Net Erosion Rates Research Articles

Assessment of soil erosion rates in a Mediterranean cultivated and uncultivated soils using fallout 137Cs

Fallout radionuclides, such as 137Cs, have been recognized as a valuable means for studying soil erosion processes. In this study, the 137Cs technique was used to assess soil erosion magnitude and to investigate the effectiveness of terrace cultivations in the High Atlas of Morocco, particularly, the Ourika watershed. 137Cs depth distribution profiles were established along slope gradients associated with land use considering cereal crop, arboriculture and native forests.Along the slope gradient, depth distribution profiles highlight dissimilarities reflecting differences in land use, topographic roughness, soil particle distribution and stoniness. 137Cs inventory decreases exponentially with depth and its penetration increases along the slope. It becomes higher at the bottom of the slope, with penetration reaching 30 cm.137Cs mass activity and inventory significantly decrease in cultivated terraces, compared to native forest. Although the pattern of 137Cs gain/loss varied according to topography, soil properties and vegetation cover, showing eroding and aggrading profiles, most samples had 137Cs values lower than the reference value, suggesting net-loss of soil as a consequence of erosion processes.The net erosion rates were estimated about 8.5 and 6.0 t ha−1 yr−1 in cereal crop and arboriculture agrosystems, respectively, whereas in the forest, the net erosion rate was lower at about 4.2 t ha−1 yr−1.Soil loss on agricultural terraces is not significantly above tolerable erosion rates for Morocco (<7 t ha−1 yr−1), particularly for arboriculture terraces. A strong correlation was observed between 137Cs and both soil organic carbon and slope gradient, in uncultivated area, whereas, there was no correlation for cultivated terraces.Terraced arboriculture systems should be encouraged for better soil preservation against water erosion in the Ourika watershed.

Assessment of shoreline alterations using a Digital Shoreline Analysis System: a case study of changes in the Yeşilırmak Delta in northern Turkey from 1953 to 2017.

The aim of our study was to determine the erosion rates and shoreline changes in the Yeşilırmak Delta (Delta) between the oldest available date (1953) before construction of the large-scale dams on the Yeşilırmak River in northern Turkey and 2017. To clearly illustrate the impacts of the dams on the Delta, we investigated the relationship between the dams and shoreline changes. To compute the net shoreline changes and erosion rates over these 64years, we used a Digital Shoreline Analysis System. To digitize the shoreline changes at the mouth of the Yeşilırmak River, 1:25000-scaled topographic maps from 1957, panchromatic aerial photographs from 1962, a KH-9 Hexagon satellite image from 1980, an Ikonos satellite image from 2002, and a WorldView-2 satellite image from 2017 were used. The west and east sides of the river mouth were divided into six main groups and assessed. Each group included a time series for the shorelines. The groupings were based on the time series. For each group, the net shoreline change (m) was calculated using the net shoreline movement (NSM), and erosion rate (m/year) was calculated using the linear regression rate (LRR). According to NSM computations, net shoreline change varied from - 235.04 to 379.18m. The greatest evolutions in net shoreline change were observed for group I, with a minimum of - 1101.11m. According to LRR analysis, the calculated erosion rate varied from - 6.83 to 19.06m/year. The greatest changes in erosion rate were observed for group II, with a minimum of - 30.63m/year. There is a close relationship between the construction dates of the dams in the Yeşilırmak Basin and the shoreline changes in the Delta.

