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Abstract: Efficient intervention to control soil erosion in rural tropical landscapes requires accurate models for predicting the spatial location and intensity of degradation. The Universal Soil Loss Equation (USLE) has commonly been applied for spatial erosion risk assessment in the tropics, but has rarely been validated using ground observations of soil degradation. As with any empirical model, application in new regions requires calibration before results are used for decision support. We evaluated USLE effectiveness for predicting erosion in a small watershed in western Kenya based on 420 georeferenced ground observations of ordinal erosion class (three categories) systematically collected from throughout the basin. Relativized model factors were parameterized using standard remote assessment methods based on interpolated spatial data layers. Inference of degradation status at cultivated sites was estimated by calibration to near infrared diffuse reflectance spectra obtained from sampled soils; diagnostic models based on spectra produced validation accuracies of 78% for three categories. Association between USLE predicted risk and observed erosion, estimated using mixed effects logistic regression to control for within-site variability, correctly classified only 38% of sites into three degradation classes and model sensitivity for delineating regions of severe degradation was only 28%. Graphical modeling was used to identify those USLE risk factors that were conditionally associated with observed degradation, and an ordinal logistic regression model, employing only these factors was developed. This alternative model, which allowed statistical flexibility in estimating effect direction and strength, correctly predicted ordinal degradation class at 54% of field sites, with 55% sensitivity for the severe degradation class. This result suggests a critical need for efficient ground-based sampling schemes to be used in conjunction with flexible statistical models based on USLE factors for future investments in erosion risk assessment in the tropics. [Copyright &y& Elsevier]

The understanding of soil erosion processes and the development of accurate erosion prediction models require understanding of detachment, deposition, and sediment transport in rills. The objectives of this study were to determine whether sediment transport capacity is a unique value for given soil, flow rate, and slope, and to determine if equilibrium sediment concentration in the rill obtained by detachment was different from that observed under depositional conditions. Experiments on a Carmi loam (fine, mixed, mesic Typic Hapludalf) simulated rill erosion under net detachment and net deposition conditions. Two discharge rates of 6 and 9 l min−1 and two sediment input regimes of 0 and excess of transport capacity were tested on soil beds with lengths of 2, 4, 6, 7, and 8 m at 7% slope. Sediment load reached steady state conditions within the 8-m distance on the rill. At 9 l min−1 discharge, 8 m length, and excess sediment added to the flow, sediment delivery was 71 g l−1 versus 31 g l−1 for the corresponding case with no sediment added. Overall, for the conditions tested, rill flow transported two times more sediment than it could detach. The flow did not reach its maximum potential transported load through detachment of soil due in part to changes in the sediment size distribution under deposition and possibly to the protective action of bedload particles moving along the rill bottom and/or changes in flow turbulence associated with sediment laden flow. [Copyright &y& Elsevier]

• 173 papers related to ephemeral gully erosion in Web of Science were reviewed. • Critical morphological features of ephemeral gully channel were different worldwide. • Topographic, climate, land management, soil, and hydraulics factors were assessed. • Fifteen methodological approaches employed to quantify gully erosion were evaluated. • Complexity of gully channel development and driving force mechanisms were discussed. Although concern for ephemeral gully (EG) erosion has been growing within the research community, knowledge gaps still need to be identified for a state-of-the-art review. This review is based on an extensive search of published literature in the Web of Science database that specifically addressed EG erosion. This review included 173 papers published in 56 scientific journals over the last 30 years. The results indicated that the main EG erosion topics were related to gully erosion processes, factors, methodology, models, and the detrimental effects of EGs. The definition of EG was identical worldwide. There is an urgency to define critical morphological values under different conditions for identifying EG channel locations in fields in order to map and model their development quickly and accurately. Topographic factors have been much more studied than other contributing factors, including climate, land management, soil, and hydraulics. Fifteen different methodological approaches or technologies, which were categorized into four kinds of methods, including field monitoring, modelling, simulation experiments, and review, have been employed to assess and quantify EG erosion. The average soil erosion rate of EGs in 36 publications increased and decreased with increase of slope and catchment areas, respectively, and the number of EGs linearly increased with catchment area. The complexity of the development of EG channels and the driving force mechanisms should also be studied intentionally, e.g., the quantification of soil detachment and sediment transport mechanisms for both surface and subsurface flow during different sub-processes, including gully head advance, sidewall collapse and bed incision. It was concluded that the interactive effects of different contributing factors on EG erosion across various spatio-temporal scales should be further researched. The comprehensive utilization or application of new methods and technologies, e.g., automatic monitoring technologies, artificial intelligence, machine learning algorithms, would be helpful to improve study efficiency and accuracy. The scientific basis of prediction models still needs further development and refinement. This review provides recommendations and suggestions to improve knowledge, prediction, and control of EG erosion. [ABSTRACT FROM AUTHOR]

