Soil Flume Research Articles

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.

A strategy for modelling sediment distribution along slope lengths of an ephemeral gully through the simulated scouring experiments

Accurately describing the soil erosion process is crucial for improving the accuracy of existing simulation models of ephemeral gully erosion. However, a laboratory experimental strategy for simulating sediment distribution along slope lengths of an ephemeral gully is lacking. This limitation greatly hinders the development of simulation models for ephemeral gully erosion. In this study, we constructed six soil flumes with 12 m in length, 0.1 m in width, and 0.5 m in depth, which were filled with cultivated loess soils. Four flow rates (5.33, 10.67, 21.33, and 42.67 × 10−3 m3 s−1 m−1), four slope gradients (5°, 10°, 15°, and 20°), and six gully lengths (2, 4, 6, 8, 10 and 12 m) were adopted to perform a series of laboratory scouring experiments. The measured sediment concentrations at the outlet ranged from 23.39 kg m−3 to 534.60 kg m−3. The trend of exponential increase in sediment concentrations along slope lengths of an ephemeral gully was similar to that observed in a rill. According to the fitting equation, the maximum sediment concentrations (Ae) ranged from 358.05 kg m−3 to 910.88 kg m−3. The attenuation coefficients (Be) indicated the rate of increase in sediment load with slope length, which ranged from 0.042 m−1 to 0.185 m−1. By combining the measured sediment loads along the slope length (8 m) with the sediment transport capacity, the sediment distribution along slope lengths of an ephemeral gully could be accurately quantified. Moreover, the results confirmed that a flume with a short slope length (4/6 m) could effectively determine the sediment distribution along slope lengths at gentle slope of 5° and 10°. However, a minimum length of 8 m was advisable at steep slope of 15° and 20°. In summary, our experimental strategy provides an efficient method for determining the sediment distribution along slope lengths of an ephemeral gully.

Response of particle detachment and hydrological processes on windward slope in laboratory flume experiments at different wind velocities and rainfall intensities

Wind-driven rain (WDR) is a natural phenomenon that causes soil erosion, a decline in land productivity, water pollution, and other environmental problems. Owing to the influence of topography on atmospheric circulation, WDR occurs most frequently on windward slopes. Studying the physical mechanisms of soil erosion on windward slopes is challenging because of the complex co-occurrence of wind and rainfall. This study aimed to investigate the effects of rainfall intensity (30, 60, and 90 mm h−1), wind velocity (0, 2, 3, 5, 7, and 8 m s−1), and induced flow hydraulic characteristics on the processes of runoff occurrence, infiltration, and sediment detachment on windward slopes based on an indoor artificial simulated WDR soil flume experiments with three replicates. The results showed that both rainfall intensity and wind velocity exerted significant effects on runoff and sediment transport on the windward slopes (P < 0.05). The contribution rates of wind velocity on runoff occurrence time, steady infiltration rate, runoff rate, and sediment detachment rate were 20.53 %, 39.44 %, 3.36 %, and 6.24 %, respectively. When the rainfall intensity was constant, the steady infiltration rate on the windward slope increased linearly with the increasing wind velocity, whereas the runoff and sediment detachment rates decreased with increasing wind velocity. The decrease in hydraulic parameters of average flow velocity (v), stream power (Ω), unit stream power (U), and unit energy of cross-section (E) influenced by wind blowing on overland flow is the main reason for the decrease in the sediment detachment rate on the windward slope. Meanwhile, the increase in flow depth (h) and shear stress (τ) caused an increase in the infiltration rate. Because of the inclusion of Ω and E in the total work done and total energy consumption of overland flow, respectively, the relationship between Ω and E of the windward water flow with the sediment detachment rate showed an exponentially increasing curve for all treatments (R2 greater than 0.97). The hydraulic parameters Ω and E of the windward water flow are the optimal factors for constructing an erosion dynamic model under WDR conditions.

