Rill Formation Research Articles

Field simulation experiment on the relationships between hydrodynamics and soil detachment rate in formed rills

The dynamics of rill erosion are crucial for accurate modelling of soil erosion and vary across rill development stages. Research has largely concentrated on the rill formation process, with limited insights into formed rills under field conditions. Given the importance of soil detachment rate in the intensity and dynamics of soil erosion, this study aims to investigate the correlations between the soil detachment rate and hydrodynamic parameters in formed rills through field experiments. Therefore, the objective of this study was to explore the relationships between hydrodynamic parameters and the soil detachment rate of formed rills through field scouring experiments. The experiments were conducted on formed rills with flow discharges of 6, 9, 12, 15, 18, and 21 L·min−1 in a field runoff plot (with length of 18, width of 1 m, and slope gradient of 17.5 %). The results showed that the soil detachment rate remained stable fluctuations over time under different flow discharges. The rill flow velocity varied from 0.191 to 0.301 m·s−1, and increased with the increase of flow discharge. Except for the low-flow discharge (6 and 9 L·min−1), the rill flows manifested as super-critical turbulent flow. The values of flow shear stress, stream power, unit stream power, and unit energy were within the scope of 8.776–12.517 Pa, 1.647–3.757 W·m−2, 0.033–0.052 m·s−1 and 0.007–0.013 m, respectively. The soil detachment rate was significantly negatively correlated with Darcy–Weisbach friction, whereas demonstrated a significant positive correlation with Reynolds number and stream power. The soil detachment rate of formed rills was accurately predicted by flow stream power using a linear function (R2 = 0.918). The results of this study provided insights into hydrodynamics and soil detachment rate of purple soil under natural rainfall conditions.

Rill formation and evolution caused by upslope inflow and sediment deposition on freshly tilled loose surfaces

Rill erosion easily occurs on tilled surfaces because the soil shear resistance is less than the runoff shear stress. However, rill erosion formation and evolution on tilled surfaces under upslope inflow conditions remain unclear. The objective of this study was to investigate the rill formation process and rill erosion amount on tilled surfaces via flow scouring under different inflow rates (4, 6, and 8 L min−1) at a 15° slope. Close-range photogrammetric technology was applied to measure the rill morphology during the experiments. The results suggested that the rill formation process due to inflow could be divided into two distinct stages, namely, before and after runoff reached the downslope end, i.e., stages I and II, respectively. At stage I, runoff washed soil particles along the downslope direction. In this process, due to limited transport capacity of runoff, washed soil particles were deposited at the runoff head and formed a soil mound, which blocked the flow path, after which the runoff direction changed within the 6.7°− 50.6° range with an average moving distance of 0.62 m. As a result, a curved rill and a series of soil mounds were left on the surface. The inflow rate affected rill morphology by influencing the runoff direction change angle and runoff moving distance between the mounds on the surface. Herein, the rill formation process is referred to as downslope-trending erosion by inflow (DTEI). At stage II, runoff reached the downslope end, and a rill channel was formed throughout the slope. Thereafter, DTEI was largely reduced, and headward erosion was strengthened. As a result, the rill morphology quickly changed, such as the rill depth and rill width, which gradually increased with ongoing headward erosion. During DTEI, the rill paths were curved due to sediment deposition, and the sediment deposition conditions varied under the different inflow rates. Therefore, the rill curvature (RC) differed (1.039 ± 0.014). The RC decreased with headward erosion progression at stage II. The total sediment yield (TSY) increased with increasing inflow rate. Under inflow rates ranging from 4 to 6 L min−1 and 6–8 L min−1, the TSY increased 2.1–2.4 times. Consequently, DTEI on tilled surfaces significantly affects the initial rill morphology and evolution at the later stage. Hence, on slopes, its role should be considered in rill erosion assessment.

