Assessing soil erosion and farmers' decision of reducing erosion for sustainable soil and water conservation in Burji woreda, southern Ethiopia.

Inadequate conservation practice affects the sustainable production of agricultural watersheds due to erosion and fertility decline. Understanding soil erosion and implementing site-specific conservation practice could enhance agriculture-based rural development. The study was aimed to document soil erosion problem and soil and water conservation effort. The specific objectives of this study were to assess soil erosion severity, practices to reduce erosion, and determinants of the decision to reduce erosion. Data were collected by interviewing 198 farm household heads, undertaking four focus group discussions, and assessing rill erosion in 10 farm fields in Morayo and Wacho sub-watersheds of southern Ethiopia. Descriptive statistics and binary logit model were applied to analyze the data. Results indicated that many of the farm households, 63% in Morayo and 83% in the Wacho sub-watershed, perceived moderate to severe soil erosion, which is characterized by big rills and small gullies on the farmlands. Rill densities of 231.4 m ha-1 and 84.1 m ha-1 in the Morayo and Wacho sub-watersheds were observed, respectively. The estimated annual soil loss due to rills was 61.2 and 23.4 Mg ha-1 in the Morayo and Wacho sub-watersheds, respectively. The soil erosion from rills alone exceeds the expected tolerable soil erosion (11 tons ha-1 year-1). Due to erosion, about 90% of farmers perceived farmland degradation as described by a progressive decline in crop yield. Farmers used to practice traditional techniques to reduce erosion and government introduced conservation measures such as soil and stone bunds. However, many farmers did not use well-promoted conservation measures such as bunds, which could have negative impact on long-term erosion control effort and sustainable implementation of the conservation options. Among the assessed explanatory variables, educational level, farm distance from home, slope of the cultivated land, and frequency of extension contact were significantly affected (p < 0.05) farmers' sustainable use of conservation measures. Development planners and policy makers are advised to consider site-specific and innovative approaches to implement conservation measures in sustainable approach in the smallholder crop-livestock mixed agriculture system.

First evidence of widespread, severe soil erosion underneath centre-pivot irrigation systems.

Centre-pivot systems are widely used for irrigation in agriculture. However, excessive water application rates under low pressure centre-pivot systems can lead to soil erosion, which degrades soil structure and increases crop vulnerability to droughts. Although efforts have been deployed to measure soil erosion underneath individual centre pivots, a large-scale systematic assessment of extent and severity of soil erosion in centre-pivot irrigated fields is currently lacking. Here we used Google Earth™ satellite images to provide first evidence of widespread, severe soil erosion in centre-pivot irrigated agricultural land. We focused on the municipality of Cristalina (6154 km2), in the Brazilian Central Highlands, where centre pivots irrigate approximately 60,000 ha of cropland. The study area is in the Cerrado biome, which is one of the most important grain-producing regions in the world and Brazil's main centre-pivot irrigation area. By mapping erosion features under centre pivots, we found that 29 % of centre-pivot fields displayed signs of rill erosion, with individual rills up to a length of 1200 m. Most erosion features were identified during the dry season of the Brazilian Cerrado, which coincided with the period of greater satellite-image availability. Moreover, we found that compacted centre-pivot-wheel tracks often triggered rill incision and that eroding centre-pivot fields displayed higher slope gradients and were better connected to surface waters than the non-eroding fields. Ultimately, the frequent identification of severe erosion features in the centre-pivot fields during the dry season indicates that irrigation causes and/or aggravates soil erosion in Cristalina and likely in other parts of the Brazilian Cerrado. This first systematic evidence of widespread soil erosion underneath centre-pivot systems highlights that irrigation erosion is an important but neglected driver of land degradation, and that urgent action is required to protect affected soils for future generations.

Factors contributing to rill erosion of forest roads in a mountainous watershed.