Impacts of slope morphological evolution on subsequent erosion for a coarse-textured soil

Assessing impacts of erosion-induced slope morphological evolution on subsequent erosion based on erosion feedback system is crucial for prevention and control of slope erosion in coarse-textured soil regions. This work aimed to reveal the effect of the evolving erosion morphology on subsequent erosion processes on the granite red soil slope through quantitative analysis. Specifically, we achieved it by conducting long-duration (120 h) simulated rainfall experiments under constant rainfall intensity of 90 mm h−1 and fixed slope gradient of 16.7 % on the soil flume packed with granite red soils to simulate field slope. Results showed that (1) with the evolution of erosion, three erosion phases (sheet erosion, rill erosion, and ephemeral gully erosion phases) varied as the dominant erosion forms; (2) with erosion development, the coverage of quartz gravels enriched on the slope surface increased first (4.60 %-49.17 %), followed by a trend of dynamic erosion–deposition equilibrium (47.68 %-56.74 %); (3) the quartz gravel cover on the surface restrained the slope erosion degree, resulting in loss of fine particles and erosive-reduction in vertical depth of inter-rill and sheet erosion areas; (4) quartz gravels entering the channels inhibited downward erosion, forcing the runoff to intensify lateral erosion, leading to gradual degradation of soil layers and the exposure of erodible subsoils under an accelerated trend. This work provides a reference for the prediction and control of slope erosion in granite red soil regions.

The effects of typical grass cover combined with biocrusts on slope hydrology and soil erosion during rainstorms on the Loess Plateau of China: An experimental study

AbstractBiological soil crust (BSC), as a groundcover, is widely intergrown with grass. The effects of grass combined with BSCs on slope hydrology and soil erosion during rainfall are still unclear. In this study, simulated rainfall experiments were applied to a soil flume with four different slope cover treatments, namely, bare soil (CK), grass cover (GC), BSC, and GC + BSC, to observe the processes of runoff and sediment yield. Additionally, the soil moisture at different depths during infiltration was observed. The results showed that the runoff generated by rainfall for all treatments was in the following order: BSC &gt; GC + BSC &gt; CK &gt; GC. Compared with CK, GC promoted infiltration, and BSC inhibited infiltration. The BSCs obviously inhibited infiltration at a depth of 8 cm. When the rainfall continued to infiltrate down to 16 and 24 cm, the effects of grass on promoting infiltration were stronger than those of BSCs on inhibiting infiltration. Compared with CK, the flow velocity of the BSC, GC and GC + BSC treatments was reduced by 62.8%, 32.3% and 68.3%, respectively. The BSCs and grass increased the critical shear stress by increasing the resistance. Additionally, the average sediment yield of GC and both treatments with BSCs was reduced by 80.8% and &gt;99%, respectively, compared with CK. The soil erosion process was dominated by the soil detachment capacity in the CK, BSC and GC + BSC treatments, while the GC treatment showed a transport‐limited process. This study provides a scientific basis for the reasonable spatial allocation of vegetation in arid and semiarid areas and the correction of vegetation cover factors in soil erosion prediction models.

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.

A contribution to rainfall simulator design – a concept of moving storm automation

Abstract. We developed an advanced-design programmable rainfall simulator (RS) to simulate a moving storm rainfall condition. The RS consists of an automated nozzle control system coupled with a pressure regulator mechanism for an operating range of 50 to 180 kPa at a drop height of 2000 mm above the soil flume surface. Additionally, a programmable mobile application was developed to regulate all RS valves. Near natural rainfall conditions were simulated at varying spatial and temporal resolutions in a controlled environment. A soil flume of 2500 mm × 1400 mm × 500 mm was fabricated to conduct different hydrological experiments. The flume was designed to record overland, subsurface, and baseflows simultaneously. This study focused on a detailed analysis of moving storms and their impact on hydrograph characteristics. Experimental results showed a considerable difference in terms of time to peak (tp), peak discharge (Qp), and hydrograph recession for two different storm movement directions (upstream and downstream). Two multiple regression models indicate a statistically significant relationship between the dependent variable (tp or Qp) and the independent variables (i.e. storm movement direction, storm velocity, and bed slope gradient) at a 5 % level of significance. Further, the impact of these moving storm phenomena reduces with the increase in the storm movement velocity.

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.

