Water Flow Erosion Research Articles

The effects of varied soil properties induced by natural grassland succession on the process of soil detachment

The changes in soil properties caused by vegetation succession might have great effects on the process of soil detachment by overland flow. This study was carried out to quantify the effects of varied soil properties induced by natural grass succession on soil detachment capacity by overland flow and soil resistance to flowing water erosion on the Loess Plateau. 300 undisturbed soil samples (without roots) were collected from ten typical grasslands, and subjected to flow scouring under six shear stresses ranging from 4.98 to 16.37 pa. The results showed that the maximum soil detachment capacity (3.80 kg m−2 s−1) was found in Astragalus melilotoides Pall. grassland, where it was 49.0 times greater than that of the minimum found in Poa sphondylodes Trin. grassland. Soil properties induced by fibrous root herbage have strong effects on the process of soil detachment. In comparison to grasslands with tap root systems, grasslands with fibrous root herbage have lower soil detachment and rill erodibility by 84.6% and 84.3%, respectively, and critical shear stress which is higher by 15.2%. Stream power was a better parameter than velocity, shear stress or unit stream power for simulating soil detachment capacity. Soil cohesion, bulk density, organic matter and median soil grain size were the main factors affecting the process of soil detachment. Rill erodibility decreased with cohesion or clay content as an exponential or power function, and increased with the median soil grain size as an exponential function. A model was developed to estimate soil detachment capacity based on hydraulic parameters and soil properties on the Loess Plateau. The result was satisfactory and the performance of model was greatly improved in comparison to previous studies (R2 = 0.77; NSE = 0.61; p < 0.01).

Responses of Bed Morphology to Vegetation Growth and Flood Discharge at a Sharp River Bend

In this study, we conducted simulations using a two-dimensional, depth-averaged river flow and river morphology model to investigate the effect of vegetation growth and degree of flow discharge on a shallow meandering channel. To consider the effects of these factors, it was assumed that vegetation growth stage is changed by water flow and bed erosion. The non-uniformity of the vegetation growth was induced by the non-uniform and unsteady profile of the water depth due to the irregular shape of the bed elevation and the unsteady flow model reliant on hydrographs to evaluate three types of peak discharges: moderate flow, annual average maximum flow, and extreme flow. To compare the effects of non-uniform growing vegetation, the change in channel patterns was quantified using the Active Braiding Index (ABI), which indicates the average number of channels with flowing water at a cross section and the Bed Relief Index (BRI), which quantifies the degree of irregularity of the cross-sectional shape. Two types of erosion were identified: local erosion (due to increased flow velocity near a vegetation area) and global erosion (due to the discharge approaching peak and the large depth of the channel). This paper demonstrated that the growth of vegetation increases both the ABI and BRI when the peak discharge is lower than the annual average discharge, whereas the growth of vegetation reduces the BRI when the peak discharge is extreme. However, under extreme discharge, the ABI decreases because global erosion is dominant. The conclusions from this study help to deepen the understanding of the interactions between curved river channels and vegetation.

Simulating and mapping the spatial and seasonal effects of future climate and land -use changes on ecosystem services in the Yanhe watershed, China.

Effective information about ecosystem services is essential to help optimize and prioritize activities that support conservation planning in the face of land use and climate changes. This study shows an approach that integrates several dissimilar models for assessing water-related ecosystem services to predict values in 2050 under three land use scenarios in the Yanhe watershed. The simulated output variables pertaining to water yield and sediment yield were used as indicators for two ecosystem-regulating services, i.e., water flow regulation and erosion regulation, which were quantified using the soil and water assessment tool (SWAT) model. The model results were translated into a relative ecosystem service valuation scale, which facilitated the analysis of spatial and seasonal changes and served as the basis for the applied mapping approach. The simulated results indicate that higher water-related regulation services were concentrated in the middle and lower reaches of rivers with high water yield and low sediment erosion. The highest water flow regulation services occurred in summer; nevertheless, this was when erosion regulation services were the lowest compared to other periods in 2050. A comparison of the three land use scenarios showed differences in the water-related regulation services. Scenario 1, with high forest coverage, had the highest erosion regulation services, but the water flow regulation services were the lowest. Scenario 3 showed the reverse pattern. Scenario 2 had intermediate water flow regulation and erosion regulation. Increasing vegetation cover in the watershed is conducive to controlling water and soil erosion but could lead to a decline in available water resources. Spatial mapping is a powerful tool for displaying the spatiotemporal differences in the water-related regulation services delivered by ecosystems and can help decision makers optimize land use in the future, with the goal of maximizing the benefits offered by ecological services in the Yanhe watershed.

