Rill Erosion Processes Research Articles

A 44-year balanced fertilizer application affected rill erosion resistance by changing humus, aggregates, and polyvalent cation

Fertilizer application can affect the physicochemical properties of soil, such as the contents of large aggregates, humus, and exchangeable cations, thereby influencing soil erosion resistance. However, the rill erosion resistance and its key influencing factors of soil following long-term balanced fertilizer application remains unclear. This study aimed to analyze the change in rill erosion resistance following 44 years of balanced fertilizer application (No changes in the type of fertilizer) to non-calcareous soils and establish a Partial Least Squares Regression (PLSR) model of rill erodibility (kd) and soil critical shear stress (τc) to changes in the physical and chemical properties of the soil. Five treatments were designed: (1) CK (no fertilizer applied), (2) N (nitrogen), (3) NP (nitrogen plus phosphorus), (4) NK (nitrogen plus potassium), and (5) NPK (nitrogen, phosphorus, and potassium). Compared to CK, the N, NP, NK, and NPK application significantly increased τc by 49.6, 96.7, 73.6, and 36.2 %, respectively. Whereas, kd increased significantly only in the NPK treatment group. The optimal partial least squares regression model showed that mean weight diameter (MWD) had the greatest positive influence on soil critical shear stress, followed by fulvic acid (FA) content, whereas water-dissolved substances had a negative influence. Long-term balanced fertilizer application can increase MWD and τc by combining micro-aggregates and humus into large aggregates. Ca2+ content had the greatest positive effect on kd. Compared with that of CK, exchangeable Ca2+ content increased significantly with NP and NPK application (7.9 and 34.3 %, respectively). Ca2+ can increase kd by binding to the polar functional groups in FA to promote the shedding of hydration shells in large aggregates. Among all treatments, the NP treatment showed the best performance for reducing kd and increasing τc. This study could contribute to the understanding of the rill erosion process and modeling in non-calcareous soils and offer a reference for agricultural erosion control treatments.

Microtopographic response of tilled loess slopes during stages of water erosion development

The tilled slopes in the loess region are characterized by artificial management practices that result in variations in microtopography, which serve as both the site of water erosion and a significant factor contributing to further slope erosion development. However, the response of microtopography during the process of erosion development on tilled loess slopes is unclear. In this study, artificial rainfall simulations were conducted to simulate the development of water erosion (splash erosion (SpE), sheet erosion (ShE), and rill erosion (RE)) under three slope gradients (5°, 10°, and 20°) and three tillage practices (artificial backhoe (AB), artificial digging (AD), and contour tillage (CT)) at a rainfall intensity of 60 mm h−1. The spatial correlation and heterogeneity of the microtopography were analysed using semivariogram functions and fractal dimensions. The response characteristics of microtopography during the water erosion development process on tilled loess slopes were analysed from three perspectives: microtopography factors, erosion equilibrium areas, and slope strips M-ΔDEM. The results indicated that the microtopography process data exhibited moderate to high spatial autocorrelation. There is a positive correlation between microtopographic factors and the slope gradient. Tilled slopes exhibit erosion equilibrium effects during the processes of splash erosion, sheet erosion, and rill erosion. These equilibrium effects are located in the longitudinal ranges of 700 to 1200 mm, 700 to 1300 mm, and 800 to 1100 mm, respectively. Soil erosion and deposition primarily occur in the middle and lower parts of the slope, with 70 % of the slope’s elevation changing within 10 mm, while the remaining changes can reach 25 mm. Among the three tillage practices, AB is the most susceptible to slope influences, which can lead to soil loss, and an optimal slope gradient exists for implementing AB measures. In the case of CT at a slope of 20°, ridge breaking is more likely to occur, but considering the combined effects of soil loss suppression and the difficulty in forming rills, it is recommended to prioritize CT as the preferred tillage practice among the three practices. This study can provide a theoretical foundation for understanding the mechanisms through which microtopography influences water erosion. Additionally, it can serve as a theoretical basis for calibrating parameters in soil erosion models for sloping cultivated lands.