Forest roads are a major source of and transport pathway for eroded sediments in mountainous watersheds. When rills develop on these roads' surfaces, they amplify sediment erosion. Best management practices can decrease sediment erosion, but in order to efficiently implement these practices it is necessary to determine which factors have the most influence on rill development on forest roads. Despite this need, there is scarce literature on rill development on forest roads. To fill this gap in knowledge, based on field survey and multivariate statistical methods including redundancy analysis (RDA) and variation partitioning analysis (VPA), we investigated unpaved forest roads in the Xiangchagou watershed in China and quantified the extent to which various factors influenced rill formation. Specifically, we studied how rill erosion intensity (REI) and rill morphological characteristics (like rill length, mean width and depth, density, and severity of fragmentation) varied along the slope of a forest road. We also introduced the concept of a road's hydrological constituents (its upslope catchment, surface, and cutslopes), and determined how much each constituent contributed to REI. We found that REI and morphological characteristics decreased moving from the upper portion of road segment downward, implying that rills developed more intensely uphill. Additionally, REI increased exponentially with rill width, density, and severity of fragmentation, and increase linearly with length and depth. Conversely, REI decreased exponentially with rill width-depth ratio. These relationships suggest that the morphological characteristics of rills could be used to predict the REI of a given road segment. Finally, we found that the road characteristics that best predicted rill formation included catchment area, cutslope area, and gravel bareness. Correspondingly, the upslope catchment, cutslopes, and road surface contributed 11.56%, 30.83%, and 8.23% of the variation in REI and morphological characteristics. The interaction between upslope catchment and road surface explained 19.89% of the variation. These results suggest that when best management practices are implemented to decrease erosion caused by forest roads in mountainous watersheds, they should integrate these hydrological constituents of a road.

Characterizing the rill erosion process from eroded morphology and sediment connectivity on purple soil slope with upslope earthen dike terraces.

Rills are critical venues for the transport of eroded sediments along hillslopes. The sediment transport efficiency and connectivity within hillslopes are affected by the spatiotemporal evolution of rill erosion and morphology. However, the effect of upslope sediment-laden inflow on rill erosion and connectivity remains unclear. This study investigated the variation in rill erosion from the eroded morphology and sediment connectivity using flume scouring experiments. Upslope sediment-laden inflow was simulated considering the upslope terrace areas of 0.15, 0.30, and 0.45 m2 and an upslope inflow of 6 L min-1. The quantity and cross-sectional depth of rills gradually decreased with increasing upslope terrace area. The cross-sectional morphology of rills changed from being V-shaped to U-shaped in the rill erosion process. All of the mean values of the morphological parameters gradually decreased with increasing upslope terrace area, in contrast to the width-depth ratio (Rw/d) and rill density (ρ), which both initially increased and then decreased. The average length, width, and depth of rills were smaller under an upslope terrace area of 0.45 m2 than those under an upslope terrace area of 0.15 m2; they decreased by 2.78 %, 20.67 %, and 33.68 %, respectively. Soil and water loss induced by rill erosion decreased with increasing upslope terrace area. Rills, as major venues for sediment transport on hillslopes, exhibited a higher sediment connectivity (IC) than that observed in interrill areas under the different upslope terrace areas. Rill development resulted in higher erosion between the upslope and downslope parts within rill channels. The variations in Rw/d and ρ were significantly correlated with runoff and eroded sediment yield, which could be used to estimate the rill erosion process under different upslope terrace areas.

Vegetation restoration restricts rill development on dump slopes in coalfields.

Severe rill erosion on dump slopes poses a great threat to the ecological environment in mining areas. Vegetation restoration is an effective measure for controlling soil erosion on dump slopes. However, few studies have identified the long-term influence of vegetation restoration on rill development on dump slopes. Therefore, we investigated the rill development characteristics of dump slopes with three typical restoration models (CK: natural restoration; ED: Elymus dahuricus; and AO: Artemisia ordosica) and three recovery time (1 y, 3 y and 5 y). The results showed that vegetation adequately controlled rill erosion on dump slopes. ED and AO could effectively control the development of rills with widths >15 cm and depths of 10-20 cm. ED vegetation restoration inhibited the development rill morphology and network better than AO. The rill erosion modulus of the ED slope and AO slope decreased by 76.29%-90.77% and 46.66%-61.49%, respectively, compared with that of natural restoration slopes with recovery time of 1 y, 3 y, and 5 y. ED controlled rill erosion better than AO, but this effect gradually weakened with recovery time. Vegetation coverage contributed 34.99% of the total variation in rill morphology and was the main factor affecting the development of rills on dump slopes. Furthermore, vegetation coverage had a more important role in controlling rill development than did the root system on dump slopes. This study provides valuable information for optimizing vegetation construction for soil loss control on dump slopes.

