Rill Flow Research Articles

Effects of longitudinal profile shape on scour and flow resistance in rills

AbstractThe literature regarding how rill longitudinal profile (concave and convex) affects soil loss and flow resistance is still lacking. The only analysis available in the literature for rills is limited by the fact that measurements were performed for a unique mean slope value sp (18%). In this article, further rill measurements were conducted on a plot with sp = 15% and complex profile shapes and were used to widen the knowledge about the influence of longitudinal profile shape on rill scour, eroded volume, and flow resistance. The findings highlighted that the concave profile has a homogeneous spatial distribution of moderate scours, whereas the scours in the convex one are deeper and more confined, but they are not placed after the slope change as found for sp = 18%. The mean scour depth, which accounts for the discharge and profile shape effects, is not (concave) or is weakly (convex) related to the flow discharge. The concave profile determined a reduction of approximately 57% of the total eroded volume when compared with the convex profile shape, confirming that a concave hillslope limits erosive phenomena. Finally, the flow resistance equation guaranteed a precise estimation of the Darcy–Weisbach friction factor.

Evaluating the effects of soil grain roughness and rill cross‐section shape on flow resistance

AbstractThe knowledge of the hydraulic characteristics of rill flows is needed to improve the understanding and the accurate modelling of upland erosion processes. Flume investigations are developed to schematize conditions useful to analyze the influence of a unique variable significant for modelling rill hydraulics, while field investigations are used to simulate morphological and hydraulic conditions of natural cases. The main aim of this investigation is to compare flow resistance measurements carried out using a rectangular rill channel (flume data) and manually generated and naturally shaped rills (plot data) for the same soils and hydraulic conditions. The two data sets allowed for establishing how the soil grain roughness and rill cross‐section shape affect flow resistance using a theoretical equation deduced by applying dimensional analysis and self‐similarity theory. The analysis developed for a fixed‐bed channel with a rectangular cross‐section demonstrated that the influence of different soil roughness on the Darcy‐Weisbach friction factor is approximately equal to ±1%, while for a rill with an irregular cross‐section the same effect is almost equal to ±2%. Therefore, for an open‐channel flow on a fixed bed, the effect of the grain size roughness on the friction factor is limited. For each examined soil, the investigation also demonstrated that the rill cross‐section shape leads to a variability of the friction factor from −20% to +30%. This result highlights that, for a fixed‐bed channel, the effect of the cross‐section shape on the Darcy‐Weisbach friction factor is much greater than that due to the soil grain size roughness. The main implication of this work is that the results deduced by laboratory measurements with regular cross‐sections, thus neglecting a complex geometry, can be affected by relevant discrepancies from those obtained by the field measurements, which better simulate natural conditions.

Factors affecting the sediment concentration in runoff from a slope during the spring thawing period

AbstractSediment concentration (S) is vital in soil erosion models. However, research on estimating S at different positions on slopes in the spring thaw period is difficult. Thus, in this research, four flow discharges, two slope gradients, and four initial thawing depths were used in combination with Rare Earth Elements (REEs) tracing techniques to investigate the effects of hydraulic parameters at different locations of freeze–thaw rill flows on S. The results show that the S on the freeze–thaw slope gradually increases from the first section to the third section of the slope, with an increase of 43.99% from the first slope section to the third slope section. The flow velocity is the most critical factor to affect the S (R2 > 0.7). The prediction equation of S on the entire slope under freeze–thaw conditions was established based on the flow turbulence intensity and relative gravity (R2 = 0.769). This study examined the S and hydraulic parameters of the freeze–thaw rill flow, which provided a reference for further research on the soil erosion model during the spring thawing period.