A coupled numerical model for water flow, sediment transport and bed erosion

Abstract In this paper, we propose an accurate Riemann-solver free method for a coupled system modeling shallow water flows, sediment transport and rapid bed erosion. The hydrodynamic model solving shallow water equations (SWEs) is coupled with the sediment transport equation and the morphodynamic model. A Godunov-type central-upwind scheme is proposed to numerically solve the resulting system based on the technique that using two different pairs of local wave speeds to estimate numerical fluxes for hydrodynamic and morphodynamic models, respectively. The proposed technique leads to accurate results for the computation of the morphodynamic model. The proposed scheme is based on computationally efficient techniques and do not require analytical expressions of the eingenstructure of the system. A well-balanced discretization of the bed-slope terms is developed in framework of the coupling approach. Finally, we demonstrate the robustness and accuracy of the proposed numerical scheme for the coupled system using several numerical tests.

Biologically-initiated rock crust on sandstone: Mechanical and hydraulic properties and resistance to erosion

Biocolonization on sandstone surfaces is known to play an important role in rock disintegration, yet it sometimes also aids in the protection of the underlying materials from rapid erosion. There have been few studies comparing the mechanical and/or hydraulic properties of the BIRC (Biologically-Initiated Rock Crust) with its subsurface. As a result, the overall effects of the BIRC are not yet well understood. The objective of the present study was to briefly characterize the BIRC from both the mineralogical and biological points of view, and especially to quantify the effect of the BIRC upon the mechanical and hydraulic properties of friable sandstone. The mineralogical investigation of a well-developed BIRC showed that its surface is enriched in kaolinite and clay- to silt-sized quartz particles. Total organic carbon increases with the age of the BIRC. Based on DNA sequencing and microscopy, the BIRC is formed by various fungi, including components of lichens and green algae. Using the method of drilling resistance, by measuring tensile strength, and based on water jet testing, it was determined that a BIRC is up to 12 times less erodible and has 3–35 times higher tensile strength than the subsurface friable sandstone. Saturated hydraulic conductivity of the studied BIRC is 15–300 times lower than the subsurface, and was measured to also decrease in capillary water absorption (2–33 times). Water-vapor diffusion is not significantly influenced by the presence of the BIRC. The BIRC thus forms a hardened surface which protects the underlying material from rain and flowing water erosion, and considerably modifies the sandstone's hydraulic properties. Exposing the material to calcination (550°C), and experiments with the enzyme zymolyase indicated that a major contribution to the surface hardening is provided by organic matter. In firmer sandstones, the BIRC may still considerably decrease the rate of weathering, as it is capable of providing cohesion to strongly weathered (and disintegrated) sandstone surfaces. However, only a near-surface zone of the sandstone is stabilized by the BIRC, and additional sources of stabilization (gravity-induced stress, inorganic cement, etc.) contribute to the resistance of the subsurface zone of sandstone exposures.

Water-related ecosystem services in Western Siberian lowland basins—Analysing and mapping spatial and seasonal effects on regulating services based on ecohydrological modelling results

For discussing and planning sustainable land management of river basins, stakeholders need suitable information on spatio-temporal patterns of natural processes and human actions. This study shows an approach to use ecohydrological modelling results for assessing water-related ecosystem services in three lowland river basins in Western Siberia. Based on the results of the ecohydrological model SWAT (Soil and Water Assessment Tool, Arnold et al., 1998), simulated output variables were used as indicators for the two regulating services water flow regulation and erosion regulation. The model results were translated into a relative ecosystem service valuation scale, which facilitates the comparison of different ecosystem services and which serves as the base for the applied mapping approach. The ecosystem service maps show different spatial and seasonal patterns. Although the daily modelling results were aggregated to seasonal maps to generate an understandable amount of information for stakeholders, differences can be depicted: during snowmelt, low regulation can be determined, especially for water flow regulation, but a very high regulation was calculated for the vegetation period during summer and for the winter period. Erosion regulation is high in two basins during all seasons. Spatio-temporal differences were modelled only for the largest basin. The SWAT model serves as a suitable quantification method for the assessment of the selected water-related regulating services.

