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The susceptibility of purple soil and intensive tillage render the land prone to erosion under heavy precipitations in a sloping cropland in southwestern China. This study aimed to improve the evaluation of the potential benefits of surface protection tillage and organic matter addition to decreasing nutrient losses. A field plot experiment under natural rainfall conditions was conducted, which employed four management practices: conventional downslope tillage system as control (CK), contour tillage (CT) with organic matter addition (CT + OM), CT with wheat straw mulching (CT + SM), and CT combining straw mulching and organic matter addition (CT + OM + SM). Runoff depth, nutrient loads, and soil erodibility were used to estimate the effects of straw mulching and organic matter addition. Results indicated that the runoff depth under CK was largest during the experimental period, with an average of 16.91 mm, and runoff coefficient average was 32%. Compared with CK, the runoff depth under CT + OM, CT + SM, and CT + OM + SM were reduced by 19%, 34%, and 50%, respectively. A significant difference in soil erodibility indicator among the four treatments was indicated (p < 0.05); CK achieved the highest value, whereas CT + OM + SM obtained the least value. In addition, the contour cultivation (i.e., CT approaches) were more sustainable than the downslope tillage system (i.e., CK). Soil erodibility under CK was 9.83 kg ha −1 mm −1 . Meanwhile, soil erodibility under CT + OM, CT + SM, and CT + OM + SM were 8.49, 6.99, and 6.87 kg ha −1 mm −1 , respectively. These values were 14%, 29%, and 30% lower than that of CK, respectively. CK was more susceptible to accelerated erosion compared with the plots with a surface cover or organic addition. This greater erodibility resulted in higher runoff, sediment yield, and associated nutrient loss for CK. The runoff-associated nitrogen losses were mainly controlled by the runoff rate and soil erodibility (p < 0.05). Variations in NO 3 − –N and NH 4 + –N concentration in runoff water were markedly affected by rainfall events and agricultural practice. A significant logarithmic correlation between NO 3 − –N load and runoff depth was identified. NO 3 − –N was proven to be the main form of inorganic nitrogen loss; therefore, fertilizer application of NO 3 − –N should be reduced in the purple soil region. Soil erodibility significantly influenced the available N losses (p < 0.01), which was best described by a positive logarithmic correlation. Soil nutrient concentration also played an important role in nitrogen loss. However, further research is needed to understand the dynamic interactions between soil erodibility as well as soil and nutrient losses. Results indicated that surface protection by CT + OM + SM is one of the good management practices to reduce soil loss by water erosion in regions with intense agricultural activity. [ABSTRACT FROM AUTHOR]

Summary Ephemeral gullies or rills are major sediment sources and the primary channel to convey runoff and sediments from hillslopes. Quantifying the detachment rate is important for understanding the erosion process of ephemeral gullies. This study suggests a method to estimate the detachment rate for use in WEPP model, using sediment distribution data along an eroding rill or ephemeral gully under controlled flume experiments, conducted at four slope gradients (8.74%, 17.62%, 26.78%, and 36.38%) and four flow rates (32, 64, 128 and 256 L min −1 ). The experimental soil materials were packed into the flume of 0.5 m wide, 0.6 m deep and 12 m long, to a thickness of 0.5 m at a bulk density of 1300 kg m −3 . Regulated water flow was introduced to soil surface at the upper end of flumes. The detachment rate was calculated from the spatial distribution data of sediment concentrations measured with sediment-laden water samples simultaneously taken along the gullies at one meter intervals. Results showed that the detachment rate was well fitted with downslope distance along the rill or ephemeral gully by an exponential equation. The regression parameters indicated that detachment rate decreases linearly with sediment load in the gully flow. The feedback relationship between sediment load and detachment rate in rill or ephemeral gullies could be adequately represented by the first-order coupling equation for loess soil under steep slopes and high flow rates. The detachment rates along the rills or ephemeral gullies were found to increase with slope gradient and flow rate and the detachment rate was positively related to unit stream power and the deficit between max sediment load and the local sediment load. [ABSTRACT FROM AUTHOR]