Experimental evidence that rill-bed morphology is governed by emergent nonlinear spatial dynamics

Past experimental work found that rill erosion occurs mainly during rill formation in response to feedback between rill-flow hydraulics and rill-bed roughness, and that this feedback mechanism shapes rill beds into a succession of step-pool units that self-regulates sediment transport capacity of established rills. The search for clear regularities in the spatial distribution of step-pool units has been stymied by experimental rill-bed profiles exhibiting irregular fluctuating patterns of qualitative behavior. We hypothesized that the succession of step-pool units is governed by nonlinear-deterministic dynamics, which would explain observed irregular fluctuations. We tested this hypothesis with nonlinear time series analysis to reverse-engineer (reconstruct) state-space dynamics from fifteen experimental rill-bed profiles analyzed in previous work. Our results support this hypothesis for rill-bed profiles generated both in a controlled lab (flume) setting and in an in-situ hillside setting. The results provide experimental evidence that rill morphology is shaped endogenously by internal nonlinear hydrologic and soil processes rather than stochastically forced; and set a benchmark guiding specification and testing of new theoretical framings of rill-bed roughness in soil-erosion modeling. Finally, we applied echo state neural network machine learning to simulate reconstructed rill-bed dynamics so that morphological development could be forecasted out-of-sample.

Factors contributing to rill erosion of forest roads in a mountainous watershed

Forest roads are a major source of and transport pathway for eroded sediments in mountainous watersheds. When rills develop on these roads' surfaces, they amplify sediment erosion. Best management practices can decrease sediment erosion, but in order to efficiently implement these practices it is necessary to determine which factors have the most influence on rill development on forest roads. Despite this need, there is scarce literature on rill development on forest roads. To fill this gap in knowledge, based on field survey and multivariate statistical methods including redundancy analysis (RDA) and variation partitioning analysis (VPA), we investigated unpaved forest roads in the Xiangchagou watershed in China and quantified the extent to which various factors influenced rill formation. Specifically, we studied how rill erosion intensity (REI) and rill morphological characteristics (like rill length, mean width and depth, density, and severity of fragmentation) varied along the slope of a forest road. We also introduced the concept of a road's hydrological constituents (its upslope catchment, surface, and cutslopes), and determined how much each constituent contributed to REI. We found that REI and morphological characteristics decreased moving from the upper portion of road segment downward, implying that rills developed more intensely uphill. Additionally, REI increased exponentially with rill width, density, and severity of fragmentation, and increase linearly with length and depth. Conversely, REI decreased exponentially with rill width-depth ratio. These relationships suggest that the morphological characteristics of rills could be used to predict the REI of a given road segment. Finally, we found that the road characteristics that best predicted rill formation included catchment area, cutslope area, and gravel bareness. Correspondingly, the upslope catchment, cutslopes, and road surface contributed 11.56%, 30.83%, and 8.23% of the variation in REI and morphological characteristics. The interaction between upslope catchment and road surface explained 19.89% of the variation. These results suggest that when best management practices are implemented to decrease erosion caused by forest roads in mountainous watersheds, they should integrate these hydrological constituents of a road.

The Influence of Short-Term Heavy Rainfall on Hydraulic Characteristics and Rill Formation in the Yuanmou Dry-Hot Valley.

