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

• A mathematical model considering the operation of 24 mega reservoirs was proposed. • The sedimentation mainly occurs in the mainstream during the flood season. • The cascade reservoirs operation leads to easier and prolonged sedimentation. The intensive construction of large dams in the upper Yangtze River Basin (UYRB) has formed large-scale cascade reservoirs and obviously altered the flow regime and sediment transport of the basin. However, few studies have quantitatively described the cumulative impact of these reservoirs on sediment interception. In this study, a flow and sediment transport model that fully considers the operation processes of 24 mega reservoirs in the UYRB was established, and the spatial and temporal variations in sedimentation from 2011 to 2080 were elucidated, assuming repeated series of daily input flow discharge and sediment concentration during 2011–2020 and monthly water operation levels of 24 reservoirs during 2018–2021 as boundary conditions. The results showed an annual average sedimentation of more than 300 Mt in the UYRB during 2011–2060, which falls to 270 Mt in the period 2061–2080, and more than 80% of the sedimentation occurs in the flood season. Nearly 80% of the total sedimentation during 2010–2080 was observed in the mainstream reservoirs, especially the four mega reservoirs in the lower Jinsha River and the Three Gorges Reservoir (TGR), which accounted for 31.9% and 22.7% of the total sedimentation, respectively. The proportion of sedimentation in mainstream reservoirs will further increase with increasing operation periods. Most reservoirs in the UYRB will not reach sedimentation equilibrium by 2080, except for reservoirs in the Min River Basin. The ratio of siltation capacity to average annual sediment yield determines the percentage of residual siltation capacity of each reservoir. The operation of the cascade reservoirs in 2013–2020 had no more immediate impact on the middle and lower Yangtze River than that in 2003–2012, when the TGR started impoundment. However, the joint operation of these cascade reservoirs has changed the spatial distribution of sedimentation in the UYRB, and will lead to prolonged impacts on downstream channel erosion. [ABSTRACT FROM AUTHOR]

[Display omitted] • The SWAT model supports better management of erosion processes. • Computational models can be used with methods of visual recording erosion. • Simple statistical models aligned to SWAT is adequate to analyze erosive processes. • The spatially distributed erosion identifies the susceptible areas to water erosion. Water-driven erosion associated with the transport and deposition of sediments in watersheds may lead to contamination problems causing significant damage to land and infrastructure. The key factors that influence erosive processes are well known and include the precipitation rate, terrain topography, soil type, and vegetation cover. The soil and water assessment tool (SWAT) considers these factors and may be used to identify areas susceptible to soil erosion and to estimate soil loss and sediment transport and deposition. To corroborate this assumption, real data on rill erosion and sediment transport from the Mogi Guaçu River Basin in southeastern Brazil were used to validate SWAT estimates. The study shows that the model is able to identify soil-eroded areas and replicate the scale and variation patterns of sediment production, although not with great precision. Hence, it can successfully be used for prioritizing areas that require action and for evaluating alternative soil management and conservation practices to mitigate erosive processes. [ABSTRACT FROM AUTHOR]

Eroded sediment is an important feature of permanent gully's development; however, obtaining the specific characteristics of the sediment transport in the gully can be difficult. Thus, a composite fingerprinting technique, incorporating uncertainty analysis, has been employed to investigate the mean relative contribution of sediment sources in permanent gullies. In this study, 31 tracers were measured at 62 different sampling sites from three layers of a gully wall [surface layer (SL), sandy soil layer (SSL), and semi-weathering rock layer (SWL)] and 36 sediment samples from the slumping deposit and alluvial fan of the gullies. The sediment source tracing procedure was therefore used to assess the relative contribution of each source. The mean relative contribution from the SL to the sediment of the slumping deposit is higher in the active gully (G1), and is relatively lower in the potentially stable (G2) and stable gullies (G3); thus there was no obvious gully bank retreat in G2 and G3. In different places of the G1 alluvial fan, the mean relative contribution of the SL, SSL and SWL ranged from 9–11, 36–43 and 46–55%, respectively. However, the sediment contribution of the SL increased in G2 and G3 due to the changes in the erosion type. The mean relative contributions of the SL were 33 ± 5 (G2) and 30 ± 3% (G3). These findings have important implications for the establishment of a scientific basis for permanent gully sediment transport management and control policies. [ABSTRACT FROM AUTHOR]