Background Ephemeral gully erosion is an important form of hillslope erosion all over the world. It is of high significance to understand its erosion mechanism. Objective A new volume replacement method is proposed to determine ephemeral gully erosion process. Styrofoam particles of hard granules were used to refill the eroded ephemeral gully sections lined with gauze sheet to the original elevation, before their volumes were measured for estimating sediment concentration distribution along the ephemeral gully. The measurement and the computational methodologies are illustrated in details. Method A series of flume experiments were conducted with a silt loam (loess) soil to study ephemeral gully erosion process under four slope gradients and three flow rates. The ephemeral gully was pre-formed before each experimental run. The volumes of the eroded ephemeral gully sections were measured and the distribution of sediment concentration along the gully was computed at 1 m intervals. For comparison purpose, the sediment concentration distribution along the eroded gully was also measured by manual sampling of the sediment-laden water flow with cups. Results The results showed that both the sediment processes estimated by the volume replacement and measured by the sampling method were very similar. The estimated and measured sediment concentrations increased with ephemeral gully length under a given flow rate. The increase rate decreased gradually as the sediment concentration gradually approached its limit. Conclusions Under different flow rates and slope gradients, the sediment concentrations at different distances, as estimated by the volume replacement method were about the same as those measured with the sampling method. The new method provides a useful and efficient way in determining the ephemeral gully erosion, and helps to understand ephemeral gully erosion process. [ABSTRACT FROM AUTHOR]

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

Sloping farmland (SpF) is an important component of cultivated land resources. In the context of food security and sustainable agricultural development, comprehensive SpF utilization has drawn wide and increasing attentions. A systematic analysis on the research progress of comprehensive SpF utilization is critical for a better understanding of the sustainable agricultural cultivation, facilitating the macroeconomic decision-making. This paper conducted a quantitative analysis of literature related to comprehensive SpF utilization over the past few decades (1946–2019) by using scientometrics methods, with the objectives to: (1) identify the research hotspots and summarize the relevant research processes in different time periods; (2) reveal the temporal evolution of those hotspots. Our results showed: 1) research on SpF dramatically increased over the past few decades (1946–2019), which can be classified into four chronological periods (I, II, III, and IV); (2) research regarding land productivity improvement and hydrological and erosion processes on SpF were dominant in the period I; (3) the research hotspots were switched to land use changes and associated soil erosion response, ecological disturbance and restoration as the social demand was intensifying in the period II; (4) the research focus of period III was aiming at the governance and rational utilization of SpF and gradually transforming to a large regional scale; (5) the intercross and penetration between disciplines had been continuously enhanced in the period IV, and the application of high and new technologies had also promoted the diversification of research methods, with aims to achieve sustainable agricultural development. Although considerable progress had been achieved around the world, more theoretical and empirical studies are required to understand the comprehensive SpF utilization and their implications to better support sustainable agricultural development. • Comprehensive sloping farmland utilization was systematically reviewed. • Research focus has changed from land productivity to multi-disciplinary fields. • Methodology is developing to be on multiple scales involved in information technology. [ABSTRACT FROM AUTHOR]