Temporal variation in soil resistance to flowing water erosion for soil incorporated with plant litters in the Loess Plateau of China

Plant litter can be incorporated into topsoil under natural circumstances by soil splash, sediment deposition, and soil-dwelling animal activities. The incorporated litter can change the mechanical properties of soil and the decomposition of the incorporated litter can improve soil structural stability. Those changes likely influence soil detachment process by overland flow. This study was undertaken to quantify the effects of incorporated plant litters into topsoil on the temporal variation in soil resistance to detachment by overland flow using natural soil samples collected from four different plots (one control and three litter incorporation treatments) and then scoured under six different flow shear stresses in a hydraulic flume. The experiment started from April 19 to October 5, 2015 for 10 times at approximately 20days sampling intervals. Soil properties and environmental parameters were also measured at each sampling time to explain the temporal variations in rill erodibility and critical shear stress. The results showed that the incorporated plant litter was effective to enhance soil resistance to flowing water erosion. Compared to bare soil, rill erodibilities of litter incorporated soils of black locust, sea buckthorn, and green bristle grass decreased by 24.3%, 33.5%, and 34.8%. The temporal variations in rill erodibility of bare and litter incorporated soils were similar. Rill erodibility decreased significantly over time as an exponential function for both bare and litter incorporated soils. The relative rill erodibility of three litter incorporated soils increased over time as a power function. The fitted critical shear stress increased exponentially over time. The temporal variations in rill erodibility could be explained by the temporal variations in soil consolidation, water stable aggregate, and litter decomposition. Rill erodibility could be well estimated by soil bulk density, water stable aggregate, and litter mass density (r2=0.92).

Effects of incorporated plant litter on soil resistance to flowing water erosion in the Loess Plateau of China

Plant litter can be incorporated into top soil via different approaches, which probably influence soil erosion processes controlled by overland flow. However, few studies have been conducted to quantify the effects of incorporated plant litter on the soil detachment process by overland flow. This study was performed to investigate the effects of incorporated litter rate on soil detachment capacity and soil resistance to flowing water erosion using undisturbed soil samples taken from 16 plots (three plant litter species by five incorporation rates, and one bare control) and were subjected to six different flow shear stresses in the Loess Plateau. The results showed that soil detachment capacity decreased exponentially with incorporated plant litter rate. A threshold of 0.35 kg m−2 of litter needed to be incorporated to provide protection of soil from overland flow erosion. The effects of litter incorporation rate on soil detachment capacity was not significant when the incorporation rate was greater than 0.35 kg m−2. Rill erodibility also decreased exponentially with the incorporated plant litter rate. The shape of plant litter fragments was hypothesised to account for the variations in the effects of different incorporated litter species on soil detachment capacity and rill erodibility. A distinguishable increasing linear trend was observed between critical shear stress and litter incorporated rate, but with a weak correlation. Critical shear stress also increased with incorporated rate and was related to soil cohesion.

Assessing potassium environmental losses from a dairy farming watershed with the modified SWAT model

Potassium (K) was intensively used to optimize agricultural crop yield. Potassium losses should be accurately quantified for efficient nutrient management. However, no hydrologic model had been developed yet to quantify daily K losses at watershed scale. The Soil and Water Assessment Tool (SWAT) model was modified (named SWAT-K) by including the main K dynamic processes (solid–liquid distribution in soil and stream, plant uptake, and transportation with water flow and soil erosion) to simulate stream K load and K budget at the watershed scale. The SWAT-K was tested on the dairy farming Shibetsu River Watershed (672km2), Eastern Hokkaido, Japan. The solid–liquid distribution coefficient for K in suspended sediment was 10mlg−1. Langmuir equation was fitted to describe the solid–liquid distribution of K in soil, which derived an affinity constant of 0.046LmgK−1 and was used directly in SWAT-K. The fitted Langmuir equation also derived an adsorption maximum for K in soil. The adsorption maximum for K in soil normalized for clay content ranged from 4 to 20gKkg−1, and the fitted value of 15.5g Kkg−1 was used in SWAT-K. The SWAT-K satisfactorily predicted the daily dissolved K load at watershed outlet, and estimated an annual dissolved K load of 27.3kgKha−1year−1. The simulated pasture K yield of 36.1 (±2.5) kgKha−1year−1 was close to the observed data of 38.0 (±3.1) kgKha−1year−1. Then the model was used to quantify K budget at watershed scale. The simulated dissolved K leaching was 15.1kgKha−1year−1, and simulated soil K surplus of 75.1kgKha−1year−1 was much higher than plant uptake of 28.4kgKha−1year−1. The large amount of leaching and soil storage indicated that agricultural K input might be excessive and reducing the K application was recommended.