A simulated study of surface morphological evolution on coarse-textured soils under intermittent rainfall events

The evolution of slope surface morphology directly affects the hydrological process and forms a dynamic feedback loop with the runoff and sediment production. However, the unique erosion processes and feedback mechanism for the surface morphological changes of coarse-textured granite soils have been rarely studied previously. Here, a series of rainfall simulation experiments (intensity of 90 mm h−1) were conducted under continual intermittent rainfall conditions (duration of 20 h) on a 0.8 m by 3.0 m soil flume packed with the four coarse-textured granite soils (sieved soils from the counties of Tongcheng, Ganxian, Changting, and Wuhua, China, defined as TCA, GXA, CTA and WHA, respectively) to measure the surface roughness, coarse grain coverage, soil erosion rate, stream power and sediment sorting. The results showed: 1) the coarse grain coverage of the four soils gradually increased with rainfall duration, with the final maximum (74.89%) for CTA soil, which corresponded to the enrichment of fine particles in the sediments; 2) the mean soil erosion rate of the four soils ranged from 0.090 to 0.76 kg m−2 min−1 with the decrease of clay content from 24.19% to 13.60% in the original soils, coinciding with the surface roughness controlled by rill erosion around the coarse grains; 3) the coarse grain cover provided a protective layer to reduce further soil loss in inter-rill areas; 4) the model of soil clay content and stream power was shown to be suitable for soil erosion prediction of coarse-textured soils (R2 = 0.93). The results highlighted soil mechanical composition and coarse grain coverage on the surface as the key factors affecting the coarse-textured soil erosion. This study facilitates our understanding of the unique erosion processes and the erosion feedback mechanism of coarse-textured soils.

Two-dimensional (2D) numerical modelling of rainfall induced overland flow, infiltration and soil erosion: comparison with laboratory rainfall-runoff simulations on a two-directional slope soil flume

Abstract This paper presents a two-dimensional (2D) numerical model of soil erosion and sediment transport resulting from rainfall induced overland flow. It is a spatial and temporal dynamic model combining physical and empirical laws and comprises: i) An overland flow module that solves the two-dimensional unsteady water flow equations on an infiltrating surface; ii) A soil infiltration module that uses a combined Horton-SCS scheme; and iii) A soil erosion and sediment transport module that solves the two-dimensional sediment transport equation, distinguishing between rill erosion, interrill erosion and sediment deposition. The performance of the model was evaluated by comparing its results with observed data from laboratory rainfall-runoff experiments on a two-directional 2.00 × 2.00 m2 soil flume set at 1% and 10% slopes in the x- and y-directions, respectively. The x-direction produced remarkably lower runoff and transported sediments than the y-direction. The numerical model significantly underestimated x-direction lower values of runoff and transported sediments. However, in the y-direction the model presented very good performance. Overall, in total terms (x- plus y-direction), the numerically simulated graphs of runoff and sediment transport were in very good agreement with corresponding experimental measurements, demonstrating the laboratory proof-of-concept of the model.

Experimental study on macropore flow effects in unsaturated soil on subsurface drainage and soil desalination*

AbstractDischarging of moisture and salt from unsaturated soil has always been a difficult problem. Therefore, a laboratory experiment in unsaturated soil, under drip irrigation, on subsurface drainage and soil desalination through subsurface drainage pipes (SDP) was conducted for using soil flume, where artificial macropore flow tubes (MFT) were deployed and connected to SDP. We conducted seven different deployment treatments of the artificial MFT, including four deployment tube densities (0, 3, 4, and 5) and two deployment arrangements (curved and straight). The results showed that no flow was observed in the SDP under the base treatment without MFT, increasing flow rates to 9.71 L, 10.7 L, and 12.3 L under the three straight line treatments and 7.81 L, 9.81 L, and 11.2 L under the three curved line treatments. Water content in both the upper and lower soil layers was almost uniform in the treatments with MFT, and an overall desalination trend was observed; the average desalination rate of curved MFT treatments was lower than that of the straight MFT treatments under the same MFT density. The findings serve as the foundation for future studies on drainage and desalination in unsaturated soil and facilitate the design of tile drainage under drip irrigation.

