infiltrating rainwater is distributed unevenly due to canopy interception and partitioning of rainfall into throughfall and stemflow. The above discussion shows that stemflow not only is the main component of rainfall but also has a great influence on surface hydrological processes. In recent decades, researchers have mainly focused on the reduction in splash erosion via the effects of crop canopies on the characteristics of falling raindrops, spatial variability of stemflow in differently managed plantations and the soil water dynamics around a stem base due to stemflow impacts (Liang et al., 2011; Levia and Germer, 2015; Fan et al., 2015b, 2015c). Meanwhile, several models have been presented to quantify the stemflow fluxes for different plant species (van Elewijck, 1989; Levia and Germer, 2015; Ma et al., 2016). However, little attention has been paid to relating stemflow to soil erosion. Furthermore, on sloping lands, rill erosion is usually attributed to concentrated flow erosion due to the soil surface roughness and heterogeneous microrelief (Helming et al., 1998; Valentin et al., 2005; Zhao et al., 2018a). Although the effects of plant stems have sometimes been mentioned and considered to be an important cause of soil erosion in some studies, these studies mainly focused on the effects of plant stems or plant residue on surface hydrodynamic forces (Levia and Germer, 2015; Liu et al., 2015; Luo et al., 2018). Certainly, few studies have tried to quantify and investigate the effects of stemflow on soil erosion processes under rainfall conditions. Therefore, it is currently unclear how stemflow affects the soil erosion process and how much soil and water loss is directly caused by stemflow. The objective of this study was to analyse the effect of stemflow on soil erosion under controlled situations and gain more comprehensive evidence that can be used to evaluate the soil and water conservation benefits of planting maize.
2.2. Experimental procedures 2.2.1. Soil preparation in the steel box The soils were air-dried at room temperature to approximately 7% water content and were then sieved through a 5 mm sieve to ensure homogeneity. The soil in each steel box was prepared as follows. First, the soil was packed into the steel box and uniformly spread by hand. Second, a wooden block was used to tamp across the soil surface to obtain a smooth surface in each steel box. The soil was packed into the steel box with a mean bulk density of 1.15 g/cm3. After the soil was prepared in the steel box, a 30 min pre-wetting rain at an intensity of 30 mm/h was immediately applied to the soil surfaces 24 h prior to the actual rainfall simulation experiment. The purpose of this pre-wetting rain was to recover soil cohesion after human disturbance and, more importantly, ensure that the initial soil moisture near the surface was at a similar level for all rainfall runs, hence allowing for a better comparison of the generated runoff and erosion associated with the different experimental treatments. In this study, the initial soil moisture near the surface (0–6 cm) was approximately 28.32% on average. 2.2.2. Rainfall simulation In previous studies, the stemflow was measured using a special collection unit that was attached to the lower part of a maize stem (Finney, 1984; Lamm and Manges, 2000; Ma, 2011; Zheng et al., 2019). These studies found that the maximum partitioning of rainfall into stemflow could account for half or more of the total rainfall amount and varied with increasing maize canopy and rainfall intensities. To enhance the applicability of the results, three rainfall intensities of 60, 90 and 120 mm/h and three stemflows of 5, 10 and 15 g/s were applied in this study. These values represent a wide range of the proportion of rainfall that is redistributed as stemflow (approximately 15%−90%) under three rainfall intensities. During rainfall, all runoff and sediment samples were collected after the runoff reached a steady state. Initially, to achieve this aim, the steel boxes were left in the rain for a specified amount of time (approximately 30 min after initiation of rainfall) to obtain a steady state of surface runoff under rainfall intensities of 60, 90 and 120 mm/h. After the runoff and sediment samples collected from the steel boxes reached an apparent steady state, which was defined as volume differences less than 2 mL for the runoff and sediment samples collected in three successive bottles (Darboux and Huang, 2005; Zhao et al., 2016), a series of runoff samples were collected in 1 L plastic bottles at 2 min intervals from the outlet of the steel box (no stemflow-Experiment A in Fig. 2).