Effects of biological soil crusts on soil detachment process by overland flow in the Loess Plateau of China

AbstractBiological soil crusts (BSCs) cover up to 60 to 70% of the soil surface in grasslands after the ‘Grain for Green’ project was implemented in 1999 to rehabilitate the Loess Plateau. However, few studies exist that quantify the effects of BSCs on the soil detachment process by overland flow in the Loess Plateau. This study investigated the potential effects of BSCs on the soil detachment capacity (Dc), and soil resistance to flowing water erosion reflected by rill erodibility and critical shear stress. Two dominant BSC types that developed in the Loess Plateau (the later successional moss and the early successional cyanobacteria mixed with moss) were tested against natural soil samples collected from two abandoned farmland areas. The samples were subjected to flow scouring under six different shear stresses ranging from 7.15 to 24.08 Pa. The results showed that Dc decreased significantly with crust coverage under both moss and mixed crusts. The mean Dc of bare soil (0.823 kg m−2 s−1) was 2.9 to 48.4 times greater than those of moss covered soil (0.017–0.284 kg m−2 s−1), while it (3.142 kg m−2 s−1) was 4.9 to 149.6 times greater than those of mixed covered soil (0.021–0.641 kg m−2 s−1). The relative detachment rate of BSCs compared with bare soils decreased exponentially with increasing BSC coverage for both types of BSCs. The Dc value can be simulated by flow shear stress, cohesion, and BSC coverage using a power function (NSE ≥ 0.59). Rill erodibility also decreased with coverage of both crust types. Rill erodibility of bare soil was 3 to 74 times greater than those of moss covered soil and was 2 to 165 times greater than those of mixed covered soil. Rill erodibility could also be estimated by BSC coverage in the Loess Plateau (NSE ≥ 0.91). The effect of crust coverage on critical shear stress was not significant. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Spatial variation in soil resistance to flowing water erosion along a regional transect in the Loess Plateau

AbstractThe factors influencing soil erosion may vary with scale. It remains unclear whether the spatial variation in soil erosion resistance is controlled by regional variables (e.g. precipitation, temperature, and vegetation zone) or by local specific variables (e.g. soil properties, root traits, land use, and farming operations) when the study area enlarges from a hillslope or catchment to the regional scale. This study was performed to quantify the spatial variations in soil erosion resistance to flowing water under three typical land uses along a regional transect on the Loess Plateau and to identify whether regional or local specific variables are responsible for these changes. The results indicated that the measured soil detachment capacities (Dc) of cropland exhibited an irregular trend along the regional transect. The Dc of grassland increased with mean annual precipitation, except for two sites (Yijun and Erdos). The measured Dc of woodland displayed an inverted ‘U’ shape. The changes in rill erodibility (Kr) of three land uses were similar to Dc, whereas no distinguishable trend was found for critical shear stress (τc). No significant correlation was detected between Dc, Kr and τc, and the regional variables. The spatial variation in soil erosion resistance could be explained reasonably by changes in soil properties, root traits, land use, and farming operations, rather than regional variables. The adjustment coefficient of Kr for grassland and woodland could be well simulated by soil cohesion and root mass density (R2 = 0.70, P &lt; 0.01), and the adjustment coefficient of critical shear stress could be estimated with aggregate stability (R2 = 0.57, P &lt; 0.01). The results are helpful for quantifying the spatial variation in soil detachment processes by overland flow and to develop process‐based erosion model at a regional scale. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Seasonal changes in surface bulk density and saturated hydraulic conductivity of natural landscapes

Soil surface bulk density (ρb) and saturated hydraulic conductivity (Ks) control many land‐surface processes such as water flow, chemical transport and soil erosion. The objective of this study was to examine seasonal changes in surface ρb and Ks in natural landscapes with few human activities. Measurements of ρb and Ks were made on undisturbed soil samples taken from the soil surface (0–0.05 m) five times from October 2007 to March 2009 along four natural transects in a small watershed on the Chinese Loess Plateau. The transects represented four landscapes with different vegetation and soil typical in this region. Results showed that ρb and Ks varied seasonally. Temporal changes in Ks generally followed the temporal patterns of ρb. According to the mean values of all landscapes, bulk density decreased by 1.6 and 1.1% and log10‐transformed Ks (Log10Ks) increased by 11.0 and 5.8% from October 2007 to March 2008 and from October 2008 to March 2009, respectively; bulk density increased by 2.1% and Log10Ks decreased by 4.9% from March to June in 2007; from June to October in 2007, bulk density decreased by 1.3% while a slight increase (1.4%) in Log10Ks was observed. Both landscape and time significantly influenced ρb and Ks, and Ks was more susceptible to temporal change than ρb. Spatial patterns of ρb and Ks did not change significantly with time. Saturated hydraulic conductivity measurements taken in different seasons can affect runoff simulation results, and Ks data measured in spring may result in underestimation of runoff in a rainy season.