Evaluating Mulch Cover with Coir Dust and Cover Crop with Palma Cactus as Soil and Water Conservation Techniques for Semiarid Environments: Laboratory Soil Flume Study under Simulated Rainfall

This paper aims to investigate the performance of mulch cover with coir dust (Cocos nucifera L.) and cover crop with Palma cactus (Opuntia ficus indica Mill.) as soil and water conservation techniques, in a laboratory soil flume under simulated rainfall. Palma cactus plants oriented at 90° and 30° angles with the slope direction were considered. Simulations comprised uniform advanced and delayed rainfall patterns. Runoff hydrographs and soil loss were monitored at the downstream end of the flume. Soil moisture and flow velocity were measured, and several hydraulic parameters of runoff were estimated. Results show that both mulch cover with coir dust and cover crop with Palma cactus were effective in reducing runoff and soil loss and increasing soil moisture content, thus being both suitable soil and water conservation techniques for semiarid environments. Coir dust was more effective than Palma cactus. Palma cactus oriented at a 90° angle was slightly more effective than Palma cactus oriented at a 30° angle. Differences between advanced and delayed rainfall patterns on the hydrological and erosive response were more pronounced for the mulch cover condition, where no runoff and soil loss were observed at the downstream end of the flume for the advanced rainfall pattern.

Development of a water erosion tracer using industrial residue as a source of rare earth elements

This study was aimed at developing a water erosion tracer from montmorillonite (MMT) chemically modified with rare earth elements (REE) as chemical signatures to understand the landscape sediment mobilization dynamics. A new route of development involving the use of an industrial residue from the REE extraction process was evaluated for its REE incorporation efficiency in the clay mineral to enable further tracer applications on a field scale. This industrial residue-modified MMT tracer was compared with a reference tracer based on the chemical modification of MMT with a commercial REE precursor. These REE-modified MMT tracers were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence spectroscopies, inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffractometry, scanning electron microscopy (SEM), and N2 physisorption measurements were performed to verify whether the MMT tracer modified with the industrial residue had the same chemical signature (REE) content and chemical stability as those of the commercial REE precursor-based tracer. Chemical stability tests under different pH conditions were carried out to evaluate possible REE content variations in the tracers, mainly over the targeted soil pH range (pH = 4.0–7.7). The clay modified with the industrial residue was further submitted to laboratory horizontal mobility tests using a soil flume. The chemical modification method was effective in producing a chemically stable MMT-based tracer that also displayed multi-chemical signature due to the use of the ClLa40-INB solution, which will potentially enhance the tracer detection in sediments. The mobility tests confirmed the suitability of the modified clay as a tracer.

Erosion characteristics of different reclaimed substrates on iron tailings slopes under simulated rainfall

Water-induced erosion of iron tailings is a serious problem affecting ecological restoration, but, little is known about how the occurrence of erosion on tailings slopes and types of reclaimed substrates that are beneficial to reducing slope erosion. This study measured the slope erosion characteristics of six reclaimed substrates including loose tailings (LT), crusty tailings (CT), tailings incorporating mushroom residues (TM), tailings incorporating soil (TS), tailings incorporating soil and mushroom residues (TSM) and soil (S) in experimental soil flumes under three simulated intermittent rainfall events, with intensity of 60, 90 and 120 mm h−1 for the first, second and third event, respectively. Significant differences (p < 0.05) were found in erosion characteristics among the six reclaimed substrates. TM had the lowest sediment yield but the highest runoff volume without obvious rills. LT, CT and TS had the highest sediment yield rates and severe slope erosion morphology. With the increased number of rainfall events, the runoff rates of the six substrates all increased, but only the sediment yield rates of LT, CT and TS increased, the sediment yield rates of other substrates increased first and then decreased. Therefore, adding agricultural organic wastes such as mushroom residues to tailings and reducing soil addition may be an effective way to reduce erosion and promote ecological restoration in soilless tailings areas.

Photogrammetric analysis tools for channel widening quantification under laboratory conditions