2. Materials and methods 2.1. Experimental setup and soils The experiments were carried out in steel boxes that were 1.3 m long, 0.25 m wide, and 0.15 m deep (Fig. 1). At the bottom of each steel box, ten holes with a diameter of 3 cm and were uniformly distributed at a 10 cm interval for draining the infiltrated water. At the downslope end of the steel box, a V-shaped drainage outlet was used to collect surface runoff and sediment samples during rainfall. The slope of the test box was 10°. A PVC tube was used to simulate a natural maize stem. In this study, only mature maize plants, which were made of PVC tubes that were 2 cm in diameter and 1 m in height, were simulated. For each steel box, a total of four stems were placed vertically and fixed to the bottom of the steel box at 30 cm intervals to allow for the introduction of stemflows to the soil surface. The 30 cm spacing interval is in accordance with the local maize planting practice. The hole at the top of each PVC 2
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Fig. 1. Schematic diagram of the experimental setup used in this study.
Fig. 2. Illustration of the experimental processes and treatments conducted during rainfall.
the same rainfall procedure was used. For each run, the steel box was prepared with fresh soil. After rainfall, the sample bottles were weighed immediately, and approximately 5 mL of saturated alum was added to each bottle to flocculate the solid fraction. Then, the samples were kept at room temperature for approximately 24 h; then, the clear supernatant was poured off, and the sediments were decanted into iron basins and ovendried at 105 °C for approximately 12 h to eliminate water weight. The dry sediments were weighed, and the sediment mass was calculated by subtracting by the tare weight of the iron basin from the weight of the dry sediments. The runoff mass was calculated by subtracting the tare weight of the sample bottle and sediment mass from the weight of the sample bottle collected during the experiment. The runoff, sediment rate and sediment concentration under the different experimental stages were calculated from these data. The sediment concentration was
After sample collection, stemflow of 5 g/s, which was simulated by a Markov bottle, added to the steel box along the surface of the PVC tube. When the surface runoff collected from the downslope outlet of the steel box reached a steady state again, six runoff and sediment samples were collected (with stemflow-Experiment B in Fig. 2). Then, the stemflow was stopped, and after a few minutes, six runoff and sediment samples were collected again (no stemflow-Experiment C in Fig. 2). Then, stemflow of 10 g/s was fed into the steel box along the surface of the PVC tube. After an apparent steady state was reached at the outlet of the steel box, six additional samples were collected (with stemflowExperiment D in Fig. 2). Experiment E (no stemflow) and Experiment F (with stemflow of 15 g/s) were then carried out using the same procedures as Experiments C and D. These runoff and sediment samples represent the soil erosion characteristics of the different stages of Experiments A to F. For each rainfall intensity of 60, 90 and 120 mm/h,
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increased. The statistical analysis showed that the mean runoff rate was significantly different (p < 0.05) among the different stemflow rates. Specifically, the mean runoff rate was 7.34 g/s at the surface without stemflow (i.e., control in Fig. 4), while the runoff rates on the surfaces with stemflow ranged from 27.78 to 62.40 g/s (i.e., Experiments B, D, and F) (Table 1). Thus, at a stemflow of 5 to 15 g/s, the runoff loss increased by approximately 278% to 750% compared to the runoff loss of the control. When subtracting the amount of stemflow fed into the steel box from the collected runoff, the actual runoff rates in Experiments B, D, and F were still three times greater than those in Experiments A, C, and E, and furthermore, these increases in runoff increased with increasing rainfall intensities. This result further implies that the stemflow contributes to surface runoff generation and significantly increases the amount of runoff losses from maize-planted lands during a rainfall event.