[Impacts of seepage flow and soil thaw depth on hillslope snowmelt erosion in Chinese Mollisol region]. / 壤中流和土壤解冻深度对黑土坡面融雪侵蚀的影响.

Snowmelt erosion is an important way of soil loss in Chinese Mollisol region. However, little is known about the effects of seepage flow and soil thaw depth on hillslope snowmelt runoff erosion. An indoor simulated experiment was conducted to analyze the impacts of seepage flow and soil thaw depth on hillslope snowmelt erosion. There were two snowmelt flow rates (1 and 4 L·min-1), two soil thaw depths (5 and 10 cm), and two near-surface hydrological conditions (with and without seepage flow). The results showed that hillslope runoff depth and soil erosion amount in the treatment with seepage flow were 1.1 to 1.2 times and 1.3 to 1.9 times of those in the treatment without seepage flow, respectively. Under two snowmelt flow rates, when soil thaw depth increased from 5 cm to 10 cm, hillslope runoff depth and soil erosion amount increased by 10.0% to 13.5% and 15.4% to 37.1% in the treatment without seepage flow, respectively. In the treatment with seepage flow, when soil thaw depth shifted from 5 cm to 10 cm, hillslope runoff depth increased by 6.5% to 8.5%, and soil erosion amount remained stable. Moreover, hillslope rill development was comprehensively influenced by seepage flow, soil thaw depth, and snowmelt flow rate, with rill erosion amount occupying more than 72% of hillslope snowmelt erosion amount. Compared with the treatment without seepage flow, flow velocity and shear stress under the treatment with seepage flow increased by 20.3% to 23.2% and 37.0% to 51.3%, respectively; but Darcy-Weisbach friction coefficient reduced by 9.0% to 21.4%, which caused an increase of hillslope snowmelt erosion. In addition, seepage flow enhanced rill development, which caused rill erosion amount to increase by 43.6% to 69.9% compared with the treatment without seepage flow, and it further resulted in the increase of hillslope snowmelt erosion amount. The main reason for soil thaw depth enhancing hillslope snowmelt erosion amount under the treatment without seepage flow was that both sloping runoff erosivity and erodible materials increased with increasing soil thaw depth. Furthermore, soil thaw depth had a significant impact on hillslope rill morphology development under the treatment with seepage flow. Rill widening process was dominated when soil thaw depth was 5 cm, whereas rill incision process was dominant when soil thaw depth was 10 cm. This study could improve the understanding of hillslope snowmelt erosion mechanism in Chinese Mollisol region and provide theoretical guidance for the development of water erosion model.

Quantifying the contributions of soil surface microtopography and sediment concentration to rill erosion.