• Susceptibility of surface to detachment always limits raindrop driven erosion. • Sediment transport associated with drop impact is always a limiting factor. • Raindrop driven erosion is both detachment and transport limited at the same time. Soil erosion involves two liked processes, detachment of particles held within the soil surface by cohesive forces and inter-particle friction, and the transport of the detached material away from the site of detachment. It is commonly thought that erosion is either detachment limited or transport limited but this is not true for surfaces where raindrop driven sediment transport occurs. When detachment occurs by raindrop impact, transport away from the site of detachment can occur in the air and in rain-impacted flow. Splash transports both fine and coarse particles aerially. In rain impacted flows, fine material travels in completed suspension and at velocities that are at, or near the velocity of the flow. Coarse material may travel by raindrop induced saltation and rolling, or if flow conditions are appropriate, by flow driven saltation and rolling. Transport by splash and by raindrop induced saltation and rolling ceases when raindrop impact ceases even if previously detached material has not been discharged. Consequently, sediment transport limits erosion whenever raindrop impact is involved in transporting detached soil material over the soil surface. However, the amount of material available for transport is controlled by the ability of the impacting raindrops to cause detachment and the ability of the cohesive soil surface to resist detachment. Consequently, the discharged of coarse material by rain-impacted flows is both detachment limited and transport limited at the same time. Raindrop driven erosion is a complex process and the spatial and temporal variations in the transport mechanisms need to be considered when experiments involving raindrop driven erosion experiments are designed and the results analysed. [ABSTRACT FROM AUTHOR]

• Ephemeral Gully (EG) Erosion is a major erosion concern and needs to be addressed. • Very few models simulate EG erosion. • A thorough review of processes behind EG simulation in existing models is discussed. • Future needs and directions to improve EG simulation processes in models are enumerated. Modeling of ephemeral gully (EG) erosion has lagged that of other soil erosion processes despite its major contribution to watershed sediment losses. Several process and semi-empirical based simulation models have been used to assess the occurrence and location of EGs, magnitude of soil losses from EGs, and degradation, aggradation, and transport of sediment through EGs, but no comprehensive EG model exists. This paper reviews these models and presents a thorough discussion of their background, general formulations, key equations, field assessments, assumptions and limitations. Most current EG models evolved from the original formulations used in CREAMS, with incremental improvements by EGEM, WEPP, AnnAGNPS-REGEM, RUSLER-EphGEE models. These models provide process-based estimation of EG processes and field-scale soil-erosion contributions, but all models, except EphGEE, require users to have a priori knowledge of EG locations and all have significant shortcomings. Several topographic index models are discussed that provide simplistic approaches to locate EGs on the landscape using only topographic features. Process-based threshold index models may provide a more robust simulation of EG location and length, though testing has been limited. EG modeling appears to still be in its infancy, with great opportunities for future research, as discussed herein, to improve the understanding and simulation of EG erosion and transport processes. [ABSTRACT FROM AUTHOR]

• 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]

A novel experimental protocol was used to characterize interrill erosion processes in both detachment-limiting and transport-limiting regimes. Small soil columns under tiny rocks marked with red crosses in the photo indicated that raindrop-impacted sheet flow seemed capable of detaching soil in the detachment-limiting regime probably because of the turbulence caused by raindrop impact. Raindrop-driven and flow-driven sediment transport were quantified for the first time. Stream power and a multiplicative empirical equation could be used to predict interrill sediment delivery for all erosion regimes precluding the need to develop different sets of predictive equations for each regime. • Raindrop-impacted sheet flow seems capable of detaching soil in interrill erosion. • A novel protocol was proposed to partition raindrop-driven and flow-driven transport. • Stream power and an empirical Eq. were best predictors for interrill sediment discharge. • Both equations can predict sediment discharge for all interrill erosion regimes. Detachment and transport processes of interrill soil erosion are not yet fully understood and quantified due to their complexity. Understanding these processes and their interaction is essential to developing effective interrill erosion prediction models. Thus, the objectives are to (1) identify detachment-limiting and transport-limiting regimes by comparing measured total soil splash with total sediment load of flow wash, (2) estimate relative importance of raindrop-driven and flow-driven transports under selected flow conditions using a novel experimental protocol, and (3) test widely used erosion equations for use in both detachment-limiting and transport-limiting regimes under the experimental conditions never before used. Sediment discharge and raindrop splash were separately collected from two plots with a 2.5-cm gap between them. Three rainfall intensities of 60, 90, and 120 mm h−1 were applied to three slopes of 9, 18, and 27% under two cover treatments of screen and tarp. Screen cover is to simulate crop canopy for dissipating raindrop energy, which is to control upslope sediment inflow by reducing raindrop detachment without modifying runoff rates. Tarp cover is to control both water and sediment inflows by varying slope length. Results showed that total raindrop splash in the entire plot during an event was less than total flow wash in the screen treatment, indicating detachment-limiting scenarios. However, total raindrop splash was greater (less) than total wash at the 9% (27%) slope in the tarp treatment, indicating transport-limiting (detachment-limiting) cases. The results indicated that raindrop-impacted flow might be able to detach soil in some cases. This finding runs counter to the common wisdom that soil detachment in interrill erosion is solely caused by raindrop impact. If raindrop-impacted flow can detach soil, the determination of limiting process by direct comparison between splash and wash loads seemed overly simplistic. Raindrop-driven transport was estimated for the first time by a means of dissipating raindrop impact energy in this study. Raindrop-driven transport was found to increase with rainfall intensity and slope gradients. Stream power and an empirical multiplication-of–factor equation performed well in predicting interrill sediment delivery under all erosion regimes, suggesting that it is unnecessary to develop distinct predictive equations for different interrill erosion regimes. [ABSTRACT FROM AUTHOR]