Response of Hydrodynamic Characteristics to Tillage-Induced Microtopography of Rill Erosion Processes under Heavy Rainfalls

Response of Hydrodynamic Characteristics to Tillage-Induced Microtopography of Rill Erosion Processes under Heavy Rainfalls

Response of erosion rate to hydrodynamic parameters in sheet and rill erosion process on saturated soil slopes

Sheet erosion and rill erosion are two important forms of slope erosion. The initiation and development of those forms are significantly affected by erosive power of water flow. Identifying main erosion forms and exploring the relationships between hydrodynamic parameters and erosion rates, especially in a saturated soil slope simultaneous the development of sheet-rill erosion, are important to reveal erosion intrinsic mechanism. The objectives of this study were to quantify erosion rates, hydrodynamic parameters, and determine the relationships between hydrodynamic parameters and the soil erosion rate at sheet and rill erosion stages of saturated soil slopes. A series of indoor soil flume simulated rainfall experiments were conducted on a saturated soil slope at three rainfall intensities (30, 60, and 90 mm h−1), four slope lengths (1, 2, 4, and 8 m), and five slope gradients (2°, 5°, 10°, 15°, and 20°). Results revealed that erosion rates at sheet and rill erosion stages on a saturated soil slope increased from 5.31 to 22.31 kg m−2 s−1 and 23.91 to 140.35 kg m−2 s−1, respectively. The values of flow shear stress, stream power, unit stream power at those two stages were within the scope of 0.064–0.151 m s−1 and 0.175–0.327 m s−1, 0.77–2.97 Pa and 1.59–6.70 Pa, 0.24–0.80 W m−2 and 0.53–1.80 W m−2, and 0.041–0.12 m s−1 and 0.07–0.36 m s−1. Hydrodynamic parameters of flow velocity, shear stress, stream power, and unit stream power at rill erosion stage were 2.01 to 2.77, 1.18 to 2.31, 1.31 to 2.51, and 1.20 to 2.57 times those at sheet erosion stage. Furthermore, erosion rates at sheet and rill erosion stages increased linearly with the increase of flow velocity, shear stress, stream power, and unit stream power under different experimental conditions. The erosion rates at those two erosion stages were accurately predicted by stream power and flow velocity (R2sw=0.86, R2rv=0.92). This study is helpful for unraveling the dynamic mechanism of sheet-rill erosion process and establishing reasonable process-based soil erosion models.

Impacts of upslope inflow and soil pipe collapse on slope water erosion in the typical Chinese Mollisol region

Understanding the effects of upslope runoff and soil pipe collapse on slope water erosion can provide scien-tific basis for preventing Mollisol degradation caused by soil erosion. We conducted an experiment to investigate the effects of upslope inflow rate and soil pipe collapse on slope water erosion and to quantify the contribution of soil pipe erosion to slope soil erosion. The experiment included three inflow rates (30, 40, and 50 L·min-1) and three near-surface soil hydrological conditions (without soil pipe: NP; with soil pipe but no pipe flow: PF0; with pipe flow: PF1). The results showed that: 1) Slope soil erosion increased with increasing inflow rates; when the inflow rate increased from 30 L·min-1 to 40 and 50 L·min-1, slope soil erosion increased by 100.0%-111.5% and 214.8%-289.2%, respectively. 2) The soil pipe occurrence and pipe flow formation aggravated the slope water erosion process. At inflow rates of 30, 40, and 50 L·min-1, slope soil loss under the PF0 and PF1 treatments were 1.4-1.6 times and 1.7-2.1 times of that under the NP treatment. The contribution of soil pipe erosion to slope soil loss was 26.7%-37.6% under the PF0 treatment and 40.5%-51.9% under the PF1 treatment. 3) Soil pipe collapse intensified the rill erosion process. Compared with the NP treatment at 30, 40, and 50 L·min-1 inflow rate, rill erosion amounts under the PF0 and PF1 treatments increased by 38.1%-66.0% and by 93.7%-128.4%, respectively. Our results suggested that increasing upslope inflow rate resulted in higher surface runoff velocity, which promoted runoff detachment and transport capacity, and then aggrandized the amount of slope soil erosion. Moreover, soil pipe collapse exacerbated rill erosion process. When the soil pipe collapsed, all surface runoff was converted to soil pipe flow, which accelerated flow velocity and slope soil erosion process, and then increased the amount of slope soil erosion.