Using 137Cs and 210Pbex measurements to explore the effectiveness of soil conservation measures in semi-arid lands: a case study in the Kouhin region of Iran

One of the most fragile agro-ecosystems in Iran is represented by dry farming lands on steep hillslopes that occupy ca. 54% of the national agricultural lands. Therefore, in order to reduce loss of fertile soil from these lands, it is important to apply effective soil conservation strategies. This study used cesium-137 (137Cs) and excess lead-210 (210Pbex) measurements to assess the effectiveness of soil conservation practices in controlling soil erosion in Kouhin, Qazvin Province of Iran. Soil samples were collected from two adjacent hillslopes with and without conservation agriculture practices. The managed site benefited from conservation practices, including controlled grazing, terraces, and contour farming for 50 years. The unmanaged site was under cultivation without agricultural conservation practices. At the managed site, both 137Cs and 210Pbex activities were measured. Only 137Cs activity was measured for the unmanaged site. Conservation practices performed at the managed site were effective in minimizing soil erosion, i.e., net soil erosion rates provided by 137Cs measurements were only 4.6 Mg ha−1 year−1 at the managed site, compared to 12 Mg ha−1 year−1 for the unmanaged site. By using the 210Pbex approach, net soil loss and sediment delivery ratio in the managed site were 12.3 Mg ha−1 year−1 and 60%, respectively. Soil redistribution magnitudes established from 137Cs inventories are associated with a period extending from 1963 to the time of sampling, while those obtained from 210Pbex measurements are related to the past 100 years. However, 210Pbex measurements are expected to be more sensitive to erosive events that occurred during the last 15 to 20 years and this could reflect the higher estimate of erosion rate provided by this radionuclide for the managed site. The increasing trend seems to suggest a possibility that the higher soil losses estimated by the 210Pbex measurements are a result of increasing climate variability during the last two decades. However, further investigation would be needed to confirm this hypothesis.

Electron Beam-Induced Carbon Erosion and the Impact on Electron Probe Microanalysis.

Electron beam-induced carbon contamination is a balance between simultaneous deposition and erosion processes. Net erosion rates for a 25 nA 3 kV beam can reduce a 5 nm C coating by 20% in 60 s. Measurements were made on C-coated Bi substrates, with coating thicknesses of 5-20 nm, over a range of analysis conditions. Erosion showed a step-like increase with increasing electron flux density. Both the erosion rate and its rate of change increase with decreasing accelerating voltage. As the flux density decreases the rate of change increases more rapidly with decreasing voltage. Time-dependent intensity (TDI) measurements can be used to correct for errors, in both coating and substrate quantifications, resulting from carbon erosion. Uncorrected analyses showed increasing errors in coating thickness with decreasing accelerating voltage. Although the erosion rate was found to be independent of coating thickness this produces an increasing absolute error with decreasing starting thickness, ranging from 1.5% for a 20 nm C coating on Bi at 15 kV to 14% for a 5 nm coating at 3 kV. Errors in Bi Mα measurement are &lt;1% at 5 kV or above but increase rapidly below this, both with decreasing voltage and increasing coating thickness to 20% for a 20 nm coated sample at 3 kV.

Cenozoic tectonostratigraphy and pre-glacial erosion: A mass-balance study of the northwestern Barents Sea margin, Norwegian Arctic

The evolution of the northwestern Barents Sea continental margin, part of a NW-SE trending mega shear zone, has been reconstructed in order to quantify the sedimentation and erosion affecting this area during and after its formation in the Paleogene–Neogene. This development was closely related to the sea-floor opening of the Norwegian-Greenland Sea. Our study incorporated 2D seismic data, well data, and information from shallow cores.During the Paleocene–Eocene, the northwesternmost Barents Sea margin was subjected to compression-transpression that led to the development of the West Spitsbergen Fold-Thrust Belt (WSFTB) and largely affected the northern part of the study area. To the south, the Vestbakken volcanic province developed in a pull-apart setting. A transition zone separates these two areas marked by a basin morphology becoming more pronounced to the south suggesting increasing subsidence and extension. Subsequently, during the Oligocene, extension and sea-floor spreading were initiated along the whole margin, resulting in the opening of the Fram Strait between Spitsbergen and NE Greenland in the Miocene.During the Paleocene, the Stappen High and a part of the NE Greenland shelf sourced sediments into the newly developing basins. The southwestern part of the WSFTB, the Stappen High, and part of the NE Greenland margin are interpreted as the main sediment source areas in the Eocene. During the Oligocene and Neogene, a larger part of the northwestern Barents Sea shelf is interpreted to have acted as source area including the Edgeøya platform. As a result of this development, the wider Barents Sea shelf itself is inferred to have been a lowland prior to the northern hemisphere glaciations.We found that the average sedimentation rate for the Paleogene–Neogene at the northwestern Barents Sea margin is about 0.034 m/k.y. This number is in agreement with the sedimentation rate reported from present-day fluvial systems and modern rates coastal erosion. By using a mass-balance approach, we have also estimated the average net erosion and erosion rate for the Paleogene–Neogene period to be ∼2440 m and 0.038 m/k.y, respectively. This erosion rate is two times higher compared to the southwestern Barents Sea margin, probably reflecting erosion of a more tectonically active northwestern margin. Thus, for the western Barents Sea margin, a general increasing trend of pre-glacial erosion northwards can be inferred. This study also suggests that more than half of the Cenozoic erosion affecting the studied part of the northwestern Barents Sea were of pre-glacial origin.