calculated as the ratio of dry sediment to runoff mass. The mean runoff and mean sediment rates for each treatment with and without stemflow were calculated based on the data of the three rainfall intensities. 2.3. Rill measurement To evaluate the effect of stemflow on rill development on the surface, the rill characteristics were also measured by hand using a ruler after each experimental stage (i.e., Experiments A-F). The rill characteristics, including rill length, width and depth, were recorded at cross-sections with a 5 cm interval from the downslope end to the top end of the steel box. 2.4. Data analysis Using runoff and sediment data obtained from the rainfall experiments, the stemflow contributions to runoff and sediment were determined as follows:
SCR =
R1 − R 0 × 100% R0
3.2. Sediment production The changes in sediment rate with different stemflows under different experimental conditions are shown in Fig. 5. Compared to the experimental conditions without stemflow (i.e., control), under the experimental conditions with stemflow, the sediment rate significantly increased. Furthermore, the statistical analysis showed that the mean sediment rates between the control and the experimental conditions with stemflow (i.e., the sediment rate in control vs. Experiments B, D and F) were significantly different (Fig. 6). These differences existed under three rainfall intensities (60, 90 and 120 mm/h). However, significant differences in sediment rates were not found among the different stemflow rates except for that between Experiments B and F at a rainfall intensity of 60 mm/h. This result occurred because the conditions of Experiments A-F were carried out as one successive rainfall event according to our experimental design. In the early stages of the experiment, such as in Experiment B, loose soil particles were widespread on the surface, so the sediment source was sufficient for transport by surface runoff, which is generated from rainfall alone. Therefore, even if the surface runoff is relatively small at a low stemflow, it can move large quantities of sediment at a maximum sediment carrying capacity. However, the opposite occurs at the later stages of rainfall, such as in Experiment F, because the soil particles spread over the soil surface were washed off by surface runoff in the early stage of the experiment. This characteristic is obvious at a high rainfall intensity. Although the sediment rate in Experiment B was larger than that in Experiments D and F, these data were all significantly greater than that in the control. Overall, soil erosion increased ten times after stemflow generation compared to that of the control (Table 1). This scenario suggests that our result is good for illustrating our study objective, which is that stemflow increases soil erosion. Under three rainfall intensity conditions, the dynamic changes in sediment concentrations under different experimental conditions (i.e., Experiments A to F) are illustrated in Fig. 7. The greatest sediment concentration was measured in Experiment B with rainfall intensities of 90 and 120 mm/h. This result occurs because exposed or/and unconsolidated soil particles on the surface are easily transported in the early stage of runoff generation. The greatest sediment concentration was measured in Experiment F with a rainfall intensity of 60 mm/h. This result occurred because, at a reduced rainfall intensity, the soil particles did not quickly wash off with runoff from the surface in the early stage of runoff generation, and when rill development later occurred on the surface, the soil particles were lost through rill flow and hence increased the sediment concentration. Overall, the changes in sediment concentration were significantly different between the experimental conditions with stemflow and the control.
(1)
where SCR represents the stemflow contribution to the generated runoff, %; R0 and R1 represent the runoff rates without stemflow (i.e., control) and with stemflow conditions, g/s, respectively. According to the experimental design, Experiments A, C and E are control measures for Experiments B, D and F, respectively, in this study. Therefore, the conditions without stemflow include Experiments A, C and E, and the conditions with stemflow include Experiments B, D and F. Similarly, the stemflow contribution to sediment generation (SCS) was calculated as follows:
SCS =
S1 − S0 × 100% S0
(2)
where SCS represents the SCS, %; S0 and S1 represent the sediment rates without stemflow (i.e., control) and with stemflow conditions, g/s, respectively. We used Microsoft Excel 10.0 for the data arrangement and plots. A one-way ANOVA was used to determine significant differences (significant at the p < 0.05 level) of sediment rate, runoff rate, sediment concentration, SCR, SCS, mean runoff rate and mean sediment rates between the different treatments. Statistical differences were determined using DPS 7.05, which is a data processing system presented by Tang and Zhang (2013). 3. Results 3.1. Runoff production The runoff generation process that was obtained after a steady state was reached is shown in Fig. 3. For all rainfall intensities, the surface runoff reached a steady state after approximately 30 min from the beginning of rainfall. The greater the rainfall intensity is, the greater the steady runoff rates. After the steady state of surface runoff was reached, the runoff rate was determined by only the rainfall intensity and stemflow. When stemflow was fed into the ground surface, the runoff rates significantly increased for all rainfall times. The runoff curves showed similar dynamic characteristics, increasing with increasing stemflow. The greater the stemflow, the greater the runoff rates. In addition, the runoff rates in Experiments A, C and E were very similar to each other (i.e., mean ± standard deviation of Experiments A, C and E were 7.34 ± 3.57, 6.96 ± 3.07 and 7.72 ± 4.40 g/s, respectively; p > 0.05), so they were combined when the stemflow contributions to runoff and sediment generation were calculated using Eqs. (1) and (2). The mean runoff rates of the different experimental conditions with stemflow and the control are shown in Fig. 4. Compared to the control, the mean runoff rates of the surfaces with stemflow significantly
3.3. Rill development Fig. 8 shows the changes in rill depth and width with increasing 4
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Fig. 3. Dynamic changes in the surface runoff with rainfall time under different stemflows and rainfall intensities (letters A to F represent different experimental treatments, as shown in Fig. 2).