Rock surface flow accelerates rill erosion of excavated slopes in karst mining areas

Rock surface flow is the secondary distribution of precipitation and inflow on the rock surface, an essential runoff generation mode in karst areas. The effects of rock surface flow on soil erosion in karst regions cannot be ignored. However, its impact on rill erosion in karst slopes remains unclear. The objectives of this study were to investigate the impact of rock surface flow on rill erosion, rill morphological characteristics, rill flow hydraulic characteristics, and hydrodynamic mechanisms. A field runoff plot (3 m long, 1 m wide and 0.3 m deep) with three inflow rates (2, 3, and 4 L/min) was subjected to runoff simulation experiments under two typical rock surface morphologies in karst areas: smooth rock surface (SRS) and karren rock surface (KRS), with bare slope (BS) as the control group. The results revealed that rock surface flow aggravated the soil and water loss of the excavated slope. Compared to BS, the runoff yield for SRS and KRS increased by 81.49 % and 61.91 %, respectively. Meanwhile, the soil loss of SRS and KRS were 2.50 times and 5.56 times greater. The rills below the rock outcrops didn’t develop into a rill network after formation, owing to the rock surface morphology. Rill depth was the best indicator of rill morphology for evaluating rill erosion on slopes below rock outcrops. The Reynolds number and Darcy–Weisbach resistance coefficient were the optimal hydraulic parameters to predict the rill erosion of the SRS and KRS, respectively. At the same time, the runoff power and flow shear stress were the best hydrodynamic parameters to describe the rill erosion mechanism of SRS and KRS. These results provide a scientific basis for further understanding the mechanism of soil erosion on karst slopes and for preventing soil erosion on karst slopes in mining areas.

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.

Modeling the sediment transport capacity of rill flow using a soil-rock mixture on steep slopes

Study regionYangkeng watershed (E118°03 ', N24°57 '), one of the typical representative areas where Benggang occurs in southern China. Study focusSediment transport capacity (Tc) plays a key role in the construction of models of soil process-based erosion, and is closely related to sediment particle properties. However, few studies have been conducted on Tc using soilrock mixtures along steep slopes. This study investigated the impacts of gravel content and gravel composition on Tc using soilrock colluvial deposits in a steep flume. New hydrological insights for the regionTc increases with the increase of gravel content and proportion of large gravel (5–10 mm), and can be simulated by slope steepness, unit flow discharge, gravel mass content and gravel composition (R2 =0.904; NSE=0.911). The mean weight diameter (MWD) can be used as a sediment characteristic parameter to simulate Tc, and the flow velocity, Reynolds number and stream power are suitable hydrodynamic parameters for predicting Tc. The new equation obtained by dimensionless analysis includes flow properties (unit flow discharge, stream power and Reynolds number) and sediment characteristics (MWD) has a good prediction effect on Tc (R2 =0.991, NSE=0.990), and the simulation accuracy is greater than that of other Tc equations deduced in this study. These results promote understanding of the relationship between Tc and sediment characteristics and contribute to further elucidating the erosion process of soilrock mixtures.

Modeling the transport of fecal coliform in ntanwaogba creek, influenced by variations in micronutrient deposition, velocity and dispersion coefficient

The study of Micronutrients in Ntanwaogba Creek were thoroughly carried out to monitor its rates of deposition at different numerous discharge location sites in the study environment, this was imperative because the rates of biological waste discharge at regular interval, based on this factor, it was necessary to conduct a comprehensive investigation of their rate of concentration at different station point of discharge. This implies that the rate of dispersions from the contaminant influenced constant discharge of waste in the creek, and based on these factors, it was determined that such comprehensive research was required. Micronutrients act as a substrate for microbial growth, but the speed at which they are injected into the rill affects how quickly they move through the system. In order to determine the effects of these two parameters on the migration rate of faecal coliform at different point sources of discharge, the study observed different growth rate at different station point in the study location. This observed condition indicates that the pollutants had a range of development speeds, including both slow and fast, which was enabled by these considerations. The system discovered that lower velocities have an effect on velocity rates with higher concentrations, and that accumulation with micronutrients increased their concentration. However, the concentration rates varied depending on the dominant characteristics of the transport under pressure at various points of discharge. In the simulation, these two parameters were used to determine the various pressure rates at different station points. Unquestionably, the study has depicted the effects of these two parameters' pressures on the movement of faecal coliform in a range of figures that correspond to the several point sources of discharge looked at. The speeds recorded at various station locations represented the pressure rates at various rates of concentration in the research environment. It has established the scope of the influence of rill flow velocity and the variance in micronutrient deposition at various point sources. On the basis of model simulation prediction results, also, the dispersions at various point sources were evaluated. Both parameters showed correlations for the best fits when the predicted and experimental values were compared for model validation.