Self‐organised patchiness and scale‐dependent bio‐geomorphic feedbacks in aquatic river vegetation

Spatial self‐organisation of ecosystems is the process by which large‐scale ordered spatial patterns emerge from disordered initial conditions through local feedbacks between organisms and their environment. Such process is considered important for ecosystem functioning, providing increased productivity, resistance and resilience against environmental change. Although spatial self‐organisation has been found for an increasing number of ecosystems, it has never been shown so far for aquatic river vegetation. Here we explore the existence of spatial self‐organisation of freshwater macrophyte patches in a typical lowland river (Belgium), showing that the underlying mechanisms for pattern formation are scale‐dependent feedbacks between plant growth, water flow and local river bed erosion and sedimentation. The mapping of vegetation patches showed that the frequency distribution of patch sizes is governed by a power‐law function, suggesting that the patches are self‐organised. Scale‐dependent feedbacks, likely to lead to this self‐organised pattern, were demonstrated with a mimic experiment. Both positive and negative feedbacks on plants were confirmed by a transplantation experiment. Placing vegetation patch mimics in the river showed experimentally that on a short range (within and behind the mimics) flow reduction and increased sedimentation occurred, while on a larger range (next to patches) the flow was accelerated and decreased sedimentation took place. By transplanting macrophytes within, next to and further away from existing patches, it was proven that the conditions within the patches favoured the survival and growth of transplants (i.e. short‐range positive feedback), while the conditions just next to patches led to decreased survival and growth (i.e. long‐range negative feedback).

Modeling of Hillslope Runoff and Soil Erosion at Rainfall Events Using Cellular Automata Approach

A novel quantitative cellular automata (CA) model that simulates and predicts hillslope runoff and soil erosion caused by rainfall events was developed by integrating the local interaction rules and the hillslope surface hydraulic processes. In this CA model, the hillslope surface was subdivided into a series of discrete spatial cells with the same geometric features. At each time step, water and sediment were transported between two adjacent spatial cells. The flow direction was determined by a combination of water surface slope and stochastic assignment. The amounts of interchanged water and sediment were computed using the Chezy-Manning formula and the empirical sediment transport equation. The water and sediment discharged from the open boundary cells were considered as the runoff and the sediment yields over the entire hillslope surface. Two hillslope soil erosion experiments under simulated rainfall events were carried out. Cumulative runoff and sediment yields were measured, respectively. Then, the CA model was applied to simulate the water and soil erosion for these two experiments. Analysis of simulation results indicated that the size of the spatial cell, hydraulic parameters, and the setting of time step and iteration times had a large impact on the model accuracy. The comparison of the simulated and measured data suggested that the CA model was an applicable alternate for simulating the hillslope water flow and soil erosion.

Numerical Analysis Of Braided Rivers and Alluvial Fan Deltas

Understanding the processes of morphological behavior in braided rivers and alluvial fan deltas is very important for river engineering purposes to manage hydraulic structures and prevent disasters from flood, and environmental purposes to maintain river ecosystem. A two-dimensional numerical model was developed to simulate braided rivers and alluvial fan deltas with erodible bed and banks. A moving boundary-fitted coordinate system was employed to calculate water flow, bed change, and bank erosion. CIP (Cubic Interpolated Pseudo-particle) method was used to calculate flow, which introduced little numerical diffusion. Sediment transport equation in the streamline and transverse wise, considering the secondary flow, was used to estimate bed and bank evolution in time. Bank erosion was simulated by following the procedure proposed by Shimizu (2002). Braided river in laboratory was reproduced for verifying the numerical model in the channel filled with nearly uniform sandy materials. Comparison of numerical results and experimental data has shown relatively good agreements. The evolution processes of the alluvial fan deltas were investigated numerically by controlling the sediment inflow, and the model reproduced the features of alluvial fan deltas, i.e., the successive aggradation, avulsion and then migration of a river.

