Sheet and interrill erosion areas are sources of soil material rich in nutrients and pollutants. The loss of soil, nutrients and other chemicals from these areas is a matter of concern both in terms of maintaining soil productivity and the health of offsite environments. Many experiments on rainfall erosion have shown enrichment of fine material, nutrients and other chemicals in the sediment discharged for sheet and interrill erosion areas, but often these results were obtained over short periods of time. A qualitative mechanistic model of raindrop-induced saltation is used to illustrate how this transport mechanism influences the composition of sediment discharged by rain-impacted flow. Initially, fast moving particles are enriched in the sediment discharge but, over time, during a rainfall event, slower moving particles become more represented. Raindrop-induced saltation promotes the storage of material on the soil surface with a coarser composition than the original soil. Winnowing of material from this storage by the development of flow-driven saltation during high-intensity events can modify the composition of the sediment discharged later by raindrop-induced saltation. Given stable soil particles, the composition of the sediment discharged at the steady state is the same as the original soil. Enrichment is a non-steady-state phenomenon and failure to recognize the transient nature of enrichment may lead to inappropriate interpretation of the implications of the results from short-term experiments. Copyright © 2011 John Wiley & Sons, Ltd.

Raindrop‐induced saltation and the enrichment of sediment discharged from sheet and interrill erosion areas

A mechanistic model was used to simulate the downstream movement of particles travelling in suspension, by raindrop induced saltation and flow driven saltation on planar surfaces ranging from 2 m to 30 m in length. Results were produced for two rainfall events, one of high intensity, the other of moderate intensity, and a mixture of particle sizes ranging from fine material to 0.9 mm sand. Initially, the composition of the sediment discharge was controlled by the composition of the soil matrix and the rate the raindrop driven transport systems were capable of transporting the particles from zones near the downstream boundary. As time progressed, material transported from upslope provided a degree of protection in these zones and controlled the composition of the sediment discharge. The protection provided reduced the discharge of fine material but had little effect of the composition of the discharge of the coarser material during a given rainfall event so that it remainder finer than that of the soil matrix. However, in the long term, the slow movement of the coarsest material into the zones controlling sediment discharge led to coarsening of the sediment discharge so that it tended to the composition of the soil matrix at the steady state. The time taken to reach that steady state may be considerable, and increases with plot length, so that many many storms may occur before the steady state condition is reached.

The influence of raindrop induced saltation on particle size distributions in sediment discharged by rain-impacted flow on planar surfaces

Simulations using a mechanistic model of raindrop driven erosion in rain-impacted flow were performed with particles travelling by suspension, raindrop induced saltation and flow driven saltation. Results generated by both a high intensity storm, and a less intense one, indicate that, because of the effect of flow depth on the delivery of raindrop energy to the bed, there is a decline in sediment concentration, and hence soil loss per unit area, with slope length when particles are transported by raindrop induced saltation. However, that decline is reversed when the critical velocities that lead to flow driven saltation are episodically exceeded during an event. The simulations were performed on smooth surfaces and a single drop size but the general relationships are likely to apply for rain made up of a wide range of drop size. Although runoff is not always produced uniformly, as a general rule, flow velocities increase with slope length so that, typically, the distance particles travel before being discharged during an event increase with slope length. The effect of slope length on soil loss per unit area is often considered to vary with slope length to a power greater than zero and less that 1·0. The simulations show that effect of slope length on sediment discharge is highly dependent on the variations in runoff response resulting from variations in rainfall duration-intensity-infiltration conditions rather than plot length per se. Consequently, predicting soil loss per unit area using slope length with positive powers close to zero when sheet erosion occurs may not be as effective as commonly expected. Erosion by rain-impacted flow is a complex process and that complexity needs to be considered when analysing the results of experiments associated with rain-impacted flow under both natural and artificial conditions. Copyright © 2009 John Wiley & Sons, Ltd.

The Impact of Slope Length on the Discharge of Sediment By Rain Impact Induced Saltation and Suspension

The effect of long-term (about 25 years) use of different farming practices on a set of soil properties and development of erosion in Lower Austria has been studied. Three tillage systems—zero, or no-till (NT); minimum, or conservation (CS); and conventional (CV)—are compared. The comparison demonstrates that the properties of Typic Argiudols (Luvic Phaeozems), formed on steep (13.2%) slopes, change depending on both the tillage type and the position on the slope. Unlike the CV tillage, the soil-saving technologies provide higher contents of nutrients, silt, and clay, as well as better water permeability and water stability of soil aggregates. Despite an almost doubled amount of lumpy fractions (>10 mm), the soil aggregate states after NT and CS tillage are estimated as “excellent”. Independently of the tillage system, all agrochemical, electrophysical, and hydrophysical parameters (except for pH and bulk density) increase downward the slope, which is associated with erosion, namely, the washout of suspended sediments by water flows. The Corg content in the soil tightly correlates with the water stability of soil aggregates (r = 0.91), the concentration of soluble humic substances and fine solids (SAK; r = 0.76), and electroconductivity (r = 0.75). An anti-erosion efficiency of tillage practice increases in the series CV–CS–NT. The NT or CS systems are recommended for the erosion-prone slopes of Alpine foothills.