developed directly to the third stem under 5 g/s stemflow (Fig. 8c). Subsequently, with the increase in runoff flow and continuous rain, headward erosion and rill wall collapse occurred continuously. The rill widened the most at the 15 g/s stemflow. Therefore, rill erosion was dominant, and lateral erosion was relatively weak at low rainfall intensities and stemflows. Accordingly, Fig. 9 shows the rill length characteristics under different stemflows at three rainfall intensities. At a rainfall intensity of 60 mm/h, the rill length of the slope under 15 g/s stemflow was longer than that of the slopes under 10 and 15 g/s stemflows. At rainfall intensities of 90 and 120 mm/h, the rill lengths showed similar characteristics among the three stemflows. That is, the rill lengths of the slope under 10 and 15 g/s stemflows were essentially equal, and both were significantly longer than that of the slope under 5 g/s stemflow.
slope length at different stemflows under three rainfall intensities. Because rill characteristics were measured at the cross-sections at 5-cm intervals, each point represents a record of rill depth and width. Missing points indicate that there was no rill erosion at that position on the soil surface, and this finding also reflects that rill erosion was discontinuous on the slope. The widths and depths of rills differed at the different slope positions. Under a rainfall intensity of 60 mm/h and stemflows of 5 and 10 g/ s, the rills on the surface were mainly discontinuous around the stem base. When the stemflow increased to 15 g/s, the discontinuous rills between the first stem and the second stem (i.e., stem #1 and #2 in Fig. 8a) connected. As a result, more sediment was lost through the rill, leading to the expansion of the rill, and the rill began to deepen (Fig. 8a). This result means that lateral erosion had started to occur (i.e., rill wall collapse). At the same time, rill erosion began to occur around the fourth stem base (i.e., stem #4 in Fig. 8a). At a rainfall intensity of 90 mm/h, rill erosion occurred continuously with increasing stemflow, and the head of the rill reached the fourth stem at 10 and 15 g/s stemflows (Fig. 8b). As the stemflow increased, the rill became wider (Fig. 8e) because both sides of the rill wall collapsed with increasing stemflow. However, the change in the depth of the rill was not obvious. In addition, the rill width fluctuated over time because the rill walls collapsed due to the abundant sediment that was not transported from the surface and was temporarily deposited in the rills. When the rainfall intensity was 120 mm/h, the rill
4. Discussion Our results show that stemflow significantly increased soil erosion, which may be related to rainwater transformation mechanisms above and below the canopy. During rainfall, rainwater is transformed into throughfall and canopy interception, and this process leads to a reduction in the impact of rainfall detachment on soils because the canopy interception process decreases the raindrop energy that reaches the ground surface, which decreases soil detachment and transportation by raindrop action under the maize canopy (Quinn et al., 1983; Fig. 4. Mean runoff rates of different stemflow conditions (Experiments B, D and F represent stemflow rates of 5, 10, and 15 g/s) and different rainfall intensities (60, 90 and 120 mm/ h) compared to the control (no stemflow); the bars represent the standard deviations (N = =3); different letters indicate significantly different means based on the least significant difference (LSD) method (at p < 0.05) for each rainfall intensity.