Accurate soil erosion monitoring provides a basis for soil erosion prediction and prevention. Channel bank erosion quantification is prerequisite to couple effectively the bank sediment supply system with fluvial sediment transport fluxes. The objectives of this study were to describe and evaluate methods for monitoring and data post-analysis of channel widening in the presence of a non-erodible layer. Technology was developed to capture 5-cm spaced cross-sections along a soil flume at 3-s time intervals. Two off-the-shelf digital cameras were positioned 3-m above the soil bed and controlled by a program to trigger simultaneously and download images to the computer. Methods utilizing color differences in images and elevation differences in DEMs were applied to detect discontinuities between channel walls and the soil bed. Channel widths were calculated by differentiating the coordinates of these surface discontinuities. A volumetric method was used to calculate flow velocity with measurements of flow depths obtained from ultrasonic depth sensors. Sediment concentration was determined by manual sampling. The results showed that different channel width calculation methods exhibited comparable outcomes and achieved satisfactory accuracy. Sediment discharge showed a significant positive linear correlation with channel widening rate, while exhibiting a 5 to 25-s time lag compared to the peak of channel widening rate. Total sediment discharge calculated by photogrammetry was 3.1% lower than that calculated by manual sampling. Flow velocity decreased with time and showed a significant negative power correlation with channel width. Advantages of the described methodology include automated high spatial and temporal monitoring resolution, semi-automated data post-processing, and the potential to be generalized to large scale river/reservoir bank failure monitoring.

Laboratory assessment of the influence of rice straw mulch size on soil loss

Abstract. Application of crop residues (mulch) over the soil surface is a common practice to control soil erosion and promote infiltration. This laboratory study aimed at investigating the effect of different rice straw mulch sizes on runoff and sediment transport. The experimental runs were conducted using a soil flume of adjustable slope and a rainfall simulator, considering bare soil and three different soil covers: 1, 2 and 5 t ha−1 application rates, for three sizes of rice straw mulch (10, 30 and 200 mm). The experimental results showed that for the same mulch application rate (by weight), the smaller mulch sizes (i.e. high surface coverage percentage) presented less soil loss. For example, 90 % soil loss reduction was achieved for smaller sizes of rice straw mulch and 80 % for the bigger size. The results of this study are an important contribution to the understanding of the soil loss process in small basins and to the definition of relevant soil conservation measures, at the plot/field scale.

Effect of rainfall moving direction on surface flow and soil erosion processes on slopes with sealing

Although rainfall characteristics play important roles in runoff generation and soil erosion, our understanding about how rainfall moving direction impacts surface flow and soil erosion on slopes considering spatial process of soil surface sealing is still poor. In this study, a series of laboratory experiments was conducted using a soil flume and a movable rainfall simulator to investigate this impact. Two rainfall moving directions (i.e. upslope-moving and downslope-moving) and two soil surface moisture conditions (i.e. dry runs with no antecedent rainfall and wet runs with antecedent rainfall) were considered in the experiments with different slope gradients and rainfall intensities. The results showed that the upslope-moving rainfalls generated more surface flow from 25% to 700% and less soil erosion than the downslope-moving rainfalls in the dry runs due to the different spatial developing processes of soil surface sealing. The difference between the runoff coefficients of the two moving direction rainfalls decreased remarkably (more than 50%) with slope gradient increasing from 15° to 25°. In the wet runs, surface flow was similar for the upslope-moving and downslope-moving rainfalls, as all of the soil surface had been sealed before the experiments. Soil erosion in the downslope-moving rainfalls was larger than that in the upslope-moving rainfalls due to higher sediment transport capability of the surface flow. For both rainfall moving directions with same rainfall conditions, the total soil erosion in a dry run was mostly larger than that in a wet run because of less erodible soil particles on the sealed slope in the wet run. The result of this study implied that different rainfall moving directions can lead to distinct spatial processes of soil surface sealing and caused large differences in surface flow and soil erosion processes on slopes.

Laboratory testing of a new thermal tracer for infrared-based PTV technique for shallow overland flows

The accurate measurement of velocities of shallow overland flows normally found in the field of hydrology is essential to understand the kinematics of these flows. In view of this, we have tested the capability of a new thermal particle tracer (cold oil droplets) under controlled laboratory conditions to measure the velocity of shallow overland flows (<2 mm depth) using the infrared-based particle tracking velocimetry (PTV) technique. Experimental apparatus comprised a soil flume 3 m long and 0.3 m wide with a feeder tank attached to the upstream end. Flow velocities acquired through infrared-based PTV were compared with corresponding values given by the volumetric discharge method, dye tracing, and conventional PTV (using polystyrene beads as tracer) for eleven flow conditions that combined different slopes (5, 7, 10 and 15%) and discharges (0.15 to 0.51 L s−1 m−1). The Reynolds number (569 to 1938) marked it as quasi-laminar flow. The results showed that acquired flow velocity given by through infrared-based PTV and conventional PTV techniques was closer to the mean velocity estimated by the volumetric discharge method than it was to dye tracing estimated velocity. The mean correction factor value (0.78) estimated by the infrared-based PTV technique was higher than the dye tracing estimated correction factor value (0.48). Furthermore, the detection of the leading edge of the dye tracer was difficult and the stagnation of polystyrene beads at lower discharges resulted in an underestimation of mean velocity. This research stresses the potential of using thermal imaging to estimate the velocity of shallow water bodies, because of the tracer's conspicuous visibility and independence from illuminating conditions.