Evaluating the effects of stream power on rill flow resistance

AbstractLimited information is currently available on how sediment transport affects rill flow resistance and the influence of hydraulic variables, as stream power, on sediment transport capacity for rill flows. In this paper, the available measurements of hydraulic variables (flow depth, channel slope, mean flow velocity, Reynolds number, Froude number, and Darcy–Weisbach friction factor) carried out by Ban et al. (Measurement of rill and ephemeral gully flow velocities and their model expression affected by flow rate and slope gradient. Journal of Hydrology, 589, 125172) and Ban (Measurements and estimation of flow velocity in mobile bed rills. International Journal of Sediment Research, 38(1), 97–104) for fixed and mobile bed rills are used to test the applicability of a theoretically deduced rill flow resistance equation based on a power‐velocity profile. The results allowed for stating that (i) the theoretical flow resistance approach can predict Darcy–Weisbach friction factor for flows over fixed and mobile beds, (ii) the stream power, dependent on flow discharge and slope, determines different flow behaviour, and (iii) the data are supportive of the slope independence hypothesis of rill velocity, for the mobile bed condition, only for the highest investigated discharge values (greater than 0.133 L s−1).

Systematic evaluation of the effects of the length, depth, and amount of incorporated maize straw on rill flow velocity

Flow velocity is an important indicator when evaluating rill erosion flow dynamics. In recent decades, incorporating straw in farmland fields has become an effective method of utilizing maize straw resources. However, the effect of straw incorporation on the rill flow velocity of sloping farmland is unclear. In this study, experimental treatments with three gradients of maize straw length (6, 9, and 12 cm), incorporation depth (9, 12, and 15 cm), and straw amount (1000, 4500, and 8000 kg ha−1) were set up. Flow velocity was measured via an electrolyte tracer measurement system, and the comprehensive effect of straw incorporation on the rill erosion flow velocity of sloping farmland was clarified using structural equation modeling. The results showed that the mean flow velocity of straw-incorporated sloping land was 0.120–0.189 m s−1, which was 13.25–44.92% lower than that of bare land. The flow path became convoluted owing to the residual straw on the soil surface, which increased the flow resistance thereby reducing the flow velocity. Straw length had the greatest effect on flow velocity (with a contribution rate of 46.67%), and significant increases in velocity were observed with increases in straw length owing to the water-collecting function of the exposed straw on the soil surface. In addition, the flow velocity increased with an increase in runoff and decreased with an increase in infiltration but had no evident response to sediment yield. Increasing the straw amount had a negative impact on soil bulk density and infiltration, resulting in an increase in rill flow velocity. The incorporation depth had a very small effect on flow velocity. The results are helpful for understanding the changes in rill flow velocity and interpreting the soil erosion process on sloping farmland with straw incorporation.

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.