Geomorphic-based mine rehabilitation coupled with AMD chemical stabilisation in sulphide-rich ore deposits and soils: insights from a pioneering intervention at the Lousal mine, Iberian pyrite Belt

ABSTRACT The Lousal mine rehabilitation project proactively planned strategic milestones around key local geomorphic landforms and geochemical characteristics. GeoFluv™ method was used to design a rehabilitation topography mimicking natural landforms, on which a built-up soil cover with chemical buffering capabilities and open limestone channels were implemented across the intervention areas. Once these landform restoration works were completed, positive effects are observed in March 2022, with the native vegetation cover (the third main component of the restoration plan) established in most of the recovered area and a visible water quality improvement to the AMD treatment system water. Potentially Toxic Elements (PTEs) concentrations have reduced significantly after the reclamation actions, especially Fe (404 to 34 mg L−1), Zn (65.7 to 15.8 mg L−1) and Cd (122 to 0.8 µg L−1) concentrations. Minor problems detected have been processes of rill erosion associated with the inflow of adjacent watersheds, and the creation of ephemeral reducing conditions resulting from the leaching of the organic amendment of the topsoil. The Lousal mine reclamation project is the first built example in Europe where the concept of geomorphic reclamation has been combined with geochemical remediation in an area with intense acid mine drainage formation. Detailed monitoring of this project’s restorative progression throughout 2024 and beyond, should offer learning opportunities and innovations which will benefit future rehabilitation projects, with comparable underlying features.

Using rare earth element tracers to investigate ridge slope erosion process in contour ridge system under extreme rainfall

Extreme rainfall is becoming more frequent and intense, which no doubt increases the risks and uncertainty of water erosion in widely-used contour ridge systems. Ridge slope erosion can increase the probability of contour failure occurrence and then induce contour ridge losing anti-erosion properties. However, there is little understanding of ridge slope erosive process controlled by contour ridge system responding to extreme rainfall. In this study, five rare earth elements were applied in different ridge segments and soil layers to monitor the successive development of ridge slope erosion process under extreme rainfall in contour ridge system. Simulated rainfall experiments at high rainfall intensity (100 mm h−1) lasting for 60 min were conducted in runoff plots with ridge tillage practices characterized by microtopography indices and ridge geometry indices. Results showed that three sub-processes were observed for ridge slope erosion, including inter-rill erosion, headward erosion, and rill erosion. Headward erosion mainly resulted from the break-point at ridge surface and sidewall collapse at ridge toe, and the pouring of ponding water in furrow greatly promoted rill development. Erosion rate was the largest during headward erosion process, and rill erosion contributed the most to eroded source. The dominated erosion process was transport-limited during inter-rill erosion process, and detachment-limited during headward and rill erosion processes. Sediment transport and deposition processes as well as their transformation occurred simultaneously after the development of headward erosion. Eroded sediment was mainly from the low part of ridge, which was transported by raindrop-impacted sheet flow or overflow. These findings provide better responding of agricultural tillage to global climate change and references for the development of physically-based erosion models.

Upslope inflow-driven variations in microtopography and size fractions effect on rill erosion in purple soil hillslopes

Upslope drainage area contributing inflow to downslope rill erosion can be affected by the dynamics of eroded sediment and soil surface microtopography. However, limited information has previously not recognized the variation of the sediment particles and microtopography on hillslopes where upslope inflow perpetually exists. A series of scouring flume experiments were performed on a 15° slope under upslope inflow rates (4, 6 and 8 L min−1) to investigate the inflow-driven microtopography and sediment particle-size distribution (PSD) during the rill erosion process. The results indicated that the surface elevation decay mostly occurred in the later stage of rill erosion. The spatial variability of microtopography increased gradually with increasing inflow rates, and obvious microtopography sharpness intensified with the development of rill erosion. The surface runoff under the 8 L min−1 inflow rate increased by 15.03 % and 90.14 % for 6 and 4 L min−1, respectively, and the sediment yield reached a maximum at the rill stabilization stage (RS). The overall particle-size distribution was mainly composed of silt-sized particles, accounting for approximately 40.97 %−50.15 % of the total eroded sediment. Considerable depletion of clay fractions occurred in the rill erosion process. High inflow rate significantly enhanced variation amplitude of critical hydrodynamic parameters (e.g., flow velocity, flow shear stress and Reynolds number). The spatial heterogeneity of microtopography was correlated with the coarser fraction of sediment PSD in rill erosion, suggesting that erosion processes were affected by the interaction between eroded sediment distribution and roughness. These findings highlight the importance of the PSD of eroded sediment and the development of soil surface structure for an in-depth understanding of the purple soil erosion mechanism at the hillslope scale.