Effectiveness of the application of rice straw mulching strips in reducing runoff and soil loss: Laboratory soil flume experiments under simulated rainfall

The use of mulch as a management tool has shown one of the highest effectiveness/cost ratios for improving agricultural soil fertility, crop productivity, soil restoration in badlands and post-fire soil erosion mitigation. Some researchers have suggested that mulching costs can be reduced by applying it in strips rather than over the entire area. However, the implications of strip-wise mulching on the effectiveness to reduce soil erosion are poorly known. This study aimed to evaluate, in laboratory experiments, the effectiveness of strip-wise mulching with rice straw in reducing runoff and soil loss for a highly erodible sandy loam soil at a steep slope of 40%. Six mulching application schemes were compared against a bare soil. The six schemes combined two surface cover rates of 50 and 70% and three spatial patterns: mulch over the entire flume length and two strips of 1/3 and 2/3 of the flume length, both located at the bottom part of the flume. The runoff-erosion experiments involved the simulation of a sequence of three rainfall events, the latter one combining the application of concentrated flow from upslope of the soil flume. Overall, mulching was more effective in reducing soil loss than runoff (50 vs. 25%) and was significantly more effective during the first rainfall event than during the following two events (83 v. 16% for runoff and 92 vs. 53% for soil loss). During the third event, mulching effectiveness dropped significantly with increasing rates of upslope concentrated flow. Overall, mulching was more effective when applied over the entire flume length than over the 1/3 and 2/3 flume’s length strips, both in terms of runoff (24 vs. 21 and 13% at 50% soil cover and 41 vs. 33 and 16% at 70% soil cover) and of soil loss (44 vs. 50 and 33% at 50% soil cover and 71 vs. 60 and 39% at 70% soil cover). Even so, these differences were not significant. Therefore, strip-wise mulching can be an effective approach to substantially reduce costs or to maximize the area that can be treated. Its main disadvantage may be that it does not avoid runoff generation and associated transport process in the slope areas where no mulch is applied.

Spatial glyphosate and AMPA redistribution on the soil surface driven by sediment transport processes - A flume experiment.

This study investigates the influence of small-scale sediment transport on glyphosate and AMPA redistribution on the soil surface and on their off-site transport during water erosion events. Both a smooth surface (T1) and a surface with "seeding lines on the contour" (T2) were tested in a rainfall simulation experiment using soil flumes (1 × 0.5 m) with a 5% slope. A dose of 178 mg m-2 of a glyphosate-based formulation (CLINIC®) was applied on the upper 0.2 m of the flumes. Four 15-min rainfall events (RE) with 30-min interval in between and a total rainfall intensity of 30 mm h-1 were applied. Runoff samples were collected after each RE in a collector at the flume outlet. At the end of the four REs, soil and sediment samples were collected in the application area and in four 20 cm-segments downslope of the application area. Samples were collected according to the following visually distinguished soil surface groups: light sedimentation (LS), dark sedimentation (DS), background and aggregates. Results showed that runoff, suspended sediment and associated glyphosate and AMPA off-site transport were significantly lower in T2 than in T1. Glyphosate and AMPA off-site deposition was higher for T2 than for T1, and their contents on the soil surface decreased with increasing distance from the application area for all soil surface groups and in both treatments. The LS and DS groups presented the highest glyphosate and AMPA contents, but the background group contributed the most to the downslope off-site deposition. Glyphosate and AMPA off-target particle-bound transport was 9.4% (T1) and 17.8% (T2) of the applied amount, while water-dissolved transport was 2.8% (T1) and 0.5% (T2). Particle size and organic matter influenced the mobility of glyphosate and AMPA to off-target areas. These results indicate that the pollution risk of terrestrial and aquatic environments through runoff and deposition can be considerable.