Rill erosion is one of the major environmental problems in the world; it is an important factor with regard to land degradation and has a serious impact on production and daily life in the region. The widely distributed Yuanmou group stratum promotes the development of rill erosion, whereby the strong time-concentrated rainfall and the alternating arid-humid climate prepare the ground for the development of rills in soils. Therefore, a study of the processes of slope rill erosion was carried out, and a gravel-soil slope in the Yuanmou dry-hot valley was chosen to simulate short-term heavy rainfall (25 mm/h) (No. 1 plot) and moderate rainfall (15 mm/h) (No. 2 plot), to study the erosion processes of soil and the dynamic characteristics of runoff involved in erosion. The study results showed that the width of runoff was significantly different between the two plots, while the depth of runoff was not significantly different. During the rill formation process, the width of the two plots first decreased and then increased with increasing washout duration, while its depth did not change significantly. Flow was the key factor in determining the hydraulic characteristics of runoff, and it had a significant or extremely significant positive correlation with hydraulic characteristics parameters, except in the case of Fr (Froude number) (r = 0.039). The total sediment content (CS) of plot No. 1 (0.158 g/cm3) was significantly different from that of plot No. 2 (0.153 g/cm3), and both CSs in the two plots decreased with increasing washout duration. The CS had an extremely significant negative correlation with τ (runoff shear force) (r = -0.863 **) and DW-f (Darcy-Weisbach drag coefficient) (r = -0.863 **) and a significant negative correlation with Re (Reynolds number) (r = -0.735 *) in the short-term heavy rainfall experiment, while the CS had a significant positive correlation with V (velocity) (r = 0.814 *), R (hydraulic radius) (r = 0.811 *) and P (unit stream power) (r = 0.811 *) in the moderate rainfall experiment. The results of this study will help guide further examination of the processes involved in the dynamic mechanisms of rill erosion on slopes under short-term heavy rainfall conditions.

Linking rill development characteristics to sediment production on different coarse-textured granite topsoils

The sheet and rill erosion phases dominate the erosion process of the catchment area and up-slope of Benggang (a special gully) area, and its evolution has a vital impact on erosion characteristics in the erosion feedback loop. Here, the rill development processes, morphological characteristics and corresponding soil loss were investigated by laser scanning on the four coarse-textured granite topsoils defined as WH, TC, GX, and CT for the counties of Wuhua, Tongcheng, Ganxian, and Changting in China, respectively. A soil flume (3 × 0.8 m) at a slope gradient of 16.7% was treated with rainfall-inflow at the upslope inflow rate of 2 L min−1 and rainfall intensity of 90 mm h−1. Results showed that soil erosion rate increased with erosion evolution (sheet erosion, rill advance, and rill maturity phases). A comparison of the four different soils exhibited that the soil erosion rate showed a downtrend with the increase of soil clay and gravel contents. Specifically, the CT soil with high clay and gravel content showed the minimal soil erosion rate (0.024 kg m−2 min−1), rill density (0.95 m m−2), headward erosion rate (0.41 cm min−1), and surface evolution rate. The maximal soil erosion rate and rill depth were observed on the WH slope, with the rill headward erosion rate reaching the maximum of 20.59 cm min−1. Despite no significant difference in the geomorphologic comentropy of the four soils, their variation trend and rate could reflect dynamic changes in erosion. The total sediment transport efficiency was improved by rill formation and development, with rill erosion contributing to about 70% of total soil loss for the four soils. This study can enrich our understanding of the rill erosion process and surface morphology of coarse-textured topsoils.

Post‐Wildfire Generation of Debris‐Flow Slurry by Rill Erosion on Colluvial Hillslopes

AbstractLandscapes after wildfire commonly experience accelerated hillslope erosion, which often contributes to the mobilization and volume of debris flows. However, quantitative studies of the erosion and its relationship to rainfall, runoff, and landscape characteristics have been limited to a narrow range of physiographic conditions. We estimated the volume and delivery rate of slurry (a water‐sediment mixture) supplied to stream channels during a post‐wildfire rainstorm that generated large debris flows in six catchments above Montecito, CA, in 2018. We mapped the distribution of rills and measured their cross‐sectional geometries to quantify the influences of runoff, lithology, and hillslope characteristics on the sediment volumes released by rill erosion, and we scaled the results up to the 19.5 km2 of burned hillslopes in the source catchments. We computed the likely rate of surface runoff during the rainstorm and developed an empirical model for the evolution of a representative hillslope‐spanning rill to illustrate the magnitude and speed of the erosion process. Rilling was the dominant form of erosion across the hillslopes of the source catchments, and the rapid evacuation and mixing of water and sediment during rill formation supplied a slurry with high solids concentrations to stream channels. Colluvium on shale formations was more continuous, finer‐grained, and probably less permeable than colluvium on sandstones, and these differences affected the extent and dimensions of rills. As a result, shale hillslopes were the dominant source of slurry to the debris flows and supplied over twice as much slurry per unit burn area as sandstones.