Sediment flux, wind erosion and net erosion influenced by soil bed length, wind velocity and aggregate size distribution

Knowledge of wind erosion as a dynamic phenomenon changing in space is of importance in measuring and modeling soil erosion. While, many erosion studies have been done on the effect of bed length on wind erosion rate, little attention has been paid to the spatial changes of net erosion rate in different soil bed lengths. The aim of this study is to examine the effect of soil bed length, surface aggregate size distribution and wind velocity on sediment flux and wind/net erosion rates. For this purpose, three soil samples were placed in seven bed lengths ranging from 1 to 7 m and subjected to different wind velocities of 6, 10 and 14 m s−1 using a wind tunnel. The result showed that total sediment flux increased at longer soil bed lengths, whereas the reverse trend was found for wind erosion and net erosion rates, which was attributed to the gradual saturation of air flow with eroded particles and the formation of armor layer. Depends on soil bed length, the measured wind erosion rate varied from 154.8 to 3783.7 g m−2 min−1. Toward the end sections of the bed length, the reduction in net erosion (34.1% to 79.1%) was higher than the reduction in wind erosion rate (18.9% to 50.0%), implying that for every length section, the measured wind erosion rates were higher than the obtained net erosion rates. Soil bed length was found to be an appropriate single variable to predict wind/net erosion rates using a power function. Furthermore, wind erosion rate (Q) was modeled as a function of bed length (L), the mean weight diameter of aggregates (MWD) and wind velocity (V), that was Q = 6 × L−0.225 × MWD−0.579 × V1.975, R2 = 0.945⁎⁎. It was also found that the soil containing coarser aggregates exhibited less sediment flux and wind/net erosion rates. The findings of this research revealed that the measurement of wind erosion on different bed lengths under laboratory conditions will result in effective discrepancies in the erosion rates, worldwide; however environmental conditions can be different in areal case. Therefore, it is necessary to pay special attention to the scale effect of soil bed length in future soil erosion studies.

Erosion and deposition processes in surface granular flows.

We report on experiments aiming at characterizing erosion and deposition processes on a tilted granular bed. We investigate the existence of the neutral angle, that is, the critical angle at which erosion exactly balances accretion after the passage of a granular avalanche of a finite mass. Experiments show in particular that the neutral angle depends on both avalanche mass and shape but is rather insensitive to the bed length. This result strongly suggests that the effective friction between the static and mobile granular phases cannot be taken as an intrinsic property that is only material dependent but should be considered a flow-dependent property. Interestingly, for a given avalanche mass, the net erosion rate increases linearly with the angular deviation from the neutral angle. We also compare our data with the predictions of the erosion-deposition model introduced by Bouchaud, Cates, Ravi Prakash, and Edwards (BCRE) [J. Phys. I 4, 1283 (1994)JPGCE81155-430410.1051/jp1:1994195]. We show that the predictions drawn from the modified version of the BCRE model proposed by Boutreux and de Gennes, in which the local erosion rate between the static and mobile phases is independent of the flow thickness, are in remarkable agreement with the experimental results.