Soil surface microtopography plays a significant role in rill erosion. In addition, upslope inflow has a large effect on downslope soil erosion processes. Experiments including four different upslope filling areas (0 m2, 0.15 m2, 0.30 m2, and 0.45 m2) with a upslope inflow rate (6 L min-1) were conducted in two 1 m × 2 m boxes on a 15° slope to examine the effects of microtopography and sediment concentration on rill erosion processes. The upslope filling areas were used to simulate different areas of earthen dike terraces. The results showed that the minimum values of soil surface elevation increased from -120 mm to -110 mm as the upslope filling area increased. The values of the simple fractal dimension (f(α)max), the singular index span (Δα) and the difference of multifractal spectrum (Δf(α)) reached minimum values in the 0.45 m2 upslope filling area. With the development of rill erosion, the soil surface microtopography tended to sharpen, and the relative elevation changed greatly. The runoff and soil loss associated with rill erosion gradually decreased as the upslope filling area increased. We identified the temporal evolution of rill erosion using Morlet wavelet analysis. The main period of temporal fluctuation of rill erosion was 28 min under different upslope filling areas. Multi-scale periods of temporal fluctuation of rill erosion emerged with the increase in upslope filling area. The Δα significantly affected the runoff and soil loss. The proportional contributions of Δα to the runoff and sediment yield were 80.95% and 77.34%, respectively. While the contributions of sediment concentration to runoff and sediment yield were 17.05% and 20.66%, respectively. The findings are important significance for better understanding rill erosion mechanisms of purple soil.

Empirical Bayes small area prediction under a zero-inflated lognormal model with correlated random area effects.

Many variables of interest in agricultural or economical surveys have skewed distributions and can equal zero. Our data are measures of sheet and rill erosion called Revised Universal Soil Loss Equation - 2 (RUSLE2). Small area estimates of mean RUSLE2 erosion are of interest. We use a zero-inflated lognormal mixed effects model for small area estimation. The model combines a unit-level lognormal model for the positive RUSLE2 responses with a unit-level logistic mixed effects model for the binary indicator that the response is nonzero. In the Conservation Effects Assessment Project (CEAP) data, counties with a higher probability of nonzero responses also tend to have a higher mean among the positive RUSLE2 values. We capture this property of the data through an assumption that the pair of random effects for a county are correlated. We develop empirical Bayes (EB) small area predictors and a bootstrap estimator of the mean squared error (MSE). In simulations, the proposed predictor is superior to simpler alternatives. We then apply the method to construct EB predictors of mean RUSLE2 erosion for South Dakota counties. To obtain auxiliary variables for the population of cropland in South Dakota, we integrate a satellite-derived land cover map with a geographic database of soil properties. We provide an R Shiny application called viscover (available at https://lyux.shinyapps.io/viscover/) to visualize the overlay operations required to construct the covariates. On the basis of bootstrap estimates of the mean square error, we conclude that the EB predictors of mean RUSLE2 erosion are superior to direct estimators.

Micro-topographic assessment of rill morphology highlights the shortcomings of current protective measures in loess landscapes.

Due to urban expansion and the rapid development of infrastructure in the loess area of China, artificial earth-fill embankments and excavated slopes are increasingly widespread in recent years. Erosion is typical in such loess slopes; however, quantitative statistical analyses of various counter-measures that affects rill erosion are still lacking. Here, we quantified rill morphology and rill erosion development in two newly constructed slopes with different engineering protection measures. We used high-resolution digital surface models (DSMs) acquired using an Unmanned Aerial Vehicle (UAV) to analyze the case areas during two-time periods. Our results from centimeter accuracy differential DSMs demonstrated that rapid rill erosion is prevalent in the study area, expressed as rill density varying between 2.03 km-2 and 8.81 km-2 at different slope surfaces (viz., erosion protected slopes [EPS], landslide protected slopes [LPS], and unprotected slopes [US]). The slope gradient responsible for rill erosion of the EPS, LPS and US are obviously different, and such information is essential for planning preventive measures in each slope type. At the EPS, the severity of erosion is maximum at the top of the ridges, whereas the gap between reinforced concrete lattice and loess deposits are of serious concern at the LPS. The current engineering measures employed in the study area are thus found ineffective for protection against rill erosion. We therefore propose an improved design by implementing an intercepting drain to the existing design for preventing further erosion.

Modelling the impacts of climate and land use changes on soil water erosion: Model applications, limitations and future challenges.

