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Article

The Effect of Vegetation Restoration on Erosion Processes and Runoff on a Hillslope Under Simulated Rainfall

by
Lele Niu
1,
Jinfei Hu
1,2,
Pengfei Li
2,*,
Guangju Zhao
3 and
Xingmin Mu
1
1
Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling 712100, China
2
College of Geomatics, Xi’an University of Science and Technology, Xi’an 710054, China
3
State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing Hydraulic Research Institute, Nanjing 210029, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(16), 2411; https://doi.org/10.3390/w17162411
Submission received: 9 July 2025 / Revised: 7 August 2025 / Accepted: 13 August 2025 / Published: 15 August 2025
(This article belongs to the Special Issue Applications of Remote Sensing and GISs in River Basin Ecosystems)
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Abstract

Determining the impact of vegetation restoration on runoff and sediment yield is crucial for formulating science-based slope management practices. The study analyzed runoff and sediment yield behavior in relation to varying vegetation cover and components, based on field-simulated rainfall experiments. The results showed that both runoff and sediment yield rates tended to decrease as vegetation cover increased. Vegetation contributed more significantly to the reduction in sediment yield than to the reduction in runoff. For a rainfall intensity of 1.5 mm·min−1, the sediment yield reduced to 37%, 73%, 78%, and 94% under the vegetation coverage of 20%, 40%, 60%, and 90%, respectively. The corresponding sediment yield reduction effects at the rainfall intensity of 2.0 mm·min−1 were 27%, 67%, 78% and 89%, respectively. At a rainfall intensity of 1.5 mm·min−1, the sediment yield reduction contributions of the litter layer, stem-leaf layer, and roots were 36%, 3%, and 51%, respectively. The corresponding sediment yield reduction contributions at the 2.0 mm·min−1 rainfall intensity were 30%, 7%, and 51%, respectively.

1. Introduction

Soil erosion is one of the most serious environmental problems, and its extent and scale may increase with global climate change [1,2,3,4]. Vegetation restoration is one of the important measures in the prevention and control of soil erosion [5,6,7,8]. In recent decades, the reduction in soil erosion and the sediment transported to rivers due to slope vegetation restoration are observed around the world [9,10,11]. Therefore, a thorough understanding of the water and sediment regulation of slope vegetation is of great significance for the optimization and adjustment of vegetation restoration measures and for regional sustainable development.
As a basic component of the catchment, the slopes are the main unit where soil erosion occurs and an important source of sediment. Rainfall runoff is the dominant driving force of soil erosion on slopes [12]. Slope vegetation reduces runoff and sediment yield by weakening the kinetic energy of raindrops, improving the physical and chemical properties of soil, increasing surface roughness, and enhancing soil permeability [13]. The regulating effect of vegetation on water and sediment varies with the change in coverage. For example, Pan and Shangguan [14] found that grassland with different coverage could significantly reduce sediment yield, and the sediment reductions were comparatively higher than the runoff reduction based on the indoor simulated rainfall. Sun et al. [15] further confirmed that the sediment reduction effects of different vegetation coverage were higher than the runoff-reducing effect. However, unreasonable vegetation coverage may also induce pernicious ecological problems such as water shortages, especially in water-scarce areas [5,16,17]. Thus, the threshold issue of vegetation coverage has also attracted widespread attention in recent years [18,19,20]. Eshghizadeh et al. [21] found that the effect of vegetation on soil loss control was better when vegetation coverage exceeded 50% in arid and semi-arid Iran. Vegetation coverage needs to reach 60% or even 65% to achieve better erosion prevention and control for the tropical mountainous areas of Central America [22]. More than 80% of vegetation coverage, by contrast, is required to ensure relatively mild erosion in the red soil hilly region of China [19]. The thresholds vary with different regions. In the Loess Plateau, the vegetation coverage has significantly increased with the implementation of the ‘Grain-for-Green’ project (GfG) since 1999. In this case, the threshold issue has also attracted widespread attention in recent years [6]. Therefore, figuring out the reasonable vegetation coverage is the key for controlling soil erosion and improving the ecological sustainability of water resource.
Vegetation can control soil erosion on slopes through different structural components such as canopy, stem–leaf, root system, and litter layer [23]. Vegetation canopy intercepts rainfall and protects surface soil by dissipating raindrop kinetic energy, thereby reducing both the direct impact and splash effects [24]. Vegetation stems, leaves, and the litter layer not only reduce raindrop splash erosion but also help disperse runoff and minimize runoff erosion [25]. The root systems can increase infiltration and improve soil structure (i.e., promoting aggregate formation and enhancing soil stability and shear strength) [26]. Both the aboveground biomass and root systems of vegetation have an impact on the surface runoff generation and erosion processes. Therefore, separating and quantifying the relative contributions of the aboveground parts and root systems of vegetation in runoff and sediment yield reduction is of great significance for clarifying the mechanism of soil erosion reduction by vegetation. Numerous studies investigated the contributions of different vegetation components to runoff and sediment load reduction based on different experimental conditions. Some studies indicated that the canopy performed better in the overland flow reduction, while the root system was more effective in controlling surface soil loss [27,28]. Vannoppen et al. [7] reported that the above parts of vegetation made a greater contribution to control splash detachment and inter-rill erosion rates, whereas roots mainly reduce rill and gully erosion rates. Zhou and Shangguan [29] and Zhao et al. [30] found that the grass roots could reduce more sediment yield than that of above-ground parts in silt loam soil.
The Loess Plateau, as the sediment source of the Yellow River, has long been a key area for the Chinese ecological and environmental construction [31]. In order to control soil erosion and improve the ecological environment, the Chinese government launched “Grain-for-Green” project in the Loess Plateau in 1999 [32]. Numerous studies showed that vegetation restoration was the key slope measurement driving the reductions in soil erosion and sediment load in the Loess Plateau [33]. However, relevant studies on runoff and sediment regulation of slope vegetation mostly focus on indoor simulation experiments with backfill soil- or field-simulated experiments based on artificially planted vegetation on slopes due to the complexity of field experimental conditions. These experiments are often questioned as the erosion processes occurring on backfill soil or the influence processes of vegetation are not able to represent field processes. This greatly limits the comprehensive understanding and integrated evaluation of the erosion reduction benefits of slope vegetation measures. Therefore, the mechanism of soil erosion prevention and control by natural vegetation covers and the regulatory role of vegetation components in runoff and sediment regulation need to be studied further in the field. To address these gaps, the objectives of this study were to (i) investigate the runoff and sediment yield processes in natural slopes with different vegetation coverage and various vegetation components, (ii) determine the contributions of different vegetation coverage to runoff and sediment yield reduction, (iii) quantify the effect of different vegetation components on runoff and sediment yield reduction.