Integrating compound-specific δ13C isotopes and fallout radionuclides to retrace land use type-specific net erosion rates in a small tropical catchment exposed to intense land use change

Retracing net erosion rates linked to land use change in tropical agricultural catchments dominated by small-holder farmers is challenging, due largely to catchment heterogeneity and uncontrolled farming practices. To tackle this problem and to complement a preceding study (Brandt et al., 2016), we introduce here an advanced approach that integrates compound-specific δ13C isotopes (CSSI) and fallout radionuclides (FRN), Excess lead-210 (210Pbex) and Cesium-137 (137Cs) to estimate past net erosion rates of dominant land use types in the mountainous catchment Chieng Khoi (207ha, Northwest Vietnam). Spatially-integrated topsoil (0 to 2cm) samples of dominant land use types (e.g., protected and secondary forests, teak, fruit orchards, maize, cassava) were collected from at least three discrete plots of each land use type within the upland area (i.e., erosion sites) of the studied catchment. In the corresponding lowland area, a representative sediment profile was localized and divided into sections of 2 to 4cm for CSSI and FRN analysis. Samples for FRN reference data were taken from undisturbed areas in close proximity. At a soil deposition site near a lakeshore, 210Pbex data determined the age and sediment accumulation rates of 19 sediment layers to a depth of 38cm. Based on 210Pbex activity, maximum sediment accumulation rates of 127tha−1y−1 were calculated, corresponding to erosion rates of about 16tha−1y−1 for the total catchment area. CSSI data confirmed that maize and cassava were the most important erosion sources during a period of dramatic land use change (1987 to 2004), when forests were cleared and high-yielding maize hybrids were introduced. Based on integrated FRN and CSSI data, net erosion rates of maize and cassava reached maximum rates of 4.8tha−1y−1 (maize), 6.2tha−1y−1 (cassava). This major finding verified the potential of integrating FRN and CSSI to accurately estimate land use type-specific net erosion rates. In conclusion, determining past sediment budgets for specific land use types provides insight into the accelerating impact of specific land use change on soil retrogression and degradation. Such knowledge is of prime importance for effective soil conservation through evidence-based land management and decision making.

Diachronic soil surveys: A method for quantifying long-term diffuse erosion?

Given the high inter-annual variability in soil erosion, representative measurements are best carried out over time scales of a few decades. In this study, a novel approach was tested that relies on diachronic, high-resolution soil mapping. For this test, a cropland-dominated watershed covering 116ha was selected in central Belgium. An initial soil survey based on augering had been carried out in 1956, with a density of 1.8augerpoints/ha. A second survey was carried out in 2015, with a density of 4.5points/ha. The results were interpreted in terms of changes in the depth of transition between successive horizons. The results indicate that a great proportion of the soils classified as non-eroded in 1956 were strongly eroded in 2015. Strongly eroded soils represented 52% of the watershed area in 2015, whereas they had represented only 8% in 1956. Some colluvial areas have been subjected to erosion and then covered again by colluvium over the 60years. Only the flatter areas near the watershed boundary appear to still be in the ‘non-eroded soils or soil subject to weak erosion’ category. A mean net erosion rate of 37t/ha per year is computed for the watershed using this method. The diachronic comparison of the pedological maps over the 60-year time interval within a context of intensive farming is a potential source of information for the calibration of landscape evolution spatial modelling. This method joins the few ones allowing a quantitative spatialization of erosion and deposition phenomena at the catchment scale.