Highlights • Applying the electrolyte tracer pulse method to measure shallow water flow velocity on hill-slope. • An equation for estimating flow velocity on the basis of the measurement distance was established. • The new equation could be used to simulate results from other experimental data sets. • The steady flow velocities and the distances when water flow reached stability were determined by the new equation. Abstract Understanding the velocity distribution of shallow water flow along hill-slope is of great significance in soil erosion studies. This study proposes a method to estimate the steady flow velocity and the distance when water flow reaches stability, using hill-slope flow velocity distribution data along a simulated rill measured by the electrolyte tracer pulse method. Laboratory experiments were performed using a flume of 12 m long, 0.1 m wide and 0.3 m height under five slope gradients (5°, 10°, 15°, 20° and 25°) and four flow discharges (2, 4, 8 and 16 L min−1). The electrolyte tracer pulse method was employed to measure the flow velocities at locations of 1, 2, 3, 4, 6, 8, 10 and 12 m from the inlet of the flowing water. The flow velocities measured by the dye tracer method served as a control. The results showed that the flow velocities measured by the electrolyte tracer pulse method initially accelerated and then reached a steady value. An equation for estimating flow velocity on the basis of the measurement distance was established. The flow velocities calculated by the equation agreed well with those measured by the electrolyte tracer pulse method. The determination (R2) and the Nash-Sutcliffe model efficiency (NSE) were greater than 0.800, except for a few cases. In addition, the equation established in this study was shown to successfully predict flow velocities measured in other studies. The steady flow velocities and the distances when water flow reached stability under different slope gradients and flow discharges were determined by this equation. When the differences between the measured flow velocity and the steady flow velocity were very small, that is, 5% and 10%, the steady distances ranged from 2.239 m to 4.772 m and from 2.843 m to 6.059 m, respectively. By comparing the steady flow velocities with the flow velocities measured by the dye tracer method, the results indicated that the steady flow velocities were 0.702–0.735 times that of the flow velocities measured by the dye tracer method at various slope gradients and flow discharges. A linear function existed between the two, and the regressed parameter k (i.e., 0.718) could be used as the correction factor between the two. In general, the equation established in this study can facilitate the prediction of the steady flow velocity. [ABSTRACT FROM AUTHOR]

Accurate estimation of sediment concentration is essential to establish physically-based erosion models. The objectives of this study were to evaluate the effects of flow discharge (Q), slope gradient (S), flow velocity (V), shear stress (τ), stream power (ω) and unit stream power (U) on sediment concentration. Laboratory experiments were conducted using a 10 × 0.1 m rill flume under four flow discharges (2, 4, 8 and 16 L min −1 ), and five slope gradients (5°, 10°, 15°, 20° and 25°). The results showed that the measured sediment concentration varied from 87.08 to 620.80 kg m −3 with a mean value of 343.13 kg m −3 . Sediment concentration increased as a power function with flow discharge and slope gradient, with R 2  = 0.975 and NSE = 0.945. The sediment concentration was more sensitive to slope gradient than to flow discharge. The sediment concentration was well predicted by unit stream power (R 2  = 0.937, NSE = 0.865), whereas less satisfactorily by flow velocity (R 2  = 0.470, NSE = 0.539) and stream power (R 2  = 0.773, NSE = 0.732). In addition, using the equations to simulate the measured sediment concentration of other studies, the result further indicated that slope gradient, flow discharge and unit stream power were good predictors of sediment concentration. In general, slope gradient, flow discharge and unit stream power seem to be the preferred predictors for estimating sediment concentration. [ABSTRACT FROM AUTHOR]