2. Materials and Methods

2.1. Study Site

The study area is located in the hilly and gully region of the Loess Plateau, which is characterized by steep gullies, suffering from extremely serious soil erosion. The simulated rainfall experiments were conducted in the Xichuan River catchment (N 36°36′, E 109°18′), one of the first-order tributaries of the Yanhe River (Figure 1). This area has a temperate continental semi-arid monsoon with an average annual temperature of 9.4 °C. The mean annual rainfall is of 500–550 mm, most of which exhibits an extremely uneven characteristic and most of the precipitation in this area falls between May and September in the form of short-duration, high-intensity rainstorms. Generally, a single heavy rain can account for 30% or even more of the annual precipitation, with the maximum rainfall intensity reaching 2.4 mm·min−1. The main surface material of the basin is fine loess soil, which is mainly composed of silt particles. The soil quality is characterized by being relatively loose and readily detachable, which, coupled with extreme rainstorms in summer, results in extraordinarily severe soil erosion. The vegetation in the study area is mainly drought grassland. The dominant species of grass are Arundinella anomala and Stipa Bungeana, and artificial vegetation mainly comprises korshinsk peashrub, robinia pseudoacacia, and Astragalus adsurgens.

2.2. Experimental Designs

Field surveys were conducted to select the representative natural grass slope in the hillslopes of Xichuan River catchment to set runoff plots. A downward-spraying rainfall equipment (15 spray nozzles) with a range of simulated rain intensity of 0.17–3.0 mm·min−1 was employed in our study. The effective raindrop descent height of this rain simulated device is 4.5 m with a raindrop diameter of 0.3–6.0 mm and the rainfall uniformity of >85%. The water used in the experiment was relatively clean, with the temperature ranging from 20 to 23 °C during the experiment. The maximum rainfall coverage area of the rain device is 5 m × 4 m (20 m2), which can accommodate the layout of two 4m × 1m runoff plots (Figure 2). Each runoff plot was surrounded by steel plate barriers, which were 15 cm above ground and 15 cm underground. The junctions between the steel plate and the ground were compacted to minimize the impact of the boundary conditions. The runoff gathering trough was set up to collect the sediment-laden flow samples at the end of the runoff plot.
Based on the natural rainstorm data of the maximum 30 min rainfall intensity in the past periods of the study area, the rainfall intensities of 90 mm·h−1 (1.5 mm·min−1) and 120 mm·h−1 (2.0 mm·min−1) were finally determined for the all rainfall simulation experiments [34]. The rainfall duration of a single simulated experiment was determined to be 45 min. All rainfall experiments were conducted between 5:00 a.m. and 7:00 a.m., when the weather conditions were relatively windless, and the evapotranspiration differences between various plots were small. In addition, the experimental area was surrounded by windbreak nets to further minimize the impact of wind on the uniformity of the rainfall. In order to keep the initial soil moisture consistent for all experimental plots, a light rainfall (20 mm·h−1 rainfall intensity) was carried out on the runoff plots before the start of the rainfall simulation. Then, another rainfall experiment was conducted when the soil moisture content returned to a level similar to the initial one. The basic physical and chemical properties of topsoil (0–10 cm) in the runoff plot are shown in Table 1.
Two different types of experimental runoff plots were established in our study. One type were plots with different vegetation components (Figure 3), which covered four treatments: original slope (OS), no-litter slope (the stems, leaves, and roots preserved) (NL), roots-only slope (RS), and bare slope (BS). Another type were the runoff plots with different vegetation coverage (0, 20%, 40%, 60%, and 90%). The vegetation was thinned to different coverage artificially. Firstly, the vegetation features in the experimental plots were obtained by taking vertical photographs above the plots with a digital camera after the plots were established. Vegetation coverage of the plots was then estimated by ENVI 5.3 software. Subsequently, the grass, including its root system, was removed randomly within the plots. Finally, the required vegetation coverage could be derived by repeated photography verification and plant removal (Figure 4). Experimental slopes with different components and bare slopes were also obtained by manually removing litter, stems, and leaves, as well as the entire plant. To minimize human influence, the rainfall experiments were conducted about three months after natural recovery.
Two duplicate plots (4 m × 1 m) were set in the coverage area of the same rain device (5 m × 4 m) for each vegetation coverage level plot and each plot with vegetation component. The relatively flat grass slope with similar slopes and same slope direction were selected for each type of runoff plot. Finally, the slope range of plots with different vegetation coverage was between 15° and 17°, and the slope of plots with vegetation components was approximately 22°.
Before the rainfall experiment, the rainfall samples were collected from different directions of the plots to calibrate the rainfall intensity (Figure 5a). Error between the measured rainfall intensity in the four directions and the target rainfall intensity should not exceed 10% to ensure that both rainfall intensity and rainfall uniformity within the runoff plots meet the test requirements. The tarpaulins were removed immediately, and timing was started after the rain intensity calibration was completed. The initial runoff generation time was recorded when the runoff was detected in the collection trough. After the runoff was initiated, the runoff and sediment samples were collected every 1 min within the first ten minutes, then at 3 min intervals, and finally at 2 min intervals for the remaining samples (Figure 5b). The rainfall experiment lasted for 45 min, and a total of 22 samples were acquired. The volume of collected sediment-laden flow samples was measured by the graduated cylinder. When the suspended sediment was deposited, the upper layer of water was removed and then transferred to iron basins and dried at 105 °C until it reached a constant weight. Then the runoff, sediment yield, and sediment concentration could be calculated.