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Table 1 Average runoff rate, sediment rate, sediment concentration and maize stemflow contributions to runoff and sediment generation during rainfall. Experiments
Stemflow rate (g/s)
Runoff rate (g/s)
Sediment concentration (g/kg)
Sediment rate (g/s)
SCR (%)
SCS (%)
A, C, E B D F
0 5 10 15
7.34 ± 3.61a 27.78 ± 4.56b 48.71 ± 4.11c 62.40 ± 3.64d
4.28 ± 2.42a 21.10 ± 9.64b 10.55 ± 0.56c 8.69 ± 3.39c
0.19 2.96 2.43 2.54
/ 278.56 ± 5.26a 563.55 ± 9.94b 750.17 ± 7.65c
/ 1503.08 ± 121.47a 1214.52 ± 76.84a 1275.64 ± 95.52a
± ± ± ±
0.18a 1.74b 0.33b 1.01b
Data (mean ± standard deviation) are the average values of three rainfall intensities;. SCR and SCS represent the maize stemflow contribution to runoff and sediment generation, respectively; SCR = =(runoff rate with stemflow- runoff rate without stemflow)/runoff rate without stemflow × 100%; SCR= =(sediment rate with stemflow- sediment rate without stemflow)/sediment rate without stemflow × 100%. The means within a column followed by a dissimilar letter are significantly different at the p = =0.05 level using the least significant difference (LSD) method (N = =3).
Fig. 5. Dynamic changes in the sediment with rainfall time under different stemflows and rainfall intensities (letters A to F represent the different experimental treatments as shown in Fig. 2).
erosion capacity of raindrops is reduced, then the total amount of soil erosion on the slope due to surface runoff will decrease accordingly. In terms of rainfall-induced erosion, the crop canopy certainly has positive impacts on sediment reduction by reducing the total rainwater that directly falls on the soil surface. As a result, soil erosion decreased on the maize-planted slope during rainfall. This result has been used in the revised universal soil loss equation (RUSLE) (Wischmeier and Smith, 1978) and explains why a low sediment rate was observed during rainfall when stemflow was not generated (Kinnell, 2005). However, from the point of view of rainwater conversion, the
Ma et al., 2015). An et al. (2012) showed that sediment concentrations were greatly reduced by 48.13% to 96.50% after raindrop impacts were eliminated. The total reduction in soil erosion is associated with changes in soil detachment and transportation due to raindrop impacts, which are important subprocesses that largely determine the total amount of sediment during the rainfall erosion process (Savat and Poesen, 1981; Mermut et al., 1997). These authors further explained that raindrop impacts are an important factor that increases the hydrodynamic forces of runoff, which are closely related to the detachment and transportation capacity of surface runoff. That is, if the
Fig. 6. Mean sediment rates of different stemflow conditions (Experiments B, D and F represent stemflow rates of 5, 10, and 15 g/s) and different rainfall intensities (60, 90 and 120 mm/ h) compared to the control (no stemflow); the bars represent standard deviations (N = =3); different letters indicate significantly different means based on the least significant difference (LSD) method (at p < 0.05) for each rainfall intensity.
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Fig. 7. Dynamic changes in the sediment concentration with rainfall time under different stemflows and rainfall intensities (letters A to F represent the different experimental treatments as shown in Fig. 2).
Fig. 8. Rill depth and width in the direction of slope length under different stemflow conditions (5, 10 and 15 g/s). 7
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Fig. 9. Rill length under different stemflow conditions; the bars represent standard deviations (N = =3); different letters indicate significantly different means based on the least significant difference (LSD) method (at p < 0.05) for each rainfall intensity.