Determination of correction factor of flow velocity on eroded rills by electrolyte tracer method with double injectors

Flow velocity is an important hydrodynamic parameter to study the rill erosion process, especially the erosion dynamic process. The dye tracer method is a common and simple method to measure the rill flow velocity. However, a correction factor is required to convert the surface flow velocity to the mean flow velocity. The reliable correction factor is vital to determine the mean flow velocity. A total of 144 flow velocities (surface and mean) were measured using electrolyte and dye tracers at the corresponding distances (4, 6, 12, and 16 m) under different inflow rates (6, 9, 12, 15, 18, and 21 L min−1) on eroded rills. Results showed that the electrolyte tracer method with double injectors can accurately measure the mean flow velocity under the condition of a long slope in the field. The Reynolds numbers varied between 1433 and 15864. And the correction factor α varied between 0.220 and 0.874 with an average value of 0.557. The correction factor was positively correlated with the rill width, depth, cross-sectional area, flow depth, and Reynolds number. Furthermore, the correlation coefficients between correction factor and hydraulic parameters were greater than those between correction factor and rill morphological parameters. Moreover, the correction factor could be predicted by the equation of inflow rate, Reynolds number, and slope length. The results verified the application of an improved double injector electrolyte tracer method on a long slope in the field, clarified the influence of rill morphology and hydraulic parameters on the correction factor, and developed a reference for mean flow velocity determined using the dye tracer method.

Response of Soil Detachment Rate to Sediment Load and Model Examination: A Key Process Simulation of Rill Erosion on Steep Loessial Hillslopes.

The rate of soil detachment by water flow indicates soil erosion intensity directly. The exact relation between soil detachment rate and actual sediment load in water flow, however, is still unclear, and the existing relationships have not been adequately tested. The aims of the present study were to investigate the response of soil detachment rate to sediment load using rill flume data with loessial soil and to quantitatively examine the soil detachment equations in the WEPP and EUROSEM soil erosion models. Six slopes were combined with seven flow discharges to measure detachment rates under seven sediment loads using a rill flume with a soil-feeding hopper. Significant differences were found among the soil detachment rate by different sediment loads in low sediment load levels, but an insensitive response of soil detachment rate to sediment load was found under high levels of sediment load. The soil detachment rate was proved to be negatively linearly correlated with sediment load. The rill detachment equation in the WEPP model predicted the soil detachment rate by rill flow very well under our experiment condition. The soil detachment equation in the EUROSEM model underestimated the detachment rates under controlled conditions, but removing the setting velocity from the equation greatly improved prediction. Further experiments that could reflect the dynamic convective detachment and deposition process need to be conducted to compare with the present examination results and to further understand rill erosion processes.