2.3. Data Analysis

The effects of different treatments of vegetation slope on the runoff and sediment yield were calculated as follows
Δ R c = R b − R i R b × 100 %
Δ S c = S b − S i S b × 100 %
where Δ R c , Δ S c are the contributions of different treatments to the reductions in runoff and sediment yield, respectively; R i is the runoff rate for the original slope, no-litter slope and roots-only slope, respectively (mm·min−1); R b is the runoff rate for bare-soil plot (mm·min−1); S i is the sediment yield rate for the original slope, no-litter slope and roots-only slope, respectively (g·m−2); S b is the sediment yield rate for bare soil (g·m−2).
The contribution of different grass components to runoff and sediment reduction were calculated as follows.
Δ C l i t t e r = Δ R c ( O S ) − Δ R c ( N L )
Δ C s t e m s + l e a v e s = Δ R c ( N L ) − Δ R c ( R S )
Δ C r o o t s = Δ R c ( R S )
Δ C t o t a l = Δ C s t e m s + l e a v e s + Δ C l i t t e r + Δ C r o o t s = Δ R c ( O S )
where Δ C l i t t e r , Δ C s t e m s + l e a v e s , Δ C r o o t s and Δ C t o t a l are the runoff rate and sediment yield rate reductions caused by litter, stems and leaves, roots, and original vegetation, respectively. The calculation formulas for the sediment reduction rate of different vegetation components are similar to those for the runoff reduction rate.

3. Results

3.1. Runoff and Sediment Yield Processes Under Different Vegetation Coverage

The temporal variations in runoff rates and cumulative runoff volume at two rainfall intensities for different vegetation coverage are shown in Figure 6. The runoff rates of the 90%-coverage slope increased relatively gently with the increase in rainfall duration at a rainfall intensity of 1.5 mm·min−1 and 2.0 mm·min−1 (Figure 6a,c). In contrast, the temporal patterns of the runoff rate under the remaining vegetation coverage were similar. The runoff rates increased sharply during the initial stage (within about 10 min), and then the growth rate slowed down and finally stabilized, with some small fluctuations under the two rainfall intensities (Figure 6a,c). Specifically, the period of sharp increase at a rainfall intensity of 2.0 mm·min−1 was shorter than that of the rainfall intensity of 1.5 mm·min−1, which was mainly concentrated within approximately five minutes after runoff was initiated. The cumulative runoff volume on the bare-soil slope was higher than that on the grassland-covered slope under the two rainfall intensity conditions (Figure 6b). Meanwhile, the cumulative runoff yield gradually decreased with the increase in vegetation coverage (Figure 6d).
The variation characteristics of the sediment yield rate differed from those of the runoff rate (Figure 7). The sediment yield rate showed a larger fluctuations than the runoff rate in the plots with low vegetation coverage, whereas the sediment yield rate was extremely low and fluctuated relatively little in the slopes with high vegetation coverage (e.g., 60% and 90%). The curves of the sediment yield rate on plots with 40%, 60%, and 90% coverage were relatively similar under the rain intensity of 1.5 mm·min−1. The overall fluctuation was relatively small, and the temporal variation was relatively stable. In contrast, the sediment yield rate fluctuated greatly and showed the characteristics of “steep rise and steep fall” for the vegetation coverage of 20% and bare-soil slope. The sediment yield rate increased rapidly, reaching its peak, with the increase in rainfall duration, and then gradually decreased and tended to stabilize. The periods of sharp growth were mainly concentrated within 5 min after the initiation of overland flow. Similarly, there was a dramatic increase in sediment yield rate for the coverage of 20% and bare-soil slope under the rain intensity of 2.0 mm·min−1, particularly in the early 5 min, and then it gradually decreased. The overall sediment yield rates of slopes with different vegetation coverage under high rainfall intensity were higher than those under low rain intensity. Especially for the bare slope, the peak value of sediment yield rate was obviously higher at a rain intensity of 2.0 mm·min−1. The cumulative sediment yield on the bare-soil slope was obviously higher than that on the grassland-covered slope under the conditions of the two rainfall intensities (Figure 7b,d). Meanwhile, the cumulative sediment yield gradually decreased with the increase in vegetation coverage.