Plot investigation on rill flow resistance due to path tortuosity

The path tortuosity t is an indicator of rill morphology accounting for the deviation of the thalweg from a straight alignment. The effect of t on flow resistance has been little investigated for rills. This paper reports the results of a plot investigation aimed to establish the suitable accuracy of the rill thalweg measurement to determine the tortuosity parameter and to test the reliability of a theoretical flow resistance law. Four rills were incised in clay soil (CS) and clay-loam soil (LS) and shaped by a clear flow discharge. The three-dimensional Digital Terrain Models were created by the Structure from Motion technique. For rills on LS, an approximate thalweg was tracked by photo-interpretation, and a specific calculation routine was applied to identify the cross sections with a constant spacing d. The actual rill thalweg was obtained as the line joining the lowest points of these cross-sections. Among the different tested d values, d = 0.075 m was chosen to determine t. For both CS and LS, the Darcy-Weisbach friction factor f featured a non-monotonic relation with t, which was explained as the result of three additive components due to bed roughness, sediment transport, and localized energy losses due to curves. The effect of the former two components on f contrasts that of the third, resulting in a linearly decreasing f-t relationship and constant flow velocity for the three lowest tortuosity values, and an increased friction factor and reduced flow velocity for the highest tortuosity value. The flow resistance law was positively tested, and the predicted friction factor was dependent on t.

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.

Response of seasonal variation in soil detachment capacity to straw incorporation in sloping farmland on the Loess Plateau

AbstractSoil detachment capacity by rill flow (Dc) is sensitive to changes in topsoil properties generated by crop straw incorporation. In the present study, we investigated the effect of crop straw incorporation on the seasonal variation in Dc, the knowledge regarding which is limited currently, and identified the key influencing factors. Three typical crop straw types (wheat, soybean, and maize) were selected for this study. Soil samples incorporated with crop straw and without crop straw were collected over 375 days (from October 1, 2020 to October 10, 2021) and were subjected to a hydraulic flume (length, 4.0 m; width, 0.2 m) to determine Dc. The topsoil properties soil bulk density (BD), soil cohesion (Coh), soil penetration resistance (PR), soil organic matter content (SOM), mean weight diameter (MWD), and straw decomposition rate (SDR) were measured to explain the seasonal variation in Dc. The results showed that crop straw incorporation effectively reduced Dc, and the Dc for the soil incorporated with straw decreased by 25.92%–38.76% compared with that without straw incorporation. Dc exhibited a significant decreasing trend over time, and the Dc at the end stage decreased by 83.00%–88.31%, compared with the initial stage. Seasonal variation in Dc was significantly correlated with BD, Coh, PR, SOM, MWD, and SDR. The measured Dc decreased with BD and Coh as a power function and with SOM and MWD as an exponential function. The influence of straw incorporation on Dc is time‐dependent and can be divided into short‐term and long‐term effects. Natural soil consolidation gradually masked the effect of straw residue with the decomposition of the incorporated straw. The comprehensive response of seasonal variation in Dc to straw incorporation was described by a power combination equation (Dc = 1.23 τ1.65 BD−2.63 Coh−1.33 SOM−0.70; R2 = 0.96). Thus, the seasonal variations in Dc with straw incorporation were mainly controlled by the changes in soil properties triggered by the decomposition of incorporated straw and natural soil consolidation. The results of this study provided a scientific reference for soil erosion prediction and control in sloping farmlands.

Experimental determination of sediment transport capacity of rill flow over sandified loess slope

Rill erosion is affected by the sand particle content in soil, especially in the wind and water erosion transition region of the Loess Plateau. The sediment transport capacity (STC) is a key parameter in rill erosion research, assessing the impact of aeolian sand intrusion on the STC of rill flow is of importance for a better understanding of rill erosion. This study aimed to assess the effect of aeolian sand intrusion on the STC on sandified loess slopes, with typical slopes and flow discharges, using a flume system which consisting of a sediment-feeding and a sediment-supply/settlement flume. The sediment feeding flume was jointed by 10° higher than that of the sediment measurement flume section. Three flow discharges (2, 4, and 8 L min−1) and four slope gradients (5°, 10°, 15°, and 25°) were used to represent the natural hydrological conditions under three intrusion rates (SIR) of aeolian sands (10%, 20%, and 50%). The results show that STC increased with slope gradient and flow discharge, and the relationship between the STC and the SIR was significantly affected by the slope gradient; the STCs decreased with the SIR on a slope of 5° but increased with the SIR on steep slopes of 15°–25°, implying a significant impact of slope gradient on the relationship between SIR and STC. The SIR of 50% resulted in the highest sediment concentration nearly 1200 kg m−3 on slopes of 25°. On sandified loess slopes of 10%, 20%, and 50% SIR, the STC were about 30%, 46%, and 57% higher than on loess slopes, indicating an increased erosion rate by sand particle intrusion into loess soil. These results highlight the impact of sand intrusion on STC of rill flow and provide deeper insights into the soil loss process on the sandified loess slope.