Sediment transport mechanisms and selective removal of soil particles under unsteady-state conditions in a sheet erosion system

Selective removal of particles and nutrients by water erosion is a key factor in soil erosion studies. Most agricultural soils are located on gentle slopes where fertility is high; however, until now, the main attention on sediment transport mechanisms was paid to high-slope gradients, where soil erosion is intense, but soils are less productive. Despite the importance of sediment size distribution (SSD) and transport mechanisms under unsteady-state conditions, few studies have been done on this issue. Higher sediment concentrations in the early stages of the runoff indicate the need to deal with unsteady-state conditions. To address this issue, sheet erosion experiments were done using a 0.2 m by 1 m tilting flume under controlled laboratory conditions. Six inflow rates (75, 100, 125, 150, 175, and 200 mL/s) were applied on two contrasting soils, including an adjacent agricultural soil and a not cultivated soil (namely Cropland soil and Control soil, respectively) at two low slope gradients (1.5% and 2%). No rill formation was observed during the experiments. The sediment-laden runoff was sampled during unsteady-state flow to determine the SSDs and sediment transport mechanisms of eroded particles for unsteady-state conditions. The results indicated different trends in the selective removal of sediment particles depending on hydraulic conditions and soil aggregate size. The contribution of suspension-saltation (SS) to the total sediment load for the Cropland and Control soils varied from 22% to 68% and from 35% to 59%, respectively, while up to 78% and 65% of soil particles were transported by bed load (BL), respectively. However, SS and BL indicated a reverse trend with stream power. The Cropland soil showed a single peak in the SSD at low stream powers because of the selective removal of fine particles (0.042 mm), whereas the SSD was shifted to a bimodal distribution at higher stream powers with the selective depletion of both fine (≤ 0.084 mm) and coarse (1.5 mm) particles. The Control soil experienced a unimodal SSD in a range of sizes between 0.109 and 0.175 mm for all stream powers. The findings of the current study highlight the need for further study on the erosion of low-slope agricultural soils, where small stream powers can remove fine and fertile soil particles.

Sand cover enhances rill formation under laboratory rainfall simulation

Soil erosion was severe in the wind-water erosion crisscross region of the northern Loess Plateau of China, where aeolian sand cover may enhance water erosion. In order to evaluate the effects of sand cover on runoff, soil loss and rill information, a series of rainfall simulation experiments was conducted in two contrasting treatments. One is a bare loess soil slope serving as a control, and another is a sand-covered loess slope (namely sand cover treatment). Nine simulated rains, 60 min each, were applied to each treatment in duplicates. The results showed that the runoff generation mechanism changed from infiltration-excess runoff in the control to subsurface saturation-excess runoff in the sand cover treatment. The runoff initiation was delayed about three folds in the sand cover treatment as compared to the control. Total runoff volumes and total soil loss in the sand cover treatment in each event were 1.1–1.2 times and 1.9–9.3 times those in the control treatment. A paired t-test showed that the runoff rates were greater in sand cover than in control in all events except for the last run at P < 0.05. The soil loss rates were greater in sand cover than in control for all events except the first rain at P < 0.05. Measured total rill length, width, and degree of rill dissection in the sand cover treatment were much greater than those in the control. The rill erosion of sand cover slope was dominated by the shallow and wider rills, while loess slope was dominated by the narrow and deeper rills. The results clearly indicated that a sand layer overlying a less pervious loess soil facilitated return flow and thus accelerated rill formation and development. This finding is in line with the common understanding that return flow in layered soil can promote rill formation and cause severe rill erosion, which is also important to understand the effect of wind-blown sand cover on water erosion in the wind-water erosion crisscross region.