The eutrophication caused by nitrogen loss from sloping farmland is a serious concern, especially in the context of increasing frequency of extreme rainfall events. The relative importance that the surface runoff and interflow processes govern the nitrogen loss under extreme rainfall events, however, is ambiguous. Moreover, this ambiguity could be further enhanced by conservation practices on sloping farmland, such as straw returning and contour tillage. To better understand these ambiguities, five simulated rainfall experiments at the intensity of 100 mm h−1 under four treatments including downslope tillage (DT), cross-slope tillage (CT), cross-slope tillage with whole straw returning (CT + WR), cross-slope tillage with crushed straw returning (CT + CR), were conducted in 2 years' maize season on a typical purple sloping farmland in the hilly area of Sichuan, China. Results showed: 1) Compared with DT, straw returning and cross-slope tillage significantly reduced surface runoff and nitrogen load of surface runoff. However the concomitant potential increase of the interflow runoff would result in the increase of nitrogen loss via interflow, offsetting the benefits of conservation practices; 2) The nitrogen loss through interflow is 6.38 ± 0.21 kg ha−1, accounting for 77.9 % of the total nitrogen loss and suggesting that interflow is the dominant process; 3) Dissolved organic nitrogen is one of the main nitrogen loss forms, accounted for 41.06 % (24.31–57.69 %) of the total nitrogen loss in surface runoff and 32.02 % (12.28–47.81 %) for interflow, should not been ignored; 4) The results of the three prediction models showed that the nitrogen loss caused by interflow drainage under extreme rainfall should not be ignored. These findings enhance our understanding of nitrogen exports induced by extreme rainfall events and provide references for nitrogen loss predictions and control in sloping farmland. [Display omitted] • Straw returning on sloping farmland increases the nitrogen loss risk via interflow. • The nitrogen loss through interflow is the dominant process under extreme rainfall events. • Dissolved organic nitrogen is one of the main nitrogen loss forms in both surface flow and interflow. • Nitrogen loss through interflow pathway should be considered when the application of conservation tillage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proposed a new method to accurately measure sediment transport capacity (STC). • The STC significantly depends on flow rate and slope of the ephemeral gully. • An empirical model was developed to predict the STC in ephemeral gully. Sediment transport capacity (STC) is an important parameter for quantifying the erosion process in ephemeral gully. It is not practical to build a flume long enough to measure the STC in ephemeral gully through traditional soil erosion. Therefore, a methodology and a criteria were proposed to adjust the sediment feeding by regulating the length of main sediment supply section to achieve measurement of STC in ephemeral gully. The STC measurements were conducted under slope gradients of 5°, 10°, 15°, and 20° and unit-width flow rates of 5.33, 10.67, 21.33 and 42.67 m3 s−1 m−1. The measured STC in ephemeral gullies ranged from 1.19 to 38.29 kg s−1 m−1 with high accuracy. Also, the measured STCs were positively and well correlated with unit-width flow rate and slope gradient. According to the values of Reynolds number and Froude number, the water flow in experimental ephemeral gully can be categorized as a turbulent and supercritical flow. A multivariate non-linear model was developed to predict the STC with slope gradients and unit-width flow rates, which performed well under experimental hydraulic conditions. The proposed methodology and measuring setup provide a new and reliable approach for measuring STC in ephemeral gully. [ABSTRACT FROM AUTHOR]

• Soil erosion due to livestock and climate in QTP was assessed using 137Cs and 210Pb ex. • Net erosion in the last 50 years was 19 t ha−1 yr−1 but 0.25 t ha−1 yr−1 over 100 years. • Erosion significantly correlated with grazing but not with climate change. • Livestock grazing accelerates soil erosion more than climate change in QTP. Livestock grazing and climate change accelerate soil erosion in the grassland of Qinghai-Tibet Plateau (QTP), China. However, the linkage of the two factors to accelerated erosion in the region has never been separated. Thus, this study was set up to determine the contribution of livestock grazing and climate change to soil erosion in the grassland of QTP. Five sites (Tuotuohe, Maduo, Maqin Dongqinggou (MQDQG), Maqin Junmuchang (MQJMC), and Maqin Jungongzhen (MQJGZ)) across 1100-km East-West transect in the Three-River Headwaters Region (TRHR) of QTP were selected for this study. The 137Cs and 210Pb ex techniques were used to determine soil redistribution rates, covering a time scale of 50 and 100 years, respectively. In the last 100 years, soil accumulation was obvious in four sites (Tuotuohe, Maduo, MQDQG and MQJMC) except one site (MQJGZ), where soil erosion occurred along the transect. In the entire transect, almost no soil erosion but soil accumulation (0.25 t ha−1 yr−1) occurred over a period of 100 years. In contrast, soil erosion significantly increased by 75 times at a rate of 18.68 t ha−1 yr−1 in the last 50 years. Net soil redistribution varied significantly from −99 t ha−1 yr−1 in MQJGZ to 14.2 t ha−1 yr−1 in MQDQG over a period of last 50 years. Accelerated soil erosion across QTP significantly correlated with livestock numbers and intensive grazing activities but not significantly correlated with precipitation. Our results suggest that livestock grazing play a key role in accelerating soil erosion more than climate change in the grassland of Qinghai-Tibet Plateau. [ABSTRACT FROM AUTHOR]