3.2. Runoff and Sediment Yield Processes on Slopes with Different Vegetation Components

The runoff rates for different treatment plots increased rapidly at the beginning; then the growth rate slowed down and tended to stabilize during the rainfall-driven overland flow experiments (Figure 8). The periods of sharp increase were mostly between 0 and 5 min, while the slow-growth stages were mainly concentrated within 5 to 15 min after the flow initiation. The runoff increasing rate of bare slope was much higher than that of other slopes. In addition, the time to stable runoff rate for bare slope was shorter than that of the other treatment slopes. The stable runoff rate was the highest in the bare plot, followed by no-litter slope and roots-only slope, while the lowest stable runoff rate appeared in the original slope. The cumulative runoff volume of original slope was significantly lower than that of the other slopes (p < 0.05) (Figure 8b,d). The difference in cumulative runoff volume between the original slope and no-litter slope at a rain intensity of 1.5 mm·min−1 was higher than that at a rain intensity of 2.0 mm·min−1, indicating that the litter of vegetation had a stronger interception effect on slope runoff under low rain intensity.
The vegetation components had a strong impact on the sediment yield and the processes under different rainfall intensities (Figure 9). The overall variation characteristics of the sediment yield on each treatment slope were relatively similar at the rain intensity of 1.5 mm·min−1. The sediment yield rate showed a trend of first increasing sharply and then stabilizing or decreasing slowly with the increase in he rainfall duration. The periods of sharp increase were mostly within 5 min after the initial flow. The variation in the sediment yield rate on the original slope was generally small. In contrast, at the rain intensity of 2.0 mm·min−1, the sediment yield rate of the three treatment slopes fluctuated greatly. The sediment yield rates increased sharply at first (approximately 5 min) with increasing rainfall duration and then gradually declined. The variation in sediment yield on the original slope was still relatively gentle, similar to that of the 1.5 mm·min−1 rain intensity. Furthermore, the sediment yield rate of the original slope was obviously lower than that of different treatment slopes under two rainfall intensities, while the sediment yield rate of the bare-soil slope surface was much higher than that of the remaining slopes. The cumulative curves of sediment yield also showed that the increasing rates of sediment yield on bare-soil slope were significantly higher than that on the other slopes (p < 0.05) under two rainfall intensities. The increasing rates of cumulative sediment yield on the original slope were significantly lower than those on the other slopes (p < 0.05).

3.3. The Contribution of Vegetation to Runoff and Sediment Reduction

3.3.1. The Effect of Vegetation Coverage on Runoff and Sediment Yield

The runoff rate and sediment yield rate presented a decreasing tendency with the increase in vegetation coverage (Figure 10). At the 1.5 mm·min−1 rain intensity, the variation ranges of the runoff rate and sediment yield rate were 0.20–0.94 mm·min−1 and 0.28–4.84 g·m2·min−1, respectively. For the rain intensity of 2.0 mm·min−1, the runoff rate varied between 0.35 and 1.19 mm·min−1, and the sediment yield rate varied between 0.65 and 5.65 g·m2·min−1. A general decreasing trend of the runoff rate was detected with the increase in vegetation coverage (Figure 10a). In contrast, the variation in sediment yield rate sharply declined as the vegetation coverage increased and then gradually slowed down when the vegetation coverage was larger than 40% (Figure 10b).
The regression results demonstrated that runoff rates corresponding to vegetation coverage had a significant linear relation for the two rain intensities (Table 2). This presented a general downward trend of the runoff rates as vegetation coverage increased. By contrast, exponential relationships were found between the sediment yield rates and vegetation coverage at rain intensities of 1.5 mm·min−1 (S = 10.805e−0.678C, R2 = 0.950, p < 0.01) and 2.0 mm·min−1 (S = 10.695e−0.552C, R2 = 0.987, p < 0.01).
For the rainfall intensity of 1.5 mm·min−1, the runoff rates decreased by 0.23 mm·min−1, 0.34 mm·min−1, 0.50 mm·min−1, and 0.74 mm·min−1 compared with the bare-soil slope under the vegetation coverage of 20%, 40%, 60%, and 90%, respectively (Table 3). The corresponding runoff reduction effects of vegetation were 24%, 36%, 53%, and 79%, respectively. The sediment yield rates decreased by 1.81 g·m−2·min−1, 3.52 g·m−2·min−1, 3.80 g·m−2·min−1, 4.57 g·m−2·min−1 for the vegetation coverage of 20%, 40%, 60%, and 90%, respectively. The corresponding sediment yield reduction effects of vegetation were 37%, 73%, 78%, and 94%, respectively. In addition, the runoff rates decreased by 0.21 mm·min−1, 0.33 mm·min−1, 0.44mm·min−1, 0.85 mm·min−1 for the vegetation coverage of 20%, 40%, 60%, and 90% at a rainfall intensity of 2.0 mm·min−1, respectively. The runoff reduction effects of vegetation were 17%, 28%, 37%, and 71%, respectively. The sediment yield rates decreased by 1.55 g·m−2·min−1, 3.76 g·m−2·min−1, 4.39 g·m−2·min−1, and 5.00 g·m−2·min−1, with the corresponding sediment yield reduction effects of 27%, 67%, 78%, and 89% under the vegetation coverage of 20%, 40%, 60%, and 90%, respectively.
As vegetation coverage increased from 20% to 40%, the sediment reduction effect of vegetation had increased by 36% and 40% for the rainfall intensities of 1.5 mm·min−1 and 2.0 mm·min−1, respectively (Table 3). However, when the vegetation coverage increased from 40% to 90%, the sediment reduction effect of vegetation increased only by 21% and 22%, respectively. The sediment reduction effect of the vegetation weakened significantly as the coverage exceeded 40%. Meanwhile, the sediment reduction effect of the same vegetation coverage was greater than its runoff reduction effect under different rainfall intensities. Moreover, the runoff reduction and sediment reduction effects of vegetation at different coverage levels all weakened as the rain intensity increased.