Impacts of freeze-thaw processes and subsequent runoff on 137Cs washoff from bare land in Fukushima

The deposited 137Cs is one of the long-lived radionuclides, that was released following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, has been hydrologically transported as particulates in the terrestrial environment of the Fukushima region. The impact of freeze-thaw processes and subsequent runoff affecting the 137Cs flux and concentration in sediment discharge were revealed in bare land erosion plot following the FDNPP accident by detailed monitoring and laser scanner measurement on the soil surface. We found that surface topographic changes due to the frost-heaving during the winter-spring period, and rill formation during the summer. We also found the evident seasonal changes in 137Cs concentration; high during the early spring and gradually decreased thereafter, then surface runoff from the plot frequently occurred during spring and autumn when rainfall was high and reached a maximum in summer. From these results, the higher 137Cs concentration in spring was caused by a mixture of unstable surface sediment following freeze-thaw processes and then transported in the early spring, but erosion amount is not significant because of the less rainfall event. The sediment with a lower 137Cs concentration, which was supplied from the rill erosion and its expansion, was wash-offed during the summer, contributing most of the flux from erosion in bare land in Fukushima region. In case, heavy rainfall occurs in the early spring, caution is required because high concentrations of cesium may flow down into the river.

Rill development and its change rate: a field experiment under constant rainfall intensity

Rill erosion is a widespread form of soil erosion on loess slope surfaces. The rill formation, evolutionary mechanism, and the quantitative expression of rill erosion dynamic changes in a micro-topographic level are currently lacking, primarily owing to lack of finer-scale measurements. This work presented the rill erosion evolutionary process and their dynamic changes at different slope gradients, (12°, 18°) in a millimeter-scale analysis. We achieved it by constructing an artificial excavation in the field and performed a soil pan with dimensions of 8 m in length and 2 m in width, which were subjected to a constant rainfall intensity of 100 mm h−1. The multiphase micro-geomorphologic changes during each rainfall phase were monitored using high-resolution terrestrial laser scanner (TLS). We noticed that the larger overland flow in the rill corresponded to more pronounced fluctuations in the cross-section lines of the rill bed. Also, with more the number of rills, the more easily the overland flow was dispersed, which required a larger slope gradient to resume the erosion and fill processes. Erosion rate and fill rate were highly correlated in general, and they increased abruptly at 70–90°. Due to the presence of swelling minerals, the loess surface expanded rapidly after encountering water in the early stage of rainfall. Therefore, the amount and rate of erosion on the slopes were underestimated. Understanding the detailed evolution process of rill erosion from a micro-perspective can provide a valuable reference for the development law of the initial stage of slope erosion in the loess region where erosion is extremely serious.

Rill Erosion and Soil Quality in Forest and Deforested Ecosystems with Different Morphological Characteristics

Rill detachment capacity is a key parameter in concentrated flow erosion. Rill erosion generally turns into gully erosion with severe environmental impacts. Changes in land use and human activities can have heavy effects in rill formation, particularly in forests subject to deforestation; soil morphology plays a significant role in these effects. However, literature reports few studies about rill detachment rates and their implications on soil quality in forest and deforested soils with different morphological characteristics. To fill these gaps, this study has evaluated the rill detachment capacity (Dc) and the main soil quality indicators in three areas (upper, middle and lower slope) of forest and deforested (for 10 years) hillslopes exposed to the north and south in Northern Iran. The variations of Dc have been measured on soil samples under laboratory conditions through a flume experiment at three slope gradients (12 to 19%) and five flow rates (0.22 to 0.67 L m−1 s−1) with four replications. The large and significant (p &lt; 0.05) difference (about 70%) detected for Dc between forest and deforested hillslopes was associated to the higher organic matter content of forest areas; as a consequence, these areas also showed higher aggregate stability, porosity, root weight density, microbial respiration and available water. In the deforested hillslopes exposed to the south, the soil erodibility was higher by 12% compared to those exposed to the north. The differences in the monitored soil quality indicators were instead less noticeable and not always significant (p &lt; 0.05). Conversely, Dc did not significantly change (p &lt; 0.05) among the upper, middle and lower hillslope areas investigated in this study. Simple but accurate models to predict the rill detachment capacity, erodibility and critical shear stress of soils from indicators of soil quality or the unit stream power using regression equations are suggested. Overall, the results can support land planners in prioritizing the actions for soil conservation in deforested hillslopes exposed to the south as well as in the extensive application of the proposed equations in erosion prediction models.