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.

Determination of sediment transport capacity by hydrodynamic variables considering subsurface water flow

AbstractSubsurface water flow can cause serious soil erosion on sloping farmland. Sediment transport capacity (Tc) data at different subsurface water flow depths are limited. The objective of this study was to develop a new equation that can accurately determine Tc under subsurface water flow impacts. Subsurface water flows were arranged at three depths: 15, 10, and 5 cm. Three flow discharges (2, 4, and 8 L min−1) and four slope gradients (5°, 10°, 15°, and 20°) were designed to conduct flume scouring experiments. The results indicate that flow velocity, shear stress, and unit energy with linear equations and stream power with power equations can be used to determine Tc, but those using the unit stream power perform relatively poorly under the different subsurface water flow depths. Among these parameters, the shear stress is the preferred predictor of Tc. The addition of prediction parameters, subsurface water flow depth, in response to Tc equations can improve the accuracy of Tc modeling. The power‐exponential equation of shear stress combined with the subsurface water flow depth best described the Tc of rill flow under subsurface water flow impacts. Subsurface water flow can increase the soil erodibility, and thus decrease the critical shear stress, resulting in a lower critical flow velocity at which sediment transport capacity can be achieved. This research provides a new equation for directly determining Tc in subsurface water flow‐impacted rill flow on purple soil slopes.

Morphological controls on surface runoff: an interpretation of steady-state energy patterns, maximum power states and dissipation regimes within a thermodynamic framework

Abstract. Recent research explored an alternative energy-centred perspective on hydrological processes, extending beyond the classical analysis of the catchment's water balance. Particularly, streamflow and the structure of river networks have been analysed in an energy-centred framework, which allows for the incorporation of two additional physical laws: (1) energy is conserved and (2) entropy of an isolated system cannot decrease (first and second law of thermodynamics). This is helpful for understanding the self-organized geometry of river networks and open-catchment systems in general. Here we expand this perspective, by exploring how hillslope topography and the presence of rill networks control the free-energy balance of surface runoff at the hillslope scale. Special emphasis is on the transitions between laminar-, mixed- and turbulent-flow conditions of surface runoff, as they are associated with kinetic energy dissipation as well as with energy transfer to eroded sediments. Starting with a general thermodynamic framework, in a first step we analyse how typical topographic shapes of hillslopes, representing different morphological stages, control the spatial patterns of potential and kinetic energy of surface runoff and energy dissipation along the flow path during steady states. Interestingly, we find that a distinct maximum in potential energy of surface runoff emerges along the flow path, which separates upslope areas of downslope potential energy growth from downslope areas where potential energy declines. A comparison with associated erosion processes indicates that the location of this maximum depends on the relative influence of diffusive and advective flow and erosion processes. In a next step, we use this framework to analyse the energy balance of surface runoff observed during hillslope-scale rainfall simulation experiments, which provide separate measurements of flow velocities for rill and for sheet flow. To this end, we calibrate the physically based hydrological model Catflow, which distributes total surface runoff between a rill and a sheet flow domain, to these experiments and analyse the spatial patterns of potential energy, kinetic energy and dissipation. This reveals again the existence of a maximum of potential energy in surface runoff as well as a connection to the relative contribution of advective and diffusive processes. In the case of a strong rill flow component, the potential energy maximum is located close to the transition zone, where turbulence or at least mixed flow may emerge. Furthermore, the simulations indicate an almost equal partitioning of kinetic energy into the sheet and the rill flow component. When drawing the analogy to an electric circuit, this distribution of power and erosive forces to erode and transport sediment corresponds to a maximum power configuration.