Changeability of rill erosion properties due to microorganism inoculation

This study aimed to assess the impacts of using soil microorganisms onrill erosion prosses. Consequently, the efficacy of individual bacteria (Ba) and combined inoculation of bacteria and cyanobacteria (Ba + Cy) extracted from a Marl region's soil microorganism bank in changing rill erosion processes was investigated. The experiment was carried out on mid-sized plots (1 × 6 m) with a 30 % slope and three replicates. Plots were subjected to a design simulated rain with an intensity of 50 mm.h−1 and a duration of 30 min. A supplementary runoff with a discharge of 2 L.min−1 was also used to mimic more extended slope conditions to motivate the rilling phenomenon. The results of the study disclosed that the control (i.e., not inoculated) and treated plots (i.e., Ba and Ba + Cy) behaved differently in the viewpoint of runoff, soil loss, flow hydraulics, and rill morphometry. Statistical analyses revealed that Ba and Ba + Cy treatments controlled all runoff and soil loss components, flow hydraulic characteristics, and rill morphologic factors significantly (p < 0.05) except the infiltrated water (p = 0.56) and the Froude number (p = 0.11) compared to the control plots. The findings implied that Ba and Ba + Cy inoculations, compared to control plots, increased infiltrated water by 1.73 and 1.55 folds and reduced soil loss by 100.31 and 273.94 folds, respectively. The Froude and Reynolds numbers declined by 2.34 and 5.77 folds when Ba was applied. At the same time, Ba + Cy decreased the Froude and Reynolds numbers by 1.24 and 20.08 folds, respectively. Furthermore, rill frequencies in the treated plots by Ba and Ba + Cy were tobogganed by 3.67 and 2.20 folds with a high effect on increasing initiation times by 17.80 and 28.29 folds compared to the control treatment. Our findings suggest endemic microorganism inoculation as an effective bioengineering technique for managing rill erosion-prone regions.

Inhibitability of soil loss and sediment concentration during consecutive rainfalls from experimental plots treated by endemic microorganisms

Consecutive rainfalls, due to changes in antecedent moisture, alter soil erosion processes, necessitating the implementation of suitable soil loss control methods. Biological soil microorganism approaches have been applied to control soil loss. However, information on the involvement of microorganisms in the soil loss and rill erosion processes has yet to be supplied. In this study, the individual and combined inoculation of cyanobacteria and bacteria was investigated during five consecutive rainfalls with an interval of three days, an intensity of 50 mm/h, and a duration of 30 min. Also, additional runoff simulation of about 2.18 ± 0.32 L/min was done in mid-sized experimental plots (i.e., 6 m × 1 m × 0.5 m) induced by rill erosion. These experiments were done at the Rainfall and Soil Erosion Simulation Laboratory of the Faculty of Natural Resources, Tarbiat Modares University, Iran. Soil loss and sediment concentration were measured during five consecutive rainfalls in the study treatments and compared with the control treatment. Results showed that the highest soil loss of 30,810.70 g occurred at the control treatment in the first rainfall. In contrast, individual and combined inoculation treatments of cyanobacteria and bacteria with the secretion of polysaccharides and resulting stability of aggregates decreased soil loss significantly (p < 0.01) by 99.65%, 99.91%, and 100.00%, respectively, compared to the control treatment. In the combined treatment of cyanobacteria and bacteria, soil loss further decreased by 68.38%, 70.33%, and 7.27% for 2 through 4, respectively, events and increased by 19.96% in the fifth event, and sediment concentration also decreased by 68.88% and 59.86% for the second and third events, respectively and increased by 4.66% and 83.30% for the fourth and fifth events, respectively. The best performance was found in the combined inoculation in consecutive rainfalls (i.e., from the second to the fifth event), and the bacteria treatment and the combined treatment did not significantly differ in performance (p = 0.94). The cyanobacterial treatment had a significant effect only for the first rainfall compared to the bacteria treatment. Bacteria therapy is advised as a treatment for conserving soil and water resources in rill erosion-prone regions due to the time and cost necessary to create the biomass required.

Characterizing the rill erosion process from eroded morphology and sediment connectivity on purple soil slope with upslope earthen dike terraces