Farmland size increase significantly accelerates road surface rill erosion and nutrient losses in southern subtropics of China

Farmland size increase (FSI), which is an increase in operational size and consolidation of fragmented croplands, is considered an effective scheme to improve agricultural productivity and reduce fertilizer use with environmental protection benefits. But FSI involving road extension can enhance rill formation on road surfaces, and increase road erosion and associated nutrient losses in a watershed. These effects have not been experimentally quantified yet. Thus, we determined the changes in road surface rill erosion and associated nutrient losses in response to FSI before (2017) and after (2018), in an intensive sugarcane production area within Nala watershed, in southern subtropics of China. We divided the watershed into three sub-watersheds (lower (S1), left (S2) and right upper (S3) regions), and surveyed three categories of roads: (i) old roads (existing roads before 2017), (ii) new roads (constructed in 2018), and (iii) old-new roads (repaired and extended old roads in 2018). The road surface rill erosion and associated nutrient losses were much higher in the lower region (S1) than the upper regions (S1 and S2) of the watershed due to variations in field density (number of farms per unit area) and traffic intensity. Field density reduced by 53 % while traffic intensity increased by 41 % after FSI. Following FSI, annual road surface rill erosion, nitrogen, phosphorus and organic carbon losses increased by 88 %, 79 %, 67 %, and 46 %, respectively. Road surface rill erosion had a significant inverse relationship with field density (R² = 0.535, P < 0.05) but positively correlated with traffic intensity (R² = 0.728, P < 0.01). The accelerated N and P losses offset the decrease in fertilizer use by 5–22 % due to FSI. Our results highlight the significance of including soil erosion in FSI scheme for environmental protection. Accelerated road erosion following FSI should be minimized by integrated soil and watershed management practices.

Soil erosion and deposition characteristics of slope surfaces for two loess soils using indoor simulated rainfall experiment

Sediment deposition is an important part of the erosion process. Slope and soil type affect the spatial distribution of erosion–deposition; however, little is known about where erosion and deposition occur specifically and how they interact with influencing factors. To address this issue, four rainfall simulation experiments were conducted in a 1 m × 5 m plot with varying slopes (i.e., 5°, 10°, 15°, and 20°) and simulated rainfall for 1 h at a rate of 60 mm h−1. For the two soils, the runoff rate and sediment concentration were correlated with a similar temporal pattern. Runoff increased gradually and ultimately approached a steady state, and the sediment concentration decreased gradually and ultimately approached a steady state, which indicated that the erosion processes of the two soils were controlled by the detachment-limited condition. The sediment concentration and erosion rate were positively correlated with slope. However, the onset of runoff was delayed for AS soil, which also had a relatively high sediment concentration. The distribution range and area of deposition for the two soils were negatively correlated with the slope gradient, and the main deposition area occurred at the bottom of the plots. In contrast, the SD soil was more susceptible to deposition, and the deposition thickness was mainly concentrated within a 0–4 mm depth; this concentration was not observed for AS soil because of its high erosion intensity. Overall, this similarity also existed in the spatial variations of silt and fine sand contents in the soil surface. The silt and fine sand-sized particles were the primary eroded particle sizes; however, selective mechanisms of these two particle sizes were obviously different in the two soils. The results indicated that the erosion–deposition characteristics and the main eroded materials differed with varying erosion intensities and soil types. These findings are essential for a more comprehensive understanding of soil erosion mechanisms under conditions without rill formation.