Spatial variation in soil, SOC, and total N redistribution on affected and non-affected slope terraces due to the 8.0 Wenchuan Earthquake in 2008 by using 137Cs technique

Using the 137Cs technique, we assessed spatial variation in soil erosion and its impact on lateral movements of soil organic carbon (SOC) and total nitrogen (total N) in a terraced series and a complex slope in the Longmenshan Fault zone, China. The complex slope derived from three terraced fields with damaged banks, resulting from the 8.0 Wenchuan Earthquakes in 2008. The terraced series and complex slope are the dominant sloping farmland patterns in this area, which represent for earthquake non-affected and affected sloping farmland, respectively. Soil loss occurred over the upper parts of slopes and deposition occurred towards the downslope boundary of each terrace. Also, soil loss occurred on upper terraces and soil accumulation on lower terraces. Net soil erosion rates for the terraced toposequence were estimated at −3.70±14.38Mgha−1yr−1, indicating the terraced toposequence played an important role in soil conservation. For the complex slope, higher soil erosion rates occurred at the summit and other slope positions with greater slope curvature, and soil accumulation occurred at the toe slope and other slope positions with lower slope curvature (15m, 25m, and 40m). In addition to water erosion, collapses resulting from earthquake and tillage erosion were also important soil erosion processes for the complex slope. Moreover, Net soil erosion rate on the complex slope (4.89±18.03tha−1yr−1) was significantly higher than on the terraced series. SOC and total N inventories showed similar patterns to the 137Cs inventory. Such results confirm that 137Cs can help to trace SOC and total N dynamics on sloping farmland in this area.

Advances in understanding of high-Z material erosion and re-deposition in low-Z wall environment in DIII-D

Dedicated DIII-D experiments coupled with modeling reveal that the net erosion rate of high-Z materials, i.e. Mo and W, is strongly affected by carbon concentration in the plasma and the magnetic pre-sheath properties. Different methods such as electrical biasing and local gas injection have been investigated to control high-Z material erosion. The net erosion rate of high-Z materials is significantly reduced due to the high local re-deposition ratio. The ERO modeling shows that the local re-deposition ratio is mainly controlled by the electric field and plasma density within the magnetic pre-sheath. The net erosion can be significantly suppressed by reducing the sheath potential drop. A high carbon impurity concentration in the background plasma is also found to reduce the net erosion rate of high-Z materials. Both DIII-D experiments and modeling show that local 13CH4 injection can create a carbon coating on the metal surface. The profile of 13C deposition provides quantitative information on radial transport due to E × B drift and the cross-field diffusion. The deuterium gas injection upstream of the W sample can reduce W net erosion rate by plasma perturbation. In H-mode plasmas, the measured inter-ELM W erosion rates at different radial locations are well reproduced by ERO modeling taking into account charge-state-resolved carbon ion flux in the background plasma calculated using the OEDGE code.

Investigations of the first-wall erosion of DEMO with the CELLSOR code

Fusion reactor systems codes (SCs) are 0.5d codes used for optimization studies towards the design of a tokamak demonstration power plant (DEMO). These codes usually comprise a description of the core plasma physics, technology aspects and reactor economy, while only a coarse description of plasma-wall interaction (PWI) aspects is included. Therefore, the new systems code extension CELLSOR (Code to Estimate the Lifetime Limited by Sputtering Of a Reactor wall) was developed in order to allow inclusion of PWI effects into reactor optimization studies. CELLSOR is foreseen to be used as a secondary tool for PWI evaluations, taking design point parameters from the European PROCESS systems code as input. CELLSOR consists of an analytical treatment of the plasma in the scrape-off layer (SOL) for fuel ions (D, T), solving the 1.5d continuity equation in fluid approximation to obtain perpendicular flux and ion density in the SOL, and a fast Monte-Carlo description of the neutral particle (D, T) behavior. The Monte Carlo (MC) implementation of the new code extension was successfully benchmarked with results from the EIRENE code. The trajectories of eroded neutral W were computed within CELLSOR ERO, an add-on code used for calculations of prompt re-deposition and self-sputtering. The damage by ions was calculated analytically for fuel (D, T), ash (He), seeding gas (N) and wall material (W), including the acceleration by a sheath in front of the wall, assuming radially constant impurity concentrations.Six exemplary DEMO SOL test-cases were defined, covering the range of predicted diffusive and convective transport strength within the SOL, and of upstream separatrix density levels sufficient for detached diverter conditions. The damage by ions (D, T, He, N, W) and by hot neutrals (D, T), being born by charge-exchange within the hot pedestal region (ΔT=−300eV/cm), was compared for the six cases. The results indicated, that in the investigated low density and weak transport cases, the recharged neutrals caused the major threat to the FW due to bad SOL screening ability, i.e. deeper plasma penetration for neutrals. In the other cases, N ions were the dominant driver for erosion due to their high concentration levels and strong sheath acceleration.Furthermore, the net erosion, incl. prompt re-deposition and self-sputtering, was investigated for variations of the wall clearance ΔSOL, which was identified as the main control parameter to keep the FW erosion in the main chamber below acceptable levels. Within the intermediate reference case, i.e. medium transport (D⊥=0.5m2s−1, v⊥=5ms−1), the net erosion rate was acceptable, if the wall clearance was at least ΔSOL ≥ 14 cm, whereas a clearance of ΔSOL ≈ 25cm was sufficient within all 6 investigated test-cases.