Rills are critical venues for the transport of eroded sediments along hillslopes. The sediment transport efficiency and connectivity within hillslopes are affected by the spatiotemporal evolution of rill erosion and morphology. However, the effect of upslope sediment-laden inflow on rill erosion and connectivity remains unclear. This study investigated the variation in rill erosion from the eroded morphology and sediment connectivity using flume scouring experiments. Upslope sediment-laden inflow was simulated considering the upslope terrace areas of 0.15, 0.30, and 0.45 m2 and an upslope inflow of 6 L min−1. The quantity and cross-sectional depth of rills gradually decreased with increasing upslope terrace area. The cross-sectional morphology of rills changed from being V-shaped to U-shaped in the rill erosion process. All of the mean values of the morphological parameters gradually decreased with increasing upslope terrace area, in contrast to the width-depth ratio (Rw/d) and rill density (ρ), which both initially increased and then decreased. The average length, width, and depth of rills were smaller under an upslope terrace area of 0.45 m2 than those under an upslope terrace area of 0.15 m2; they decreased by 2.78 %, 20.67 %, and 33.68 %, respectively. Soil and water loss induced by rill erosion decreased with increasing upslope terrace area. Rills, as major venues for sediment transport on hillslopes, exhibited a higher sediment connectivity (IC) than that observed in interrill areas under the different upslope terrace areas. Rill development resulted in higher erosion between the upslope and downslope parts within rill channels. The variations in Rw/d and ρ were significantly correlated with runoff and eroded sediment yield, which could be used to estimate the rill erosion process under different upslope terrace areas.

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.

Flow-driven soil erosion processes in a calcareous semiarid soil: Rill length and flow rate impacts

Understanding the dynamic mechanisms of rill erosion is vital for accurately modeling of soil erosion. In this research, rill erosion was experimentally studied using different rill lengths. A flume, 11 m long and 0.2 m wide adjusted to 3% slope, was used for constructing rectangular rills of 0.05 m width and six different lengths (1 to 12 m). The experiments were carried out on a clayey calcareous soil from a semiarid region under two flow rates of QL = 0.025 and QH = 0.045 L s−1. Temporal sediment concentrations (C) were measured at the rill outlet for 40 min and the sediment size distribution (PSD) was recorded twice during each test. The common decreasing trend of C was affected by both flow rate and rill length. The formation of head-cuts along the bed of long rills was an important erosion process affecting C and PSD. The compiled data were used to determine the exponential models' parameters of both C and soil detachment rate based on the steady, unsteady, and event-average values. While, the equilibrium sediment transport occurred by increasing rill length beyond 7 m under QL, the tested rills under QH were not sufficiently long for the flow to reach its maximum transport capacity at the gentle slope used and for the calcareous soil tested. Based on the sediment PSD, the selective transport of coarse particles was evidenced. Selectivity transport was affected by rill length reached its maximum at the rill length of 7 m.The findings provides better understanding of rill erosion processes for calcareous soils on gentle slopes in semiarid regions. Further research is needed for better understanding of the rill erosion processes on short gentle slopes to provide experimentally supports for the existing theoretical concepts of rill erosion models.

A comparative experimental study of rill erosion on loess soil and clay loam soil based on a digital close-range photogrammetry technology

Accurately describing rill development is important for building physical process models of soil erosion. The objectives of this study were to investigate the evolution processes of rill erosion morphology, their impacts on runoff and sediment yield, and the relationship between flow hydraulic characteristics and rill erosion dynamic mechanisms on slopes of two soil types, namely, clay loam soil (CLS) and loess soil (LS), under two successive rainfall events at a rainfall intensity of 120 mm h−1. We used digital close range photogrammetry technology to obtain overhead images of two experimental soil bins (5 m long, 1 m wide and 0.5 m deep with a slope gradient of 20°) at 5-minute time intervals to monitor the dynamic processes of rill erosion. The results showed that rill erosion processes exhibited important impacts on sediment and runoff and its hydrodynamic characteristics. Compared with LS, runoff formed and rills developed earlier in the CLS slope. Soil with high clay content was more prone to form rills, but less susceptible to severe soil erosion than LS. Once rills began to develop in LS, soil loss increased and the morphology of rills changed rapidly, as did runoff rate. The flow velocity for the two soils decreased gradually with time and increased along the flow direction. The trend of Froude number (Fr) decreased with time, whereas the Darcy-Weisbach resistance coefficient (f) exhibited an opposite trend. The Reynolds number (Re) of LS increased first and then decreased, while Re of CLS decreased gradually with time. Sediment yield and runoff were positively correlated with f, but significantly negatively correlated with Fr. Moreover, Re was significantly negatively correlated with sediment yield, but not with runoff. These results are of great importance for soil loss prediction.