• First validation of a daily large-scale sediment model using remote-sensing data. • Remotely-sensed reflectance can help sediment dynamics modeling in ungauged basins. • Better spatiotemporal sediment patterns using surrogate observations. Sediment models are important for estimating soil erosion, sediment transport, and deposition at different scales and for disparate scenarios; however, the availability of in-situ measurements may not be sufficient for the appropriate evaluation of model performance. For this reason, this study investigated how surrogate data (water quality [WQ] and remote sensing) could be used as a proxy for suspended sediment concentration (SSC) in order to evaluate the performance of a large-scale erosion and sediment transport model. To achieve this, the daily SSC was simulated, using the Large Scale Sediment Model (MGB-SED), a semi-distributed and conceptual model in which the Modified Universal Soil Loss Equation (MUSLE) is used to estimate sediment yield and an advection-diffusion equation is used to route suspended loads along rivers. Four experiments were performed that compared simulated SSC against: (1) in-situ SSC; (2) turbidity; (3) total suspended solids (TSS); and (4) surface spectral reflectance (SSR). Correlation analysis showed that r -values above 0.5 were found in 62%, 91%, 89% and 83% of the SSC, turbidity, TSS and SSR stations, respectively. Spatially, the model was evaluated in smaller drainage areas, using WQ data. Temporally, the number of SSR observations was greater than the number of in-situ measurements, allowing a better analysis of the temporal dynamics of the simulated SSC. Also, the SSR data improved the spatial coverage and could be employed for even large-scale, ungauged basins. These results bring a new perspective to the application of sediment models in poorly or ungauged basins, since surrogate data can improve the spatial and temporal suspended sediment coverage when compared to traditional SSC measurements, which is a common issue in large-scale sediment modeling. [ABSTRACT FROM AUTHOR]

• Prescribed fire promoted herbaceous growth in bare interspaces. • Litter enhanced infiltration and provided protection against erosion. • Increased vegetation cover altered flow paths and reduced runoff and soil loss. • Reduced soil surface connectivity post-fire improved hydrologic function. • Hydrologic function continues to improve 9 yr after fire. Woody species encroachment into herbaceous and shrub-dominated vegetations is a concern in many rangeland ecosystems of the world. Arrival of woody species into affected rangelands leads to changes in the spatial structure of vegetation and alterations of biophysical processes. In the western USA, encroachment of pinyon (Pinus spp.) and juniper (Juniperus spp.) tree species into sagebrush steppes poses a threat to the proper ecohydrological functioning of these ecosystems. Prescribed fire has been proposed and used as one rangeland improvement practice to restore sagebrush steppe from pinyon-juniper encroachment. Short-term effects of burning on the ecohydrologic response of these systems have been well documented and often include a period of increased hydrologic and erosion vulnerability immediately after burning. Long-term ecohydrologic response of sagebrush steppe ecosystems to fire is poorly understood due to lack of cross-scale studies on treated sites. The aim of this study is to evaluate long-term vegetation, hydrologic, and erosion responses at two pinyon-juniper-encroached sagebrush sites 9 years after prescribed fire was applied as a restoration treatment. Thirty-six rainfall simulation experiments on 6 m × 2 m plots were conducted for 45 min under two conditions: a dry run (70 mm h−1; dry antecedent soils) and a wet run (111 mm h−1; wet antecedent soils). Runoff and erosion responses were compared between burned and unburned plots. Overall, increases in herbaceous cover in the shrub-interspace areas (intercanopy area between trees) at both sites 9 years post-burn resulted in runoff- and erosion-reduction benefits, especially under the wet runs. While the initially more degraded site characterized by 80% bare ground pre-burn, registered a higher overall increase (40% increase) in canopy cover, greater post-fire reductions in runoff and erosion were observed at the less degraded site (57% bare ground pre-burn). Runoff and erosion for the wet runs decreased respectively by 6.5-fold and 76-fold at the latter site on the burned plots relative to control plots, whereas these decreases were more muted at the more degraded site (2.5 and 3-fold respectively). Significant fragmentation of flow paths observed at the more-degraded site 9 years post-fire, suggests a decreased hydrologic connectivity as a mechanism of runoff and erosion reduction during post-fire recovery. [ABSTRACT FROM AUTHOR]