Numerical simulation for braided rivers with erodible banks

Understanding the process and mechanism of morphological behavior in braided rivers is very important for river engineering purposes to manage hydraulic structures and prevent disaster from flood, and environmental purposes to maintain river ecosystem and landscape. A two-dimensional numerical model was developed to simulate braided rivers with erodible bed and banks composed of well sorted-sandy materials. A moving boundary-fitted coordinate system was employed to calculate water flow, bed change, and bank erosion. CIP (Cubic Interpolated Pseudo-particle) method was used to calculate flow, which introduced little numerical diffusion. Sediment transport equation in the streamline and transverse wise, considering the secondary flow, was used to estim ate bed and bank evolution in time. Bank erosion was simulated when the gradient in the cross-sectional direction of the banks was steeper than the submerged angle of repose because of the bed erosion in the vicinity of banks. Braided river in laboratory was reproduced for verifying the numerical model in the channel filled with nearly uniform sandy materials. Comparison of numerical results and experimental data has shown relatively good agreements.

Numerical Simulation of Relatively Wide, Shallow Channels with Erodible Banks

A two-dimensional numerical model was developed to simulate relatively wide, shallow rivers with an erodible bed and banks composed of well-sorted, sandy materials. A moving boundary-fitted coordinate system was used to calculate water flow, bed change, and bank erosion. The cubic interpolated pseudoparticle method was used to calculate flow, which introduced little numerical diffusion. The sediment-transport equation for the streamline and transverse transport was used to estimate bed and bank evolution over time, while considering the secondary flow. Bank erosion was simulated when the gradient in the cross-sectional direction of the banks was steeper than the submerged angle of repose because of bed erosion near the banks. The numerical model reproduced the features of central bars well, such as bar growth, channel widening due to divergence of the flow around the bars, scour holes at the lee of the bars, and the increase of bar size with time. These features were in accordance with the observations for laboratory experiments. It also reproduced the features of braided rivers, such as the generation of new channels and abandonment of old channels, the bifurcation and confluence of channels, and the lateral migration of the channels. The model showed that the sediment discharge rate fluctuated with time, one of the dynamic features observed in braided channels.

Scale effects on specific sediment yield in the Yellow River basin and geomorphological explanations

Based on data from 199 stations in the Yellow River drainage basin for which more than 5 years of data are available, the relationship between specific sediment yield ( Y s) and drainage area ( A) has been studied. This relationship for the Yellow River basin is different from those for many other rivers of the world, both at the scale of whole basin and at local scales. With increasing basin area, the specific sediment yield increases, reaches a maximum, and then declines. The non-linear variation in the Y s– A relationship can be explained by, first, surface material distribution; second, adjustment of the basin at macro time- and space-scales; and the third, the variation of energy expenditure with drainage-basin scale. As the loess deposits in the high-relief headwater areas is thin, it can be exhausted much more rapidly by flowing water erosion than in other areas, so the underlying erosion-resistant bedrock may be exposed much earlier. In many rivers in the study area, bedrock is exposed in the upper part of the drainage basin, and loess appears in the middle part, with a thickness that increases downstream to a peak, followed by a decline. Due to the influence of these spatial patterns of surface material distribution, a spatial pattern of specific sediment yield appears. The non-linear Y s– A relationship may be interpreted as an indication that the fluvial system of the Yellow River is still at the stage of strong adjustment to the environmental change of Pleistocene–Holocene transition, especially to the change of dominant geomorphic agency from wind to flowing water. This non-linear relationship can also be explained by the variation of stream power with the drainage basin scale. The stream power increases with drainage area to a peak value, and subsequently decreases, a trend that is similar to the trend of the Y s– A relationship.

Soil erosion processes in the loess plateau of Northwestern China

The soil erosion processes in the Loess Plateau may be divided into three types: namely, waterflow erosion; gravitational erosion; wind erosion. The waterflow erosion is most widely distributed and is the main erosion action in the Loess Plateau. The main factors dominating the occurrence and development of the soil erosion in the Loess Plateau are: 1. rainfall; 2. topography; 3. vegetation; 4. soil character. The energy of erosion action depends upon the rainfall and topography, but erodiblity depends upon the vegetation and soil properties. The degree of soil erosion in the Loess Plateau changes with variations of interaction of erosion and anti-erosion measures.