Variations in soil detachment by rill flow during crop growth stages in sloping farmlands on the Loess Plateau

Near soil-surface characteristics change significantly with crop growth in sloping farmlands and thereby induce the temporal variations in soil detachment by rill flow. In this study, variations in soil detachment capacity by rill flow (Dc) and soil erosion resistance to rill flow as reflected by rill erodibility (Kr) and critical shear stress (τc) during crop growth stages were quantified in sloping farmlands of the Loess Plateau, China. The soil samples were collected from four different plots (one control and three crop plots—soybean, corn, and millet) and subjected to flume experiments under different shear stress (τ) levels (5.16–15.61 Pa). The near soil-surface characteristics including soil moisture content (SMC), soil bulk density (BD), soil cohesion (Coh), mean weight diameter (MWD), soil organic matter content (SOM), root length density (RLD), root surface area density (RSAD), and root mass density (RMD), were measured to explain the variations in soil detachment during crop growth stages. The results showed that SMC fluctuated greatly, whereas BD, Coh, MWD, SOM, RLD, RSAD, and RMD increased gradually with crop growth. Measured Dc and fitted Kr significantly decreased with crop growth during one growing season. The mean values of Dc for soybean, corn, and millet decreased by 61.19–67.23%, Kr decreased by 55.53–61.36%, and τc increased by 16.0–18.31% compared to those for the bare soil control. The decrease in Dc and Kr were significantly affected by BD, Coh, SOM, MWD and RLD, RSAD, and RMD (p < 0.01). Seasonal variations in Dc were well simulated by τ, BD, Coh, and RLD (R2 = 0.96, NSE = 0.96). The findings demonstrate that it is essential to consider the seasonal variations in soil detachment for optimizing soil and water conservation measures in sloping farmlands.

Soil detachment by sediment-laden rill flow interpreted using three experimental design methods

Soil detachment by rill flow is a key sub-process of rill erosion on hillslope areas; however, many studies have focused on soil detachment by clear rather than by sediment-laden rill flow and most studies have used the time- and labor-consuming full-factorial design. This study used three experimental design methods, including the full-factorial design, orthogonal design, and Taguchi method for indoor flume experiments to explore the process of soil detachment by sediment-laden rill flow. We analyzed and quantified the effects of the slope, flow discharge, and sediment load on the soil detachment rate by sediment-laden rill flow, and compared the results of the three experimental design methods to evaluate the applicability of the orthogonal design and Taguchi method in indoor flume experiments. The results showed that the flow discharge and slope were positively correlated with the soil detachment rate, while the sediment load was negatively correlated with it. The main effects of slope, flow discharge, and sediment load on the soil detachment rate, which were obtained from the three methods, had the same trends. The contribution rates of the flow discharge, slope, and sediment load were 46.20–47.83, 34.42–40.88, and 7.45–12.87%, respectively. The contribution rates of each factor obtained from the three methods were slightly different, but the overall ranked order was the same. The Taguchi method and the orthogonal design can effectively reduce the number of tests and ensure low relative errors. The prediction of the soil detachment rate from the Taguchi method was closer to the full-factorial design than the orthogonal design. Therefore, the Taguchi method proved was most efficient for flume experiments on soil detachment processes by the sediment-laden rill flow.

Rill flow velocity affected by the subsurface water flow depth of purple soil in Southwest China

Rill flow velocity affected by the subsurface water flow depth of purple soil in Southwest China