Sheet Erosion Processes Research Articles

Impacts of biocrusts on sheet erosion and its detachment- and transport-limited processes: A case study from the Loess Plateau, China

Understanding sheet erosion processes and improving estimates of soil erosion on biological soil crustal (biocrustal) slopes requires a deep understanding of the factors influencing sheet erosion and its detachment- and transport-limited processes. Simulated rainfall experiments were conducted on plots of biocrust to simultaneously measure the rates of splash detachment and sheet erosion with 0%, 20%, 40%, 50%, 60%, and 80% of coverage at 90 mm h −1 rainfall intensity and 15° slope. The splash detachment rate decreased rapidly during the first 5 min of rainfall and stabilised as rainfall continued, while the sheet erosion rate increased gradually and eventually became relatively stable. The rates of splash detachment and sheet erosion decreased substantially as biocrusts coverage increased and average values of them can be effectively weakened when biocrusts coverage increased from 20% to 40%. Transport-limited processes, dynamic equilibria between detachment- and transport-limited processes, and detachment-limited processes all occurred on biocrustal slope within the same rainfall event. The mean duration of transport-limited processes was longer (16.3 min vs. 9.1 min) when bare slopes were covered by biocrusts, and the latest change in the time from transport-limited to detachment-limited processes appeared at 40% biocrusts coverage. The existence of biocrusts could effectively affect the sheet erosion and its detachment- and transport-limited processes, especially when the coverage was close to 40%. These results can improve our understanding of impacts of biocrusts on sheet erosion processes, and aid an effective way of reducing sheet erosion in arid and semiarid regions. • The effects of biocrusts on sheet erosion were explored. • Detachment- and transport-limited processes occurred on biocrustal slope. • Increasing biocrusts coverage reduced rates of splash detachment and sheet erosion. • Biocrusts with a cover close to 40% effectively affected sheet erosion processes.

Response of Sheet Erosion to the Characteristics of Physical Soil Crusts for Loessial Soils

The influences and quantifications of soil crust traits on the infiltration, hydrodynamic of runoff, and erosion rate of sheet erosion under the combined effects of raindrop impact and sheet flow scouring need further study. Loessial soil from the Loess Plateau was tested to produce different antecedent crusts under simulated rainfall intensities (0.5, 1.0, 1.5, 2.0, and 2.5 mm/min, typical storm intensity in the area), and then the effects of antecedent crusts on sheet erosion processes were quantified at a rainfall intensity of 1.5 mm/min. The results showed that the bulk density and hardness of antecedent crusts were higher than those of soil. Particle sizes of crusts were smaller than those of soil at light rain intensity but larger under heavy rain intensity. The bulk density, hardness, and particle size D50 of the antecedent crust were all positively correlated with rainfall intensity, being well described by linear equations (R2 > 0.87), while the thickness was negatively linearly correlated with rainfall intensity (R2 = 0.88). Although the existence of antecedent crusts could decrease the infiltration and increase the runoff, resulting in the high flow velocity and stream power, antecedent crusts could still effectively reduce sheet erosion. The reductions in the average infiltration rate and average erosion rate and the increases of average flow velocity and stream power all increased with the increment of bulk density of antecedent crust. Relationships could be all well described by linear positive correlations (R2 > 0.79). When the bulk density of crust was enhanced by 27∼29%, the flow velocity and stream power could be increased by 8∼29% and 15∼70%, and the sheet erosion could be reduced by 61∼73%. The existence of crust could effectively reduce sheet erosion. These results could help understand the mechanism of the erosion process in the presence of physical crusts.