The Impact of No-Till, Conservation, and Conventional Tillage Systems on Erosion and Soil Properties in Lower Austria

It has been revealed that, in the years when the freezing depth of leached chernozems on slopes in the southern Cis-Ural region is in the range of 30–38 cm, no surface runoff is observed in the spring regardless of the snow cover, soil wetting, or vegetation; the critical freezing depth under forest is half that on the open surfaces. When the freezing depth is deeper, the critical volume of the spring runoff is mainly determined by the meteorological conditions, the water permeability of the soil, and its protection by plants.

Snowmelt-induced soil erosion on gentle slopes in the southern Cis-Ural region

Socioeconomic transformation together with climate change in recent decades significantly affected the geography of agricultural erosion in European Russia. Calculations of erosion rate and soil loss from slopes using logical-mathematical erosion models within different landscape zones and administrative regions revealed spatial-temporal regularities in the dynamics of these parameters and made it possible to assess the role of changes in the main natural and anthropogenic factors of erosion. A universal significant reduction in the mass of soil material washed from tilled slopes is revealed on the background of multidirectional changes in erosion rate.

Dynamics of agricultural soil erosion in European Russia

Field studies and model experiments, as well as theoretical considerations, suggest that bed sediments represented by soil aggregates in overland flows on slopes exert a considerable influence on the intensity of erosion processes. In this context, one of the key problems in the development of adequate erosion models is the problem of the rate of destruction of such aggregates in the flows. The results of experimental studies of the destruction of aggregates of chernozemic soils are analyzed. It is found that the destruction of soil aggregates in the flow proceeds in two stages. During the first stage, the aggregates are rapidly broken apart into smaller fragments. During the second stage, these fragments are subjected to abrasion. An equation describing the destruction of aggregates upon their movement in the flow in dependence on the aggregate size and the distance of aggregate transport is suggested. The effect of some groups of soil microorganisms on the aggregate resistance to the destruction is shown.

Destruction of soil aggregates in slope flows

The effect of different factors and preparation conditions of monofraction samples from the arable horizon of leached chernozem on soil erodibility and its relationship with soil tensile strength (STS) has been studied. The exposure of samples at 38°C reduces their erodibility by two orders of magnitude. The drying of samples, on the contrary, increases their erodibility. It has been shown that erodibility decreases during the experiment. It has been found that the inoculation of soil with yeast cultures (Naganishia albida, Lipomyces tetrasporus) reliably increases the STS value in 1.5–1.9 times. The sterile soil is eroded more intensively than the unsterile soil: at 4.9 and 0.3 g/(m2 s), respectively. The drying of soil followed by wetting to the initial water content (30%) has no significant effect on the STS value in almost all experimental treatments.

Experimental Study of Factors Affecting Soil Erodibility

The close almost functional relationship of the erosion rate and, hence, the erodibility of model soil samples with the temperature of the water used in the experiments has been shown. This suggests that the rupture of bonds between the particles of eroded soil samples is due to the electrostatic forces appearing between the monomolecular water layers around the adjacent soil aggregates similarly oriented with respect to the soil solid phase rather than to the hydraulic forces. The erosion parameters of the samples also strongly depend on the soil moisture. The lowest erosion rate of the heavy loamy chernozem samples is observed at an initial water content of 22–24%. The erosion rate increases and the variability of the results is reduced with both decreasing and increasing the initial water content.

Effect of the water temperature and soil moisture on the erodibility of chernozem samples: A model experiment

The soil erosion rate was studied on chernozemic soil samples with different densities in the flows with different contents of suspended sediments. It was shown that the soil loss was strongly reduced in the presence of suspended sediments. The reason was the silting up of interaggregate pores by sediments, which increased the forces of adhesion between aggregates and, hence, decreased the erodibility of the soil samples. An empirical equation was proposed to estimate the effect of suspended sediments on the erodibility of the soil.

L. F. Litvin

Papers

Dynamics of agricultural soil erosion in European Russia

Destruction of soil aggregates in slope flows

Experimental Study of Factors Affecting Soil Erodibility

Effect of the water temperature and soil moisture on the erodibility of chernozem samples: A model experiment

Effect of suspended sediments on soil erodibility