The world is experiencing serious soil losses. Soil erosion has become an important environmental problem in certain regions and is strongly affected by climate and land use changes. By selecting and reviewing 13 extensively used soil water erosion models (SWEMs) from the published literature, we summarize the current model-based knowledge on how climate factors (e.g., rainfall, freeze-thaw cycles, rainstorms, temperature and atmospheric CO2 concentrations) and land use change impact soil erosion worldwide. This study also provides a critical review of the application of these 13 SWEMs. By comparing model structures, features, prediction accuracies, and erosion processes, we recommend the most suitable SWEMs for different regions of the globe (Asia, Europe, Africa and the America) based on the evaluations of 13 SWEMs. Future soil erosion could be simulated using the RUSLE, LISEM, WEPP v2010.1, SWAT, EPIC, KINEROS and AGNPS models in Asia; the RUSLE, WEPP v2010.1, SWAT, EPIC, WATEM-SEDEM, MEFIDIS, AGNPS and AnnAGNPS models in Europe; the RUSLE, LISEM, SWAT, and AGNPS models in Africa; and the WEPP v2010.1, SWAT, EPIC, KINEROS, AGNPS and AnnAGNPS models in America. Finally, the limitations and challenges of the 13 SWEMs are highlighted.

Understanding erosion processes using rare earth element tracers in a preformed interrill-rill system.

Tracking sediment source and movement is essential to fully understanding soil erosion processes. The objectives of this study were to identify dominant erosion process and to characterize the effects of upslope interrill erosion on downslope interrill and rill erosion in a preformed interrill-rill system. A coarse textured soil with 2% clay and 20% silt was packed into a physical model of a scaled small watershed, which was divided into eight topographic units and was tagged with eight rare earth element (REE) oxides. Three 30-min rains were made at the sequential intensities of 60, 90, and 120mmh-1, and runoff and sediment were collected every 2min at the outlet. REE concentration in sediment was measured and used to estimate source contributions after fine-enrichment correction. Results showed that interrill erosion rate and sediment concentration increased with downslope distance, indicating that sediment transport might have controlled interrill erosion rates. In contrast, rill erosion rate was limited by rill detachment and development process. Rill erosion contributed most soil loss; however, the proportion decreased from 78 to 61% as rainfall intensity increased and rill network matured over three rains. Interrill erosion was more sensitive than rill erosion to rainfall intensity increases. The former was mostly affected by rainfall intensity in this experimental setup, while the latter was controlled by flow discharge, gradient, and rill evolution stage. The greatest sediment concentration and delivery rate occurred in the stage of the fastest rill development. The increased sediment delivery from interrill areas appeared to suppress rill detachment by concentrated flow. This study enhanced our understanding of interrill and rill erosion processes and provided the scientific insights for improving soil erosion models.

An experimental study of rill sediment delivery in purple soil, using the volume-replacement method.

Experimental studies provide a basis for understanding the mechanisms of rill erosion and can provide estimates for parameter values in physical models simulating the erosion process. In this study, we investigated sediment delivery during rill erosion in purple soil. We used the volume-replacement method to measure the volume of eroded soil and hence estimate the mass of eroded soil. A 12 m artificial rill was divided into the following sections: 0-0.5 m, 0.5-1 m, 1-2 m, 2-3 m, 3-4 m, 4-5 m, 5-6 m, 6-7 m, 7-8 m, 8-10 m, and 10-12 m. Erosion trials were conducted with three flow rates (2 L/min, 4 L/min, and 8 L/min) and five slope gradients (5°, 10°, 15°, 20°, and 25°). The eroded rill sections were refilled with water to measure the eroded volume in each section and subsequently calculate the eroded sediment mass. The cumulative sediment mass was used to compute the sediment concentration along the length of the rill. The results show that purple soil sediment concentration increases with rill length before eventually reaching a maximal value; that is, the rate of increase in sediment concentration is greatest at the rill inlet and then gradually slows. Steeper slopes and higher flow rates result in sediment concentration increasing more rapidly along the rill length and the maximum sediment concentration being reached at an earlier location in the rill. Slope gradient and flow rate both result in an increase in maximal sediment concentration and accumulated eroded amount. However, slope gradient has a greater influence on rill erosion than flow rate. The results and experimental method in this study may provide a reference for future rill-erosion experiments.