Response of soil detachment rate by raindrop-affected sediment-laden sheet flow to sediment load and hydraulic parameters within a detachment-limited sheet erosion system on steep slopes on Loess Plateau, China

The response of soil detachment rate by raindrop-affected sediment-laden sheet flow to sediment load and hydraulic parameters was investigated within a detachment-limited sheet erosion system on steep slopes to understand sheet erosion processes fully and derive an accurate experimental model. An experiment was conducted at slopes of 12.23%, 17.63%, 26.8%, 36.4%, 40.4% and 46.63% under rainfall intensities of 48, 60, 90, 120, 138 and 150 mm h−1, respectively, by using simulated rainfall. Results showed that the soil detachment rate by raindrop-affected sediment-laden sheet flow decreased as the sediment load by sheet flow increased, and the decrease was a power function of sediment load by sheet flow with NSE = 0.58, MSE = 0.0099 and R2 = 0.58. In addition, the soil detachment rate by raindrop-affected sediment-laden sheet flow increased as a linear function of shear stress, stream power and unit stream power. Shear stress and stream power could be used to predict the soil detachment rate by raindrop-affected sediment-laden sheet flow accurately through a linear equation. Stream power (R2 = 0.87, MSE = 0.003 and NSE = 0.87) was a better predictor of soil detachment rate by raindrop-affected sediment-laden sheet flow than shear stress (NSE = 0.83, MSE = 0.004 and R2 = 0.83). However, prediction based on unit stream power (NSE = 0.43, MSE = 0.01 and R2 = 0.43) was poor. These findings can improve our understanding and modelling of sheet erosion processes on steep slopes in the loess region of China.

Effects of soil type and rainfall intensity on sheet erosion processes and sediment characteristics along the climatic gradient in central-south China

Soil erosion poses a major threat to the sustainability of natural ecosystems. The main objective of this study was to investigate the effects of soil type and rainfall intensity on sheet erosion processes (hydrological, erosional processes and sediment characteristics) from temperate to tropical climate. Field plot experiments were conducted under pre-wetted bare fallow condition for five soil types (two Luvisols, an Alisol, an Acrisol and a Ferralsol) with heavy textures (silty clay loam, silty clay and clay) derived separately from loess deposits, quaternary red clays and basalt in central-south China. Rainfall simulations were performed at two rainfall intensities (45 and 90mmh−1) and lasted one hour after runoff generation. Runoff coefficient, sediment concentration, sediment yield rate and sediment effective size distribution were determined at 3-min intervals. Runoff temporal variations were similar at the high rainfall intensity, but exhibited a remarkable difference at the low rainfall intensity among soil types except for tropical Ferralsol. Illite was positively correlated with runoff coefficient (p<0.05). Rainfall intensity significantly contributed to the erosional process (p<0.001). Sediment concentration and yield rate were the smallest for the tropical Ferralsol and sediment concentration was the largest for the temperate Luvisol. The regimes (transport and detachment) limiting erosion varied under the interaction of rainfall characteristics (intensity and duration) and soil types, with amorphous iron oxides and bulk density jointly enhancing soil resistance to erosive forces (Adj-R2>88%, p<0.001). Sediment size was dominated by <0.1mm size fraction for the Luvisols and bimodally distributed with the peaks at <0.1mm and 1–0.5mm size for the other soil types. Exchangeable sodium decreased sediment size while rainfall intensity and clay content increased it (Adj-R2=96%, p<0.01). These results allow to better understand the climate effect on erosion processes at the spatial-temporal scale from the perspective of soil properties.

Gully evolution and geomorphic adjustments of badlands to reforestation

Badlands and gullied areas are among those geomorphic environments with the highest erosion rates worldwide. Nevertheless, records of their evolution and their relations with anthropogenic land transformation are scarcer. Here we combine historical data with aerial photographs and tree-ring records to reconstruct the evolution of a badland in a Mediterranean environment of Central Spain. Historical sources suggest an anthropogenic origin of this badland landscape, caused by intense quarrying activities during the 18th century. Aerial photographs allowed detection of dramatic geomorphic changes and the evolution of an emerging vegetation cover since the 1960s, due to widespread reforestation. Finally, tree-ring analyses of exposed roots allowed quantification of recent channel incision of the main gully, and sheet erosion processes. Our results suggest that reforestation practices have influenced the initiation of an episode of incision in the main channel in the 1980s, through the hypothesized creation of disequilibrium in water-sediment balance following decoupling of hillslopes from channel processes. These findings imply an asymmetry in the geomorphic response of badlands to erosion such that in the early evolution stages, vegetation removal results in gullying, but that reforestation alone does not necessarily stabilize the landforms and may even promote renewed incision.