Dynamic Coupling of Boltzmann Plasma Model to Surface Erosion Model for Kinetic Treatment of Plasma-Material Interactions

We present an innovative coupled Boltzmann–binary collision approximation (BCA) method for the simulation of the near-wall plasma in the presence of a material-releasing wall. The method is based on a full-f multispecies Boltzmann solver for the plasma (charged and neutral species) coupled to a modification of the classical BCA code TRIDYN. Both the plasma ions and the impurities are treated as Boltzmann kinetic species, allowing high resolution even at very disparate densities, particle fluxes, drift velocities, and energy fluxes. From the distribution functions, all the fluid moments (density, heat flux, etc.) and the net and gross erosion rates are derived. An example of calculation of a helium plasma facing a beryllium wall is reported, showing the evolution of the phase-spaces of ions, neutrals, and material impurities in the near-wall region at nominal ITER conditions.

View factor modeling of sputter-deposition on micron-scale-architectured surfaces exposed to plasma

The sputter-deposition on surfaces exposed to plasma plays an important role in the erosion behavior and overall performance of a wide range of plasma devices. Plasma models in the low density, low energy plasma regime typically neglect micron-scale surface feature effects on the net sputter yield and erosion rate. The model discussed in this paper captures such surface architecture effects via a computationally efficient view factor model. The model compares well with experimental measurements of argon ion sputter yield from a nickel surface with a triangle wave geometry with peak heights in the hundreds of microns range. Further analysis with the model shows that increasing the surface pitch angle beyond about 45° can lead to significant decreases in the normalized net sputter yield for all simulated ion incident energies (i.e., 75, 100, 200, and 400 eV) for both smooth and roughened surfaces. At higher incident energies, smooth triangular surfaces exhibit a nonmonotonic trend in the normalized net sputter yield with surface pitch angle with a maximum yield above unity over a range of intermediate angles. The resulting increased erosion rate occurs because increased sputter yield due to the local ion incidence angle outweighs increased deposition due to the sputterant angular distribution. The model also compares well with experimentally observed radial expansion of protuberances (measuring tens of microns) in a nano-rod field exposed to an argon beam. The model captures the coalescence of sputterants at the protuberance sites and accurately illustrates the structure's expansion due to deposition from surrounding sputtering surfaces; these capabilities will be used for future studies into more complex surface architectures.

Magnetic sheath effect on the gross and net erosion rates due to impurities

Simulations of impurity trajectories in deuterium plasmas in the vicinity of the surface are performed by taking into account the magnetic sheath in conditions relevant for ITER and WEST. We show that the magnetic sheath has a strong effect on the average impact angle of impurities in divertor conditions and that it can lead to an increase of at the gross erosion maximum for neon (Ne+4) compared to the case when only the cyclotron motion is considered. The evaluation of the net erosion has been undertaken by retaining local redeposition of tungsten (W). We investigate how it is affected by the sheath magnetic potential profile. The largest effect is however observed when an energy distribution is considered. In this case the number of particles that manage to exit the sheath is larger as it is dominated by the more energetic particles. The comparison with other work is also discussed. The application to a scenario of the WEST project is finally performed, which exhibits a moderate, however non negligible, erosion of the plasma facing components.