3.3.2. The Effect of Vegetation Components on Runoff and Sediment Yield

The runoff rate and sediment yield rate of the bare slope were 1.12 mm·min−1 and 14.07 g·m−2·min−1 at a rainfall intensity of 1.5 mm·min−1 (Table 4). Compared with the bare-soil slope, the average runoff rate and sediment yield rate of the original slope decreased by 0.51 mm·min−1 and 12.64 g·m−2·min−1, respectively. The corresponding reduction rates of runoff and sediment yield were 45% and 90%, respectively, which were the sum of the of water and sediment reduction contribution in the above-ground and underground parts of the vegetation. The average runoff rate and sediment yield rate of the roots-only slope decreased by 0.51 mm·min−1 and 12.64 g·m−2·min−1, respectively. The contribution rate of roots to runoff and sediment yield reduction were 12% and 51%, respectively. The runoff and sediment yield of no-litter slope increased by 0.3 mm·min−1 and 5.06 g·m−2·min−1 compared with the original slope, respectively, indicating that the corresponding contribution rates of litter layer to runoff and sediment yield reduction were 26% and 36%. The contribution rate of stem–leaf to runoff reduction (7%) and sediment yield reduction (3%), was the lowest compared to the litter layer and root of vegetation. To sum up, at a rainfall intensity of 1.5 mm·min−1, the runoff reduction contributions of the litter layer, stem-leaf layer, and roots were 26%, 7%, and 12%, respectively, and the sediment yield reduction contributions were 36%, 3%, and 51%, respectively. For the rainfall intensity of 2.0 mm·min−1, the runoff reduction contributions and sediment yield reduction contributions of the litter layer were 17% and 30%, respectively, which is a certain reduction compared with the rainfall intensity of 1.5 mm·min−1. The contributions of stem–leaf to runoff and sediment yield reduction were 10% and 7%, respectively, which shows a certain improvement compared to the rainfall intensity of 1.5 mm·min−1. The contribution of roots to runoff and sediment yield reduction at a rainfall intensity of 2.0 mm·min−1 showed practically no difference compared to that at 1.5 mm·min−1, with contribution values of 11% and 51%, respectively.

4. Discussion

4.1. The Runoff and Sediment Yield Regulation Effects of Vegetation with Different Coverage

The runoff generation and sediment yield of slopes is an extremely complex hydrological process, and the variation in vegetation coverage is one of the important factors affecting these processes. The vegetation will form a natural protective layer on the ground when vegetation coverage reaches a certain level, which in turn prevents raindrops from directly impacting the soil surface. This can effectively weaken the splashing effect of raindrops on the surface soil, thereby reducing soil loss on the slope [25]. Similar findings for different areas and plant types also indicate that the soil loss reduction by vegetation restoration ranged from 39% to 95% [27,28]. However, the sediment reduction effects of different vegetation coverage vary greatly [14]. Our results suggested that the reduction effects of vegetation on sediment yield were relatively limited when the grass coverage on the slope was less than 20% under different rainfall intensities. Specifically, at a 2.0 mm·min−1 rain intensity, the contribution of the vegetation on sediment yield reduction was only 27% compared to the bare-soil slope. In contrast, the contribution of the vegetation on sediment yield reduction was as high as 78% when the coverage exceeded 60%. The variation characteristics of sediment yield with vegetation coverage have been verified in many studies. For example, Sun et al. [35] reported that the sediment yield could be reduced by more than 90% when vegetation coverage reached 60%. Sun et al. [15] also concluded that the increase in vegetation coverage could significantly reduce sediment yield—the sediment concentrations were reduced by 81.7–97.8% in grassland with coverage ranging from 20 to 90%. Therefore, vegetation restoration can effectively reduce the soil erosion of slopes and improve the regional ecological environment greatly.
In our study, the sediment yield rate generally showed an initial increasing trend followed by a decrease on bare-soil slopes and some slopes with low vegetation coverage under both rainfall intensities. In contrast, for slopes with high vegetation coverage, the sediment yield first increased and then tended to stabilize (Figure 9). Previous studies reported that the sediment yield of slope was mainly caused by the splash erosion of raindrops during the early stage of rainfall [36,37]. The soil erosion resistance was relatively weak at this stage and the surface layer of slopes existed abundant original loose particles, which provided an adequate material source for the sediment yield [38,39]. Thus, the sediment yield rates of bare-soil slopes and some low-cover slopes showed a sharp increase during the early stage of runoff generation under the two rainfall intensities. Meanwhile, the dispersion ability of splash erosion on soil aggregates increased with the increase in rainfall intensity, which could provide more movable soil particles for the water flow on the slope. Therefore, the peak of sediment yield rate was higher at the rain intensity of 2.0 mm·min−1. The topsoil was gradually saturated, and the runoff rate of the slope gradually tended to stabilize as the rainfall duration increased (Figure 10). However, the loss rate of soil particles along with slope runoff decreased to a certain extent due to the reduction in loose soil particles on the slopes. On the other hand, with the emergence of thin-layer water flow on the slope, the occurrence of splash erosion was suppressed, which reduced the influence of splash erosion on the topsoil. Therefore, the sediment yield rate of the slope reached its peak and then gradually decreased as rainfall continued.