Overland runoff erosion dynamics on steep slopes with forages under field simulated rainfall and inflow

AbstractAlthough numerous studies have acknowledged that vegetation can reduce erosion, few process‐based studies have examined how vegetation cover affect runoff hydraulics and erosion processes. We present field observations of overland flow hydraulics using rainfall simulations in a typical semiarid area in China. Field plots (5 × 2 m2) were constructed on a loess hillslope (25°), including bare soil plot as control and three plots with planted forage species as treatments—Astragalus adsurgens, Medicago sativa and Cosmos bipinnatus. Both simulated rainfall and simulated rainfall + inflow were applied. Forages reduced soil loss by 55–85% and decreased overland flow rate by 12–37%. Forages significantly increased flow hydraulic resistance expressed by Darcy–Weisbach friction factor by 188–202% and expressed by Manning's friction factor by 66–75%; and decreased overland flow velocity by 28–30%. The upslope inflow significantly increased overland flow velocity by 67% and stream power by 449%, resulting in increased sediment yield rate by 108%. Erosion rate exhibited a significant linear relationship with stream power. M. sativa exhibited the best in reducing soil loss which probably resulted from its role in reducing stream power. Forages on the downslope performed better at reducing sediment yield than upslope due to decreased rill formation and stream power. The findings contribute to an improved understanding of using vegetation to control water and soil loss and land degradation in semiarid environments.

Multi-temporal UAV data for assessing rapid rill erosion in typical gully heads on the largest tableland of the Loess Plateau, China

Quantitative estimation of rill erosion research plays a significant role in the understanding of rill formation and their development mechanisms, which is lacking presently. In this paper, rill erosion is quantified to analyze and evaluate the spatial and temporal distribution in typical gully heads of the Loess Plateau of China. Although several studies have carried rill erosion quantification, the variability analysis of rill erosion rates at different slope position is limited primarily because of the unavailability of high resolution elevation models. Using the low-altitude photography with Unmanned Aerial Vehicles (UAVs), digital surface models (DSMs) with centimeter accuracy were generated, and then geometrical parameters of a total of 1,129 rills at tableland-slope (T-S), hillslope (S), and slopes affected by artificial vegetation (SV) in the study were counted. Then, spatial and temporal development as well as erosion rates of the rills at the three sites were compared. Although many studies revealed the erosion law at the S position, few studies have linked the development of rills at the T-S position, and artificial afforestation at the SV position exacerbates the rill erosion. Furthermore, based on the centimeter-resolution DSMs obtained by the two-stage UAV photography, we found that the slope gradient which was the most susceptible to rill erosion were 5-10°, 35-50°, and 65-80° at T-S, S and SV sites, respectively. At the SV, rill incision was the most sensitive to slope changes, and rill incision potential proved to be the greatest. Generally, the higher the rill erosion depth, the smaller the frequency of occurrence, but this trend was mitigated at the SV position. Therefore, urgent action is needed to control rill erosion caused by artificial afforestation on hillslopes to alleviate and eliminate the adverse effects of vegetation planting on rill erosion on the Loess Plateau of China.

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

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

Fish‐scale pit effects on erosion and water runoff dynamics when positioned on a soil slope in the Loess Plateau region, China

AbstractSoil moisture construction is a very important way to reduce soil erosion and maintain crop growth in arid and semiarid regions of China. A microcatchment technique for soil erosion prevention widely used on soil slopes on the Loess Plateau are fish‐scale pits (FSPs). This technique is combined with afforestation techniques to store runoff, interrupt the dynamics of soil erosion, and help mitigate frequent water deficiencies encountered by forest trees. This study investigated the effects of FSPs on soil erosion dynamics including rill formation and runoff. A three‐dimensional (3D) laser scanner was used to evaluate the erosion resistance of a 15° slope treated with FSPs arranged in a triangular pattern and the hydraulic characteristics of the flow on this slope during several intermittent simulated rainfall events. The following results were obtained. (a) The FSP‐treated slope displayed eight progressive stages of erosion: splash erosion, sheet erosion, scouring by water streams, formation of the scour pits, rill erosion, down‐slope erosion, up‐slope erosion, and collapse of the pit walls. (b) As the rainfall duration increased, the runoff velocities at various locations on the slope fluctuated, but generally, the runoff velocities were significantly higher in the down‐slope positions than for the midslope and upslope positions. When the cumulative rainfall duration reached approximately 58 min and the total rainfall reached approximately 88.5 mm, the ability of the FSPs to intercept and store runoff rapidly decreased. In the first two rainfall events, both the runoff reduction benefits (RRB) and sediment reduction benefits (SRB) were positive, but following the third rainfall event, the SRB and RRB of the FSPs were negative. The accuracy of erosion parameters extracted using the 3D laser equipment and the ArcGIS software in comparison with the measured rill erosion parameters all less than 10%. The relative error between the measured sediment and the calculated sediment is within 5.66–22.13%. (c) During the rainfall process, the flow on the upslope was constantly in the laminar regime, but after the FSPs were completely filled with water, the laminar flow on the downslope transitioned into a turbulent flow. (d) As the cumulative rainfall duration increased, the degree of topographic relief and the number of rills increased, as did the measurements of surface roughness and flow resistance. In conclusion, the microcatchment methods reduced the rill erodibility and enhanced the soil's resistance to concentrated flow erosion.