Gross and net erosion of tungsten in the outer strike-point region of ASDEX Upgrade

We have investigated net and gross erosion of W in the outer strike-point (OSP) region of ASDEX Upgrade with the help of marker probes during low-density/high-temperature L-mode discharges. Post mortem analyses indicate net-erosion rates of 0.04–0.13 nm s−1, with the highest rates measured close to the OSP. Re-deposition was some 30–40% of gross erosion, which is lower than what has earlier been obtained spectroscopically (∼50–60%), possibly due to the special plasma conditions of our experiment and intense flux of W atoms originating from the main chamber. Gross erosion was also estimated by passive emission spectroscopy, and around the OSP the results matched with post mortem data. However, the spectroscopic erosion profile in the poloidal direction was much steeper than the post mortem one. Preliminary ERO simulations have predicted net erosion of the same order of magnitude as experimental results but reproducing the poloidal erosion/re-deposition profiles requires further work.

Simulation of gross and net erosion of high-Z materials in the DIII-D divertor

The three-dimensional Monte Carlo code ERO has been used to simulate dedicated DIII-D experiments in which Mo and W samples with different sizes were exposed to controlled and well-diagnosed divertor plasma conditions to measure the gross and net erosion rates. Experimentally, the net erosion rate is significantly reduced due to the high local redeposition probability of eroded high-Z materials, which according to the modelling is mainly controlled by the electric field and plasma density within the Chodura sheath. Similar redeposition ratios were obtained from ERO modelling with three different sheath models for small angles between the magnetic field and the material surface, mainly because of their similar mean ionization lengths. The modelled redeposition ratios are close to the measured value. Decreasing the potential drop across the sheath can suppress both gross and net erosion because sputtering yield is decreased due to lower incident energy while the redeposition ratio is not reduced owing to the higher electron density in the Chodura sheath. Taking into account material mixing in the ERO surface model, the net erosion rate of high-Z materials is shown to be strongly dependent on the carbon impurity concentration in the background plasma; higher carbon concentration can suppress net erosion. The principal experimental results such as net erosion rate and profile and redeposition ratio are well reproduced by the ERO simulations.

Quantifying sheet erosion in agricultural highlands of Sri Lanka by tracking grain-size distributions

Abstract The reduction of productivity in arable lands of the tropical highlands through intensified soil erosion is a major reason for food scarcity in many developing countries. Identifying soil erosion hot spots in highland agricultural croplands and monitoring the effectiveness of soil conservation are crucial for improving land management practices. This paper develops a low cost and efficient method to quantify the rate of soil loss caused by splash and sheet erosion, in-situ at the plot scale. This method assumes that textural differences between the topsoil and the underlying soil within the plot are due to erosion of the top layer. Grain size analyses conducted in agricultural soils of the Upper Uma Oya Catchment (UUOC) of Sri Lanka provided data to quantify splash or sheet erosion and deposition. Net soil loss from the plot was calculated based on rates of area weighted soil loss and deposition. The net splash and sheet erosion rate in the plot was 20 t ha−1 yr−1, which is comparable with previous soil erosion results obtained in the area (25 t ha−1 yr−1) at similar temporal and spatial scale and under the same agricultural use. This study suggests that approximately half of the total erosion in the UUOC area results from splash and sheet erosion, whereas the rest is driven by concentrated flow through rill and gully erosion. This method is potentially transferable to other croplands as a robust approach for rapid measurement of splash and sheet erosion.