4.2. The Effect of Different Vegetation Components on Soil Erosion

Grass covers can significantly reduce soil erosion on the slope [40]. They can improve soil infiltration conditions more rapidly and better reduce soil erodibility compared with other vegetation types [41]. The soil erosion control of slopes by grass covers is mainly achieved through the combined action of the above-ground stems and leaves, the surface litter layer, and the underground root system. The stem-leaf and litter layer of the grass cover trap rainfall runoff, increase surface roughness, and improve runoff infiltration [27,42]. The root system of grass covers enhances the shear strength of the soil through physical combination with soil particles and aggregates [43]. Meanwhile, the root system can further increase soil infiltration and improve soil structure and aggregate stability [7,44].
Our results demonstrated that the stem-leaf layer, the surface litter layer, and the underground root system all played important roles in reducing soil erosion on the slope to a certain extent, although their contributions to sediment yield reduction varied. The contribution differences might be caused by the different erosion processes influenced by various vegetation components. For instance, Gyssels et al. [26] pointed out that vegetation roots were more effective in controlling rill erosion and gully erosion, while the above-ground parts of vegetation were more effective in weakening raindrop splash erosion and interrill erosion. Our results demonstrated that the contribution rates of roots to soil erosion reduction were as high as 51% under the two rainfall intensities, far higher than that of the stem-leaf and the litter layer. This was in line with previous studies [28,29]. In addition, Mamo and Bubenzer [45,46] pointed out that soil erosion susceptibility decreased sharply with the increase in root density in the soil layer. Gyssels et al. [47] also verified that the soil erosion effect of runoff weakened with the increase in root biomass density. Therefore, based on our results and related research results, it can be concluded that if the slope vegetation is in the early growth stage or the above-ground part of the grass cover is harvested for livestock breeding, protecting the root system and increasing root density might also be a feasible erosion control strategy. Furthermore, considering the significant role of roots in soil erosion reduction on slopes, the selection of artificial vegetation with a higher root density during the ecological restoration process is also an effective method for soil erosion prevention and control. However, some studies have reported that the sediment yield reduction effect of the root part was almost the same as that of the above-ground part of the grass cover for the grass slope [48]. In contrast, Gyssels et al. [26] found that the erosion reduction effect of the vegetation root system was smaller than that of the vegetation canopy, which is consistent with our findings. The discrepancy in vegetation types and soil types may account for the contribution difference of various vegetation components.
Our results showed that the sediment yield rates fluctuated with the rainfall process under the two rainfall intensities, with the sediment yield rate of the bare-soil slope surface fluctuating the most (Figure 9). This is consistent with the finding of Van Oost et al. [49], who suggested that the erosion rate of the bare-soil slope showed an obvious fluctuating state. This can be attributed to side wall collapse, which causes the blockage or unblocking effects on the transport of sand-containing water flow due to the repeated alternating connection of the rills. This inevitably leads to the occurrence of high and low sediment yield values during the erosion and deposition processes.
For the grassland slopes of the Loess Plateau, biological crust is another important factor affecting soil erosion of the slopes. Biological crusts are mainly composed of algae and mosses. The biological crust develops rapidly in the early stage of vegetation restoration when the grass is covered by the stem-leaf layer and the litter layer, which can effectively reduce the impact of rainfall splash erosion on the surface soil [50]. In our study, there was also a certain amount of biological crust on the grass cover slope, which also played a rather important role in controlling soil erosion and sediment yield on the slope.

4.3. Limitations

This study is subject to several limitations. We only carried out the independent rainfall simulation on the natural slope without taking runoff from the drainage area into account. Thus, this experimental design could only characterize the processes of rainfall–runoff–erosion–sediment transportation, while the confluence scouring effect of upslope runoff could not be involved [51]. In addition, limited by the size and range of the rainfall equipment, the experimental plots in this study were relatively long and narrow. The erosion process of rainfall runoff and the sediment transport path may differ from natural slopes. The design of the experiment plot needs to be further optimized to better characterize the real slope erosion process.
For the natural rainfall, the rainfall intensity is not a fixed value during a rainfall process but present multiple types. As reported by Mohamadi et al. [52], the rainfall types could be classified as increasing rainfall intensity, increasing then decreasing intensity, decreasing intensity, decreasing then increasing intensity. This study uniformly generalized rainfall intensity of this rainfall, which was inconsistent with the vast majority of natural rainfall processes. Thus, our results are only applicable to the runoff and sediment yield processes under the two fixed rainfall intensities. Except rainfall types, the slope gradient of experimental plots in this study is relatively gentle, and the vegetation belongs to the naturally restored grass. The processes of runoff and sediment yield in steep slope areas and artificial vegetation areas are still unclear. Therefore, further comparative studies on the response of runoff and sediment yield to vegetation restoration under different terrain features, vegetation types, and rainfall conditions are also needed to enhance the applicability and credibility of the results.
Furthermore, due to the root systems of some vegetation being relatively deep in this study, removing the roots completely would cause significant damage to the slope surface. There were still some deep roots on the bare plots that have not been completely removed in this study. Meanwhile, experiments with different rainfall intensities were conducted successively in the same plot, which might have had a certain impact on the initial sediment characteristics of the plot. All these factors inevitably had a certain impact on our study results.