Camponotus japonicus burrowing activities exacerbate soil erosion on bare slopes

As soil ecosystem engineers, ants considerably affect soil physical and chemical properties, and further accordingly affect soil erosion. However, few study was made on studying the effects of ant-burrowing activities on soil erosion previously. This study quantified the impacts of ant (Camponotus japonicus)-burrowing activities on runoff and soil erosion rates. Simulated laboratory rainfall experiments were undertaken on six soil tanks filled with clay-loam soil classified as a cumulic anthrosol, three of which introduced with ant colonies for two days and the other three were without ant colonies, under 40, 80, and 120 mm h−1 of rainfall intensities and a slope of 15°. Results showed that ants made mounds with a bulk density of 0.75 g cm−3, which was lower than that of the soil matrix, i.e. 1.34 g cm−3. Soil erosion rates for the tanks with ants were 6.78, 36.90, and 62.00 g m−2 min−1 under three rainfall intensities of 40, 80, and 120 mm h−1, which were much higher than those of 3.94, 23.49, and 44.48 g m−2 min−1 for the tanks without ants. Ant nests reduced the runoff rate by 31%, 20% and 13% compared with those without ants and enhanced the soil water storage within 90 cm depth. Ant nests played a positive role in soil water conservation due to the large nest entrance diameter and the continuous macropore network. However, the ant mounds provided a loose erodible material and changed micro-topography of the slope surface, thereby accelerating the rill formation and exacerbating soil erosion. This study can help understand the effects of burrowing insects on soil erosion, which is an important environmental problem on the Loess Plateau.

Runoff from biocrust: A vital resource for vegetation performance on Mediterranean steppes

AbstractDryland vegetation is limited by water scarcity, and usually appears in the form of sparsely distributed patches within a heterogeneous unvegetated matrix often covered by biological soil crust. Biocrusts usually act as runoff sources, whereas vegetation acts as sinks, reinfiltrating most of the run‐on from upstream biocrusted areas. Alteration of biocrusts by human disturbances or climate change may exert a strong impact on soil erosion, promoting rill formation, increasing flow connectivity between source areas and reducing run‐on inputs to vegetation, strongly affecting ecosystem functioning. The role of biocrust runoff in vegetation productivity, which to date has not been studied in depth, must therefore be understood to predict the consequences of such changes. In this study, we analysed the response of Machrochloa tenacissima to run‐on exclusion for two years and compared it to the performance of plants of the same species receiving runoff from biocrusted source patches. The results showed that plants receiving run‐on had more photosynthetically active biomass, net C uptake rates and water‐use efficiency than plants under run‐on exclusion. Differences between treatments became wider over time, especially after rainfall, when the differences in water availability were greatest. These results lead to the conclusion that run‐on is a crucial water input for dryland vegetation. Any alteration of unvegetated areas usually covered by biocrust may have important effects on vegetation productivity similar to those from changes in precipitation pattern, and would result in a new, eco‐hydrological equilibrium of the whole ecosystem.