Quantifying the sheet-rill erosion process along a saturated soil slope

The sheet and rill erosion process on a saturated soil slope is different from that on an unsaturated soil, resulting in different erosion masses. Nevertheless, few studies have investigated sheet erosion and rill erosion along a saturated soil slope. Quantifying and comparing erosion masses of sheet and rill erosion along a saturated soil slope are important to understanding soil erosion mechanisms and control soil loss. Based on indoor soil flume simulated rainfall experiments, the erosion process characteristics from sheet erosion to rill erosion were studied, and the erosion masses were measured under combined conditions of different rainfall intensities (30, 60, and 90 mm/h) and slope gradients (2°, 5°, 10°, 15°, and 20°). The fitting method of sheet erosion (FSM) and an analytical method of rill erosion (ARM) were utilized to determine the erosion mass in different erosion stages from sheet to rill erosion. The results showed that the FSM and ARM methods were applicable to estimate erosion masses during the erosion process from sheet to rill erosion. The relationship among the rainfall intensity, slope gradient and erosion mass in the sheet erosion and rill erosion states could be expressed by a power function. The erosion masses in the sheet and rill erosion states were more sensitive to the rainfall intensity than to the slope gradient. Furthermore, the erosion masses in the rill erosion state were approximately 2.31–5.42 times higher than those in the sheet erosion state under the combined conditions of three rainfall intensities (30, 60, 90 mm/h) and five slope gradients (2°, 5°, 10°, 15°, and 20°). Relatively higher slope gradients and rainfall intensities corresponded to greater amplitudes of the variation in erosion masses in the sheet and rill erosion states than those under relatively lower slope gradients and rainfall intensities. The results provide simplified and accurate experiments and methods for quantifying erosion masses in different erosion stages between sheet and rill erosion and clarifying the erosion process from sheet to rill erosion.

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.

Effect of subsurface water flow depth on the rill erosion process on purple soil slopes

Understanding rill erosion processes on purple soil slopes with subsurface water flow at different depths is of great significance to determine rill erosion model parameters. The purposes of this study were to quantify the influences of subsurface water flow at different depths on the rill erosion process. Laboratory experiments werecarried out to measure sediment delivery under 5°, 10°, 15°, and 20° slope gradients, 2, 4, and 8 L min−1 flow rates and 0, 5, 10, and 15 cm subsurface water flow depths (SWFDs) below the soil surface at eight slope lengths of 1, 2, 3, 4, 5, 6, 7, and 8 m. The results showed that the rill erosion prediction model is applicable to rills on purple soil slopes with subsurface water flow at different depths. The effect of the slope gradient on the sediment concentrations is greater than that of the flow rate under various SWFDs. Slopes with shallower subsurface water flow depths from the soil surface produced higher soil erosion and sediment delivery, and the sediment concentrations of rill erosion under the impacts of SWFDs of 5, 10, and 15 cm from the soil surface were 61%, 50%, and 41% of that of the saturated soil slope, respectively. Moreover, the rill erosion process can be well simulated with sediment concentration data from 1 to 4 or 1–8 m rill lengths when the maximum sediment concentration at the sediment transport capacity level is determined. Our results provide a simplified and accurate method for predicting erosion processes with few data and reveal rill erosion processes under the effect of subsurface water flow.

Mathematical method for physics-based rill erosion process using detachment and transport capacities

Quantification of rill erosion processes is of great importance in both model parameter estimations for process-based rill erosion models and in model performance verification. This study presents a mathematical method to determine a physics-based rill erosion process derived from the feedback relationship of transport capacity and detachment capacity. Experimental data sets were used to determine transport capacities under steep slope gradients of 15°, 20°, and 25° and the detachment capacities. The estimated transport and detachment capacities were then used to determine the sediment delivery processes under different hydraulic regimes. The sediment concentrations along the rill determined with the mathematical method were compared with the experimental measurements to verify the methodology and the mathematics. Results showed that the mathematical model results agreed well with the experimental data in references. The predicted detachment capacity calculated by the new method was capable of predicting saturated, unsaturated, and thawed slope, but incapable of partially thawed soil. This study not only supports the analytical solution to the differential equation of rill erosion, but also verifies that the experimental method was fit well with the mathematical concept. The new method provides a useful and efficient way to quantify rill erosion processes.