4.4. Implications

Globally, the effective management of water resources in ecologically fragile areas (such as arid and semi-arid regions) is the key to ensuring the water resources needed for regional food production and ecosystem functions. The slope vegetation can reduce soil erosion effectively, which has been verified by our results and previous studies [7,53]. However, it also inevitably leads to a decrease in slope runoff during the processes of erosion control and sediment reduction. For instance, the sediment yield of the catchment has decreased significantly in recent years with the continuous recovery of vegetation on the Loess Plateau. The runoff has also decreased accordingly with the reduction in the sediment in the Yellow River. Related studies have concluded that the total annual runoff in the main basins of the Yellow River have decreased by 33.9% since the implementation of Grain for Green Project [54]. Similarly, Sun et al. [55] pointed out that the runoff in the basins decreased by more than 50% after the implementation of Grain for Green Project on the Loess Plateau. Liang et al. [31] analyzed the contribution rate of vegetation restoration measures to the reduction in runoff in 14 river basins of the Loess Plateau by using the elasticity coefficient method. The results showed that the average contribution rate of ecological restoration projects to the variation in runoff in each basin was as high as 62%. Therefore, slope vegetation restoration not only reduces erosion and sediment, but also intercepts a large amount of water resources, thereby affecting river runoff. Our study found that the sediment yield reduction contribution of the slope with 60% vegetation coverage was as high as 78% compared with the bare-soil slope surface at a rainfall intensity of 2.0 mm·min−1, while the reduction effect of slope runoff was only 37%. Nevertheless, as the vegetation coverage increased to 90%, its contribution to sediment reduction only increased by 11% compared with 60% coverage, while the reduction in runoff increased by 34%. Such a high vegetation coverage will lead to a sharp reduction in the runoff volume of the Loess Plateau, which is known as the “cost of sediment reduction” [5]. Therefore, in order to effectively regulate the sediment production on the slope and reduce the impact of vegetation restoration on the runoff volume at the same time, it is necessary to clarify the critical coverage threshold of slope vegetation further. In addition, our results found that the root system could sharply reduce the soil erosion and sediment yield while having a relatively small impact on runoff. Therefore, plants with well-developed root systems should be selected when choosing species for vegetation restoration measures on the Loess Plateau.

5. Conclusions

A series of field rainfall simulation experiments were conducted to investigate the responses of runoff and sediment yield to slope vegetation. Two types of experimental plots were designed to determine the influence of vegetation coverage on runoff and sediment yield and study the differences in erosion control by different components of vegetation. The main findings are as follows. The runoff and sediment yield showed a decreasing tendency with the increase in vegetation coverage. Specifically, the runoff reduction by vegetation was relatively uniform with the increase in vegetation coverage, whereas the change rate of sediment yield gradually slowed down when the vegetation coverage was larger than 40%. The sediment yield has already decreased by approximately 70% under the vegetation coverage of 40% for the two rainfall intensities. In contrast, the effect of vegetation on runoff was relatively weak—the runoff reduction rate was only 37% under the vegetation coverage of 60% and the rainfall intensity of 2.0 mm·min−1. The effects of root system to sediment yield reduction were much higher than those of the litter layer and the stem-leaf layer. The sediment yield reduction contributions of roots reached up to 51% at a rainfall intensity of 1.5 mm·min−1 and 2.0 mm·min−1. In contrast, the corresponding contributions of the stem-leaf layer were 3% and 7%, respectively. This suggested that the root system still had a great effect on the sediment yield reduction after the stem–leaf was manually removed. This study only explored the influence of natural recovery grass of gentle slopes on the runoff and sediment yield under the fixed rainfall intensity. Further experiments under different vegetation types and slope gradients as well as the dynamic rainfall intensity are needed to enhance the understanding of the influence of vegetation restoration on sediment yield and runoff. Overall, our results have implications for future research on the in-depth understanding of slope erosion and the establishment of science-based slope management practices.

Author Contributions

Conceptualization, J.H. and P.L.; methodology, L.N. and J.H.; validation, J.H. and P.L.; formal analysis, L.N.; investigation, L.N., J.H., and G.Z.; resources, X.M.; data curation, L.N. and J.H.; writing—original draft preparation, L.N. and J.H.; writing—review and editing, L.N., J.H., P.L., G.Z., and X.M.; visualization, L.N., J.H.; supervision, P.L.; project administration, G.Z. and P.L.; funding acquisition, J.H., G.Z. and P.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (42207407, U2243211).