A 13‐year record of erosion on badland sites in the Karoo, South Africa

AbstractLand degradation in South Africa has been of concern for more than 100 years with both climate change and inappropriate land management (overgrazing) being proposed as primary drivers. However, there are few quantitative studies of degradation and, in particular, few of erosion by water. Badlands, taken here to be the landform which results from extreme erosion, have been notably neglected. We report on 13 consecutive years of erosion pin measurements of badland erosion on 10 study sites in the Sneeuberg uplands of the eastern Karoo in South Africa. The study sites are on Holocene colluvium which mantles footslopes. They have been subject to overgrazing for at least 100 years, c. 1850–1950. Currently they are lightly grazed by sheep. The area receives about 500 mm rainfall per year. The sites are remote, with only informal, farmer‐operated, daily raingauges nearby. The nearest sub‐daily raingauge is c. 55 km distant. Also we report on an analysis of the erosion pin data which focuses on establishing the origins and context of the badlands, including the relationship between study sites and adjacent valley‐bottom gully systems; compare erosion rates on our study sites with rates determined by erosion pins on other badland sites; and discuss the implications of these erosion rates for landscape development and off‐site impacts. Net erosion rates on the study sites are relatively high compared with global badland rates and range from 3.1 to 8.5 mm yr‐1 which may be extrapolated to 53 to 145 t ha yr‐1 (using a measured bulk density of 1.7 g cm‐3). However, comparisons with badland sites elsewhere are difficult because of different measuring methodologies, lithologies, climate and dominant processes. Erosion rates on the study sites are strongly influenced by rainfall amounts and, in particular, by daily rainfall events which exceed ~10 mm: this is the threshold intensity at which runoff has been observed to commence on badlands. Of significance, but of lesser influence, is weathering, mainly by wetting and drying: this prepares bare surfaces for erosion. However, questions remain regarding the role of site characteristics, and of processes at each site, in determining between‐site differences in erosion rate. Crude extrapolation of current rates of erosion, in conjunction with depths of incision into the badlands, suggests that badland development started around 200 years ago, probably as a response to the introduction of European‐style stock farming which resulted in overgrazing. We assume, but cannot quantify, the additional influence of periods of drought and burning in the erosional history of the area. Intermittent connection of these badlands to valley‐bottom gullies and therefore to small farm dams and ultimately to large water storage reservoirs increases their impact on local water resources. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Validating the Use of 137Cs Measurements to Derive the Slope Component of the Sediment Budget of a Small Rangeland Catchment in Southern Italy

AbstractThe sediment budget is a key concept and tool for characterizing the mobilization, transfer and storage of fine sediment within a catchment. Caesium‐137 measurements can provide valuable information on gross and net erosion rates associated with sheet and rill erosion that can be used to establish the slope component of a catchment sediment budget. However, there is a need to validate the use of 137Cs measurements for this purpose, because their reliability has sometimes been questioned. The study reported focuses on a small (3·04 ha) steepland (mean slope 37%) catchment in Southern Italy. It exploits the availability of information on the medium‐term sediment output from the catchment provided by the construction of a reservoir at its outlet in 1978 and the existence of estimates of soil redistribution rates derived from 137Cs measurements made on 68 replicate soil cores collected from the slopes of a substantial proportion of the catchment in 2001, to validate the use of 137Cs measurements to construct the slope component of the catchment sediment budget. An additional 50 replicate soil cores were collected from the catchment slopes for 137Cs analysis, to complement the data already available. Nine cores collected from the area occupied by the reservoir were used to estimate the mean annual sediment input to the reservoir. In the absence of evidence that the poorly developed channel system in the catchment was either a significant sediment source or sink, it was possible to directly compare the estimate of net soil loss from the catchment slopes (7·33 Mg ha−1 y−1) with the estimate of sediment output from the catchment provided by the reservoir deposits (7·52 Mg ha−1 y−1). Taking account of the uncertainties involved, the close agreement of the two values is seen as providing a convincing validation of the use of 137Cs measurements to both estimate soil redistribution rates and as a basis for constructing the slope component of the sediment budget of a small catchment. Copyright © 2015 John Wiley &amp; Sons, Ltd.