development in the upslope area is equivalent to headward erosion, which has been widely studied in previous publications (Gómez et al., 2003; Wells et al., 2009); the process of rill development in the downslope area is due to downcutting and scouring actions from concentrated flow. Different from the slope without stemflow, the observed flow lines were formed between the spaces of stems after stemflow was input to the slope with stemflow. As a result, sediment transport was mostly concentrated in these flow lines, and more importantly, concentrated flow increased with the addition of rainfall runoff; thus, detachment and the transport capacity of concentrated flow increased (Rauws and Govers, 1988; Slattery and Bryan, 1992; Polyakov and Nearing, 2003). Finally, flow lines gradually developed to rills in the downslope area. An obvious characteristic is that the initial rills did not connect, but in addition to the increases in stemflow and rainfall duration, headward erosion accelerated; the rills that were previously separated gradually became connected, forming a connected flow path; hence, the length of the rill increased. Meanwhile, the depth and width also increased with the increase in stemflow-induced lateral erosion. The development rates of rills showed different characteristics between high and relatively low stemflows. The greater the stemflow, the longer the rill length. This result is confirmed by the results reported by Sun et al. (2013) because high stemflow is equal to high surface flow, which promotes the generation of rills on the soil surface. In this study, no significant differences in rill length were measured between 10 and 15 g/s stemflows under 90 and 120 mm/h rainfall intensities. This result occurred because our data were obtained from a slope with a small area (1.3 m length). Under high stemflow and rainfall intensity conditions, rills quickly developed from the first stem to the fourth stem. After that, the rill width and depth mostly changed. Therefore, the depth and width of rills exhibited apparent fluctuations along the slope direction (Fig. 8). Based on our experimental observations, we concluded that a potential rill erosion area exists around the stem base due to stemflow. The main mechanism by which stemflow contributes to rill development can be illustrated as an idealized drawing in Fig. 10. During a rainfall event, canopy interception flows down to the ground along stems, thus forming stemflow. Then, a portion of the stemflow infiltrates the soil along crop roots and becomes soil water; meanwhile, another portion of stemflow becomes surface flow and flows downslope. The stemflow that becomes surface flow contributes to the total water loss from the surface and increases the detachment and transportation capacity of surface runoff; finally, rill development occurs on the soil surface around the stem base with prolonged rainfall. As a result, a potential rill erosion area was formed on the soil surface around the stem base in this study. The mechanism through which maize
impact of the crop canopy on soil erosion was different before and after stemflow generation. When the flow from canopy interception travels down through the stem, stemflow is generated and contributes to surface runoff from the slope. At this point, the transportation of soil by surface flow increases on the slope, and this process represents a driving condition for soil erosion intensification. Furthermore, rill erosion is also a key reason for the increased soil erosion on maize-planted slopes due to stemflow. During rainfall, under increased crop canopy cover, the amount of canopy interception storage gradually increases with the decrease in the amount of throughfall. Therefore, a large amount of rainwater finally reaches the ground via the stem. A laboratory experiment reported by Ma (2011) suggested that the largest proportion of the rainwater volume to the total rainfall reached approximately 70%. When stemflow reaches the soil surface, it contributes to the formation of concentrated flows around the stem base but not overland flows. Such concentrated flows result in increased soil detachment in the area around the stem base, contributing to rill occurrence, ultimately intensifying soil erosion on sloping farmland. Therefore, the sediment rates observed during rainfall in Experiments B, D and F with stemflow were greater than those in Experiments A, C and E without stemflow (i.e., control in Fig. 6). This result implies that stemflow can significantly change the surface hydraulic characteristics and hence impact the soil erosion process. In addition, the high variations in the sediment rate observed with different amounts of stemflow may be related to the rainfall erosion processes. Many studies have suggested that the sediment detachment process varies before and after rill development (Góvers 1991; Bradford and Huang, 1994; Zhao et al., 2018a). After rill erosion occurred, the rill development process continued to advance as stemflow increased. In the early stage of rill development, the surface soil was extensively denuded, and the slope sediment yield increased dramatically. However, when rill development was complete, rill erosion was relatively weakened because surface water could flow from the slope along the rill in a short time, and sediment loss along the slope was relatively reduced, which was the main reason for the above changes in sediment rate and sediment concentration (Gómez et al., 2003; Zhao et al., 2018b). Rill erosion occurs on the soil surface after stemflow appears because concentrated flow is generated around the stem base due to stemflow. Under steady state runoff conditions, the flow velocity and flow amount increase on the surface near a stem, increasing soil detachment and runoff transportation (Merten et al., 2001; Zhang et al., 2003). In addition, from our observations of rill development in the experimental processes, rill erosion occurred on the soil surface after a short stemflow time, and a rill incision first began to occur around the stem base and then extended downslope and upslope. The process of rill 8
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Fig. 10. Schematic diagram of rill erosion due to the impact of stemflow.
