Results of previous experimental assessment of soil erodibility, using the methodology of washing out model samples by a water stream at different installations (hydraulic flume, jetting unit, and water tube), were usually strongly variable. To solve this problem, a new approach to the formation of samples was elaborated. It differs from the previous one by loading the sample through a removable side wall of the horizontally placed cartridge. The layered loading and compaction of samples remains unchanged. The normal position of sample layers to the flow contrary to the parallel position practiced formerly enabled us to obtain a more uniform sample resistance to the flow. The use of a Poseidon measuring unit developed to define mean depth of the wave flow provided not only many-fold increase in the number of measurements of washout intensity during the experiment (from 1 to 14–15), but also enabled us to assess the quality of sample preparation, which increased accuracy and reliability of the study. The percentage standard deviation for erodibility was almost always lower in case of the horizontal loading as compared to the vertical one both between and within the series (by 13 and 12

The erodibility of model soils has been studied using the methodology of washing out soil samples with a water flow in a hydrodynamic tube. Compared to the traditional method of vertical loading of cartridges, the new method of forming soil samples using horizontal loading has made it possible to obtain a more homogeneous resistance of samples during their erosion with the water flow; the layers of soil aggregates were formed perpendicularly to the water flow. The use of the Poseidon measuring device has made it possible to obtain erosion rates in real time, evaluate the quality of sample preparation, and analyze the data for their rejection, in particular, at the beginning and end of the experiments. In addition, the use of the device has allowed us to skip some operations needed to clarify the data of the experiments at their early termination (the consideration of the remainder of a sample, which requires its drying to determine the amount of eroded soil by the mass difference). The data files generated by the device at the end of the experiments significantly simplified their processing and considerably reduced the required time. The coefficient of variation in the erosion rate decreased by an average of 12

The article presents results of experiment and its theoretical justification aimed to study the mechanism and intensity of the early stage of gully head formation. The experiment was carried out using the jet installation that allows to change the angle of the water stream from 0.5 to 90° with the flow velocity of up to 6 m/s. The flow angle was changed with a 10° step, while the flow velocity was maintained in the range of 1.01–1.04 m/s. The intensity of soil erosion positively correlates with the flow angles (angles of attack) in the range from zero up to 40°. When the angle was increased to 50–90°, the intensity continuously declined. The lowest absolute values of erosion intensity were observed when the water flow was normal to the soil surface. There are two main reasons why the intensity of soil erosion changes with changing angle of stream flow; the first is hydraulic, the second is hydro-mechanical. The former determines weakening and destruction of inter-aggregate bonds by the water penetrating into the soil; the latter – by the combined vectors of forces of hydrodynamic head and forces keeping the particle (aggregate) in place. The experiment showed that the maximum impact of water jet on a separate soil particle occur at angle of 41°. The flow in a gully happens occasionally and the results obtained should be attributed to the early stage of erosion: the stage before formation of a so called gully headcut.

Socioeconomic reforms in agrarian sector together with climate changes induce spatial changes in the anthropogenic factors of agricultural soil erosion and soil erosion patterns in the Asian part of Russia. Comparison of pre- and post-reform quantitative parameters of erosion rate and soil loss from arable slopes was performed using logical-mathematical erosion models for different administrative regions of the Asian part of Russia. Significant spatially-differentiated decrease in the annual soil loss from arable slopes was revealed in most of the administrative regions, except for the Altai krai and Amur oblast. On the arable lands of other administrative regions, the decrease in the annual soil loss varied within 25–50% of the soil loss in 1960–1990. The maximum decrease was observed in arid landscapes of the republics of Tyva and Buryatia and in the Zabaykalsky (Transbaikal) krai (–73–93%). Spatial changes in the rate of erosion are less prominent on currently cultivated land; a significant growth in the rate of agricultural soil erosion has taken place in