Reply to “The new assessment of soil loss by water erosion in Europe. Panagos P. et al., 2015 Environ. Sci. Policy 54, 438–447—A response” by Evans and Boardman [Environ. Sci. Policy 58, 11–15

The new assessment of soil loss by water erosion in Europe (Panagos et al., 2015a) was commented by Evans and Boardman (2016), who raised not only concerns related to the spatial differences outlined by our work compared to their visual semi-qualitative assessment conducted in Britain during the late eighties, but also generally to the suitability, validity and scientific robustness of the applied modelling approach. The objective of the pan-European assessment using the Revised Universal Soil Loss Equation (RUSLE) was not to outcompete any regional- or national-scale modelling, but to harmonize and improve our knowledge and our understanding of current soil erosion rates by water across the European Union. The focus of such a modelling project is on the differences and similarities between regions and countries beyond national borders and nationally adapted models. In order to do so, a state-of-the-art large-scale spatially distributed modelling exercise using harmonized datasets and a unified methodology to suit the pan-European scale was carried out. We reply that the semi-qualitative approach proposed by Evans and Boardman (2016) is not suitable for application at the European scale because of work force and time requirements, input data accessibility issues, accuracy of field-based estimates, subjectivity of soil loss estimates during the aerial and terrestrial photo interpretation, impossibility of upscaling or downscaling, inadequate representation of sheet erosion processes, lack of spatial and temporal representativeness, and lack of detailed description expressing the risk level. As such, their methodology has limited applicability, with today’s financial resources it is not feasible at European or at national scale and, most important, cannot respond to policy requests regarding scenarios of climate and land cover/use change. In contrast to Evans and Boardman (2016), we do know that RUSLE, like probably any other approach, is not able to reproduce “reality”. The latter is actually a misjudgment which has been extensively discussed 20years ago. Modelling in general and large-scale modelling specifically can per se not aim at an accurate prediction of point measurements, but tests our hypothesis on process understanding, relative spatial and temporal variations, scenario development and controlling factors (Oreskes et al., 1994). As such, our approach can be offered as a helpful tool to policy makers at pan-European scale. We are confident that the simple transparent structure of RUSLE as well as the discussion of the uncertainties of each modelling factor will help to supply objective guidance to policy makers.

Experimental study on hydro-dynamic mechanism of sheet erosion processes on loess hillslope

Experimental study on hydro-dynamic mechanism of sheet erosion processes on loess hillslope

Process identification of soil erosion in steep mountain regions

Abstract. Mountainous soil erosion processes were investigated in the Urseren Valley (Central Switzerland) by means of measurements and simulations. The quantification of soil erosion was performed on hill slope scale (2·20 m) for three different land use types: hayfields, pastures with dwarf shrubs and pastures without dwarf shrubs with three replicates each. Erosion rates during growing season were measured with sediment traps between June 2006 and November 2007. Long-term soil erosion rates were estimated based on Cs- 137 redistribution. In addition, soil moisture and surface flow were recorded during the growing season in the field and compared to model output. We chose the WEPP model (Water Erosion Prediction Project) to simulate soil erosion during the growing season. Model parameters were determined in the field (slope, plant species, fractional vegetation cover, initial saturation level), by laboratory analyses (grain size, organic matter) and by literature study. The WEPP model simulates sheet erosion processes (interrill and splash erosion processes, please note that no rill erosion occurs at our sites). Model output resulted in considerable smaller values than the measured erosion rates with sediment traps for the same period. We attribute the differences to observed random gravity driven erosion of soil conglomerates. The Cs-137 measurements deliver substantially higher mean annual erosion rates, which are most likely connected to snow cover related processes such as snow gliding and avalanche activities.