Data Availability Statement

Please contact Pengfei Li (pengfeili@xust.edu.cn) for access to the data.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. The location of experimental plots in the Xichuan River catchment.
Figure 1. The location of experimental plots in the Xichuan River catchment.
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Figure 2. The experimental runoff plots and rainfall device for simulating rainfall.
Figure 2. The experimental runoff plots and rainfall device for simulating rainfall.
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Figure 3. The experimental plots with different treatments. (a) The original slope, (b) the slope with no litter, (c) the root slope, (d) the bare-soil slope.
Figure 3. The experimental plots with different treatments. (a) The original slope, (b) the slope with no litter, (c) the root slope, (d) the bare-soil slope.
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Figure 4. Partial view of the experimental plots under different vegetation coverage: (a) 20%, (b) 40%, (c) 60%, (d) 90%.
Figure 4. Partial view of the experimental plots under different vegetation coverage: (a) 20%, (b) 40%, (c) 60%, (d) 90%.
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Figure 5. The experiment process. (a) Rainfall intensity calibration, (b) runoff sample collection.
Figure 5. The experiment process. (a) Rainfall intensity calibration, (b) runoff sample collection.
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Figure 6. Temporal variation in runoff rates (a,b) and cumulative runoff (c,d) under different vegetation coverage and rainfall intensity.
Figure 6. Temporal variation in runoff rates (a,b) and cumulative runoff (c,d) under different vegetation coverage and rainfall intensity.
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Figure 7. Temporal variation in sediment yield rates (a,c) and cumulative sediment yield (b,d) under different vegetation coverage and rainfall intensity.
Figure 7. Temporal variation in sediment yield rates (a,c) and cumulative sediment yield (b,d) under different vegetation coverage and rainfall intensity.
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Figure 8. The variation in runoff rates (a,c) and cumulative runoff (b,d) with time under different treatments and rainfall intensities.
Figure 8. The variation in runoff rates (a,c) and cumulative runoff (b,d) with time under different treatments and rainfall intensities.
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Figure 9. The variation in sediment yield rates (a,c) and cumulative sediment yield (b,d) with time under different treatments.
Figure 9. The variation in sediment yield rates (a,c) and cumulative sediment yield (b,d) with time under different treatments.
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Figure 10. The variation in runoff rates (a) and sediment yield rates (b) with vegetation coverage under different rainfall intensities.
Figure 10. The variation in runoff rates (a) and sediment yield rates (b) with vegetation coverage under different rainfall intensities.
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Table 1. Summary of the basic physical and chemical properties of the tested topsoil (0–10 cm).
Table 1. Summary of the basic physical and chemical properties of the tested topsoil (0–10 cm).
Soil
Type		Bulk
Density		Soil Porosity (%)Mechanical Composition (%)
Inactive PorosityMicroporosityMacroporositySandSiltClay
Loessial
soil		1.1 ± 0.17.42 ± 1.24 8.65 ± 1.72 26.88 ± 2.45 11.49 ± 0.25 62.24 ± 0.33 26.27 ± 0.28
Table 2. Relationships between runoff rates/sediment yield rates and vegetation coverage under different rainfall intensities.
Table 2. Relationships between runoff rates/sediment yield rates and vegetation coverage under different rainfall intensities.
Rainfall Intensity
(mm·min−1)		Fitted EquationR2p
1.5R = −0.1752C + 1.10950.9840.01
1.5S = 10.805e−0.678C0.9500.01
2.0R = −0.1931C + 1.40790.9380.01
2.0S = 10.695e−0.552C0.9870.01
Note: R is runoff rate (mm·min−1); S is sediment yield rate (g·m−2·min−1); C is vegetation coverage (%).
Table 3. Average runoff rates and sediment yield rates as well as reduction efficiency for different grassed plots with different coverages under two rainfall intensities.
Table 3. Average runoff rates and sediment yield rates as well as reduction efficiency for different grassed plots with different coverages under two rainfall intensities.
Rainfall
Intensity
(mm·min−1)		Vegetation Coverage
(%)		Runoff Rate
(mm·min−1)		Sediment Yield Rate
(g·m2·min−1)		ReductionContributions (%)
Runoff
(mm·min−1)		Sediment Yield
(g·m−2·min−1)		RunoffSediment Yield
1.500.944.84----
200.723.030.231.8124%37%
400.611.320.343.5236%73%
600.451.050.503.8053%78%
900.200.280.744.5779%94%
2.001.195.65- -
200.994.100.211.5517%27%
400.861.890.333.7628%67%
600.751.260.444.3937%78%
900.350.650.855.0071%89%
Table 4. Average runoff rates and sediment yield rates as well as reduction efficiency for different plots with different treatments under two rainfall intensities.
Table 4. Average runoff rates and sediment yield rates as well as reduction efficiency for different plots with different treatments under two rainfall intensities.
Rainfall
Intensity
(mm·min−1)		TreatmentRunoff Rate
(mm·min−1)		Sediment Yield Rate
(g·m2·min−1)		ReductionContributions (%)
Runoff
(mm·min−1)		Sediment Yield (g·m−2·min−1)RunoffSediment Yield
1.5BS1.1214.07----
OS0.611.430.5112.6445%90%
NL0.916.490.217.5819%54%
RS0.996.950.137.1212%51%
2.0BS1.2621.73----
OS0.772.530.4919.238%88%
NL0.999.160.2712.5722%58%
RS1.1210.720.1411.0111%51%
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MDPI and ACS Style

Niu, L.; Hu, J.; Li, P.; Zhao, G.; Mu, X. The Effect of Vegetation Restoration on Erosion Processes and Runoff on a Hillslope Under Simulated Rainfall. Water 2025, 17, 2411. https://doi.org/10.3390/w17162411

AMA Style

Niu L, Hu J, Li P, Zhao G, Mu X. The Effect of Vegetation Restoration on Erosion Processes and Runoff on a Hillslope Under Simulated Rainfall. Water. 2025; 17(16):2411. https://doi.org/10.3390/w17162411

Chicago/Turabian Style

Niu, Lele, Jinfei Hu, Pengfei Li, Guangju Zhao, and Xingmin Mu. 2025. "The Effect of Vegetation Restoration on Erosion Processes and Runoff on a Hillslope Under Simulated Rainfall" Water 17, no. 16: 2411. https://doi.org/10.3390/w17162411

APA Style

Niu, L., Hu, J., Li, P., Zhao, G., & Mu, X. (2025). The Effect of Vegetation Restoration on Erosion Processes and Runoff on a Hillslope Under Simulated Rainfall. Water, 17(16), 2411. https://doi.org/10.3390/w17162411

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