Abstract: In order to assess the extent of sediment connectivity between uplands and lowlands and to quantify the processes of in-channel deposition and remobilization, measurements of suspended sediment fluxes were conducted in a nested rural catchment of the Mexican Volcanic Belt. Data were collected over one year at three upland sites (3 to 12km2) and two downstream stations (390–630km2). Our results show that a structural discontinuity in the catchment (i.e. abrupt slope decrease at the junction between piedmonts and the alluvial plain from 2 to 10% to<0.1%) could be compensated by functional continuity during floods. Direct conveyance of fine sediment to the outlet occurred when a high stream transport capacity was reached. Erosion of the streambed was observed on various occasions and accounted for up to 50% of the flux leaving the catchment during one event. Conversely, temporary in-channel storage was apparent on other occasions, amounting to up to 52% of the flux recorded upstream during one storm. These two distinct behaviours were approximately equally distributed along the rainy season and strongly driven by the extent of coupling between surface and subsurface water. This work indeed highlights the role of baseflow spatial variations in determining the extent of lowland sediment conveyance. Riverbed erosional processes occurred when large differences in pre-event baseflow values (i.e. at least a twofold longitudinal increase) were observed between the 5-km distant lowland stations. Our findings outline the importance of systematically taking into consideration the baseflow parameter in research focusing on fine sediment transport across scales. [Copyright &y& Elsevier]

Abstract: Distributed erosion models are potentially good tools for locating soil sediment sources and guiding efficient Soil and Water Conservation (SWC) planning, but the uncertainty of model predictions may be high. In this study, the distribution of erosion within a catchment was predicted with a semi-empirical erosion model that combined a semi-distributed hydrological model with the Morgan, Morgan and Finney (MMF) empirical erosion model. The model was tested in a small catchment of the West Usambara Mountains (Kwalei catchment, Tanzania). Soil detachability rates measured in splash cups (0.48–1.16gJ−1) were close to model simulations (0.30–0.35gJ−1). Net erosion rates measured in Gerlach troughs (0.01–1.05kgm−2 per event) were used to calibrate the sediment transport capacity of overland flow. Uncertainties of model simulations due to parameterisation of overland flow sediment transport capacity were assessed with the Generalized Likelihood Uncertainty Estimation (GLUE) methodology. The quality of the spatial predictions was assessed by comparing the simulated erosion pattern with the field-observed erosion pattern, measuring the agreement with the weighted Kappa coefficient of the contingency table. Behavioural parameter sets (weighted Kappa >0.50) were those with short reinfiltration length (<1.5m) and ratio of overland flow power α to local topography power γ close to 0.5. In the dynamic Hortonian hydrologic regime and the dissected terrain of Kwalei catchment, topography controlled the distribution of erosion more than overland flow. Simulated erosion rates varied from −4 to +2kgm−2 per season. The model simulated correctly around 75% of erosion pattern. The uncertainty of model predictions due to sediment transport capacity was high; around 10% of the fields were attributed to either slight or severe erosion. The difficult characterisation of catchment-scale effective sediment transport capacity parameters poses a major limit to distributed erosion modelling predicting capabilities. [Copyright &y& Elsevier]

• Spatio-temporal variations of effective sediment sizes were investigated on field plots. • Sediment size was more susceptible to erosional degradation under low intensity rainfall. • Erosional degradation impact exhibited a unimodal variation with soil structure stability. • Rainfall intensity, pH and bulk density contributed jointly to sediment size (p < 0.001). • This study facilitated an understanding of sediment transport mechanisms on eroded lands. Land degradation by erosion alters soil properties to a varying extent and definitely influences the subsequent soil erosion. However, little is known about the impact of erosion-induced land degradation on the erosion process from the aspect of sediment size characteristics, which may not only indicate aggregate breakdown and sediment transport mechanism, but also influence sediment deposition. Here, the effective sediment size distribution (undispersed) and its dynamics during sheet erosion on different eroded lands (non, moderately and very-severely eroded) were studied under field rainfall simulation on five types of soils (Calcic Luvisols, Ferric Luvisols, Plinthic Alisols, Plinthic Acrisols and Acric Ferralsols) formed separately on loess deposits, quaternary red clays and basalt from temperate to tropical climate. The rainfalls at the intensities of 45 and 90 mm h−1 were simulated on pre-wetted bare fallow plots (3 × 0.8 m2, 10° slope). The effect of erosion degree on the effective sediment size showed a unimodal trend from temperate Luvisols to tropic Ferralsols, coupled with the influence of rainfall intensity to a varying extent. Under the low-intensity rainfall, except for Ferralsol, the moderate erosion degree increased the sediment size by −49% to 72% relative to non-eroded lands, in contrast to a decrease of 6–64% for the very-severe erosion degree. The high-intensity rainfall was shown to lessen the erosion degree effect for Luvisols, magnify it for Alisol, and change it to positive for Acrisol. Sediment transport was dominated by suspension-saltation of <0.10 mm size fraction for Luvisols regardless of erosion degree due to the weak soil structure, and shifted from both rolling (0.5–2 mm) and suspension-saltation on the non/moderately eroded lands to only suspension-saltation on the very-severely eroded lands for the other soil types. Rainfall intensity, pH and bulk density contributed jointly to sediment size (Adj-R2 = 0.76, p < 0.001), and exchangeable sodium showed a positive effect on its temporal variation (Adj-R2 = 0.36, p < 0.001). The varying effects of land degradation by erosion on sediment size and transport mechanism at the soil pedon and regional scales merit consideration in land management and rehabilitation as well as erosion modeling. [ABSTRACT FROM AUTHOR]