width and depth of rills with prolonged rainfall duration. As a result, stemflow increases the amount of loosened sediment on a slope more than that on a slope without stemflow. Therefore, whether stemflow is produced represents a key factor that influences the soil erosion process during rainfall events. In other words, if stemflow is generated, soil erosion will increase because of rill erosion; otherwise, soil erosion is small. In natural situations, stemflow is generated from the redistribution of crop canopy rainfall; thus, the relative proportions of rainwater to stemflow differ with the growth of plants. Thus, the contribution of stemflow to soil erosion should be taken into account in soil erosion predication to provide a more reliable assessment of sloping lands. For example, in the case of extensive crops, such as maize, the estimation of water erosion risk from simple data such as vegetation cover may lead to inaccurate results. In addition, the effect of stemflow on soil erosion was assessed for only mature plants and proved to be significant in this study. To achieve a better understanding of how stemflow affects soil erosion, our results need to be further extended for a wide range of experimental situations. In future studies, it may be important to discuss the effect of stemflow on soil water changes, soil surface hydrological dynamics and soil particle detachment and transportation in natural field conditions.
stemflow influences soil and water loss from the ground surface is by impacting the rainfall characteristics through rainfall redistribution, which leads to alterations in rainfall erosivity and a change in the dominant erosion type from sheet erosion to rill erosion under the maize canopy. Our results suggest that stemflow contributes to soil erosion on cultivated lands. This result provides another perspective for researchers to understand the mechanisms of soil erosion on cultivated lands with crop cover. Although our results reveal the role of stemflow in soil erosion, limitations exist because the experiments were conducted under laboratory situations and the maize stems were substituted with PVC tubes. This simplification resulted in differences in the generation of stemflow between our experiment and natural situations in the field. In our experiment, rill erosion occurred once the stemflow was added to the soil surface. However, rill erosion is very complex in natural situations because there are many factors other than stemflow that can affect soil erosion, such as soil surface roughness, rainfall duration, and soil properties. Most importantly, in natural situations, plant residue cover usually exists in maize lands (Wilson et al., 2008; Gerrits et al., 2010). Therefore, the effect of stemflow on soil erosion is also affected by surface cover. In our study, all experiments were conducted on the soil surface without considering surface cover. Therefore, high sediment rates were measured in all experimental stages with stemflow; however, future studies should determine how much higher these rates are than the actual situation. At this point, our data are not complete enough to fully interpret the processes and mechanisms for the effect of maize stemflow on surface runoff and sediment production on agricultural lands. Our results and experimental setup will help us design new erosion experiments and improve stemflow-induced erosion research to further understand the effects of maize crops on the process of erosion on agricultural lands.
Declaration of Competing Interests 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. Acknowledgements This study was financially supported by the National Natural Science Foundation of China (41867014, 41601293), the Science and Technology Project of Guizhou Province (QKHJC[2016]1027, QKH [2016]ZC2835, QKHJC[2017]1041), and the Excellent Young Science and Technology Talents Program of Guizhou Province (QKHPTRC[2019]5671).
5. Conclusions This study showed an example of how stemflow contributes to surface soil erosion. An experimental setup including a soil box and stemflow simulation device was built and run. The results from our laboratory studies show that stemflow has a significant effect on soil erosion processes and soil erosion amounts. The contribution of stemflow to concentrated flow can trigger the occurrence of rill erosion around the stem base and further promote the expansion of the length,
References 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, 517–526. https://doi.org/10.1097/SS.0b013e3182639de1.
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