A New Splash and Sheet Erosion Equation for Rangelands

Soil loss rates predicted from erosion models for rangelands have the potential to be important quantitative indicators for rangeland health and for assessing conservation practices. Splash and sheet erosion processes on rangelands differ from croplands, where the process is conceptualized in part as interrill erosion. Previous interrill equations were developed from cropland soils where interrill erosion was conceptualized and modeled for small plots, which are not generally large enough to encompass the relative high spatial heterogeneity of rangelands. Also, interrill erosion is usually modeled as a function of rainfall intensity (I) and runoff rate (q) such that I and q are independent of each other. Splash and sheet erosion is the dominant type of erosion on most undisturbed rangeland hillslopes where there is adequate vegetation, and these important erosion processes need to be addressed to develop an appropriate rangeland erosion model. In this study, we developed a new equation for calculating the combined rate of splash and sheet erosion (Dss) using a large set of rainfall simulation data from the western United States. The equation we propose: Dss = KssI1.052q0.592, where Kss is a splash and sheet erosion coefficient, takes into account a key interrelationship between I and q revealed in the data. This equation was successfully evaluated using independent sets of multiple‐intensity experimental data. The new equation should enable improved estimation of water erosion on rangelands in the western United States and in other parts of the world.

Long-term net soil erosion as determined by 137Cs redistribution in an undisturbed and perturbed tropical deciduous forest ecosystem

Erosion is the main cause of soil degradation in tropical regions, where the lack of methods for long-term studies is the principal constraint to addressing soil erosion problems. Recently, the analysis of 137Cs redistribution within the landscape has been used for assessing long-term soil erosion and net deposition. In the present study, measurements of 137Cs distribution were used to calculate long-term soil erosion in a Mexican tropical deciduous ecosystem under undisturbed forest and pasture conditions. Sheet erosion processes caused 137Cs redistribution within the landscape. The crests had significantly higher 137Cs activity than midslopes and lower concentration than the footslopes. There was no clear relationship between 137Cs redistribution and local topographic variables in our study (that is, slope). Soils in a gentle midslope had lower 137Cs activity than those in a steeper midslope. However, hill morphology explained 137Cs redistribution within landscape, that is, high 137Cs activity was associated with sites at the base of hillslopes. Thus, net erosion was found to be strongly influenced by hill morphology. Calculated erosion and deposition rates for the undisturbed watershed were 13.2 Mg ha −1 yr −1 and 4.9 Mg ha −1 yr −1, respectively. Net erosion within the pasture conversion plots was strongly influenced by rainfall erosivity during the year following perturbation. High net erosion was associated with plots with high annual erosivity immediately after forest burn. This suggests that the first year after slash and burn is critical for susceptibility to soil erosion. Based on erosion rates calculated in the present study, the top 5 cm of soil could be removed in only 25 years. This represents soil productivity loss, as this top layer represents the principal soil nutrient pool for the Chamela region.

Chapter IV. Sediment Sources and Sediment Yields

Methods and procedures are analyzed for determining erosion rates from water-borne sediments, along with sediment yield or deposition rates at locations downstream from the erosion source. Emphasis is placed on the erosion types and processes which cause the most engineering problems, either by virtue of the large sediment quantities involved or by the locally severe damages incurred. Accelerated erosion and deposition rates from man's activities are of special concern; these derive from sheet erosion processes on tilled lands and from such specialized activities as strip mining, urban development, construction of highways and utilities, and sheet erosion on any portion of the land surface that is denuded or inadequately protected. Erosion and deposition rates caused by channelization or runoff waters in upland gullies and downstream drainageways are also considered. Present bases for estimating these rates, on both a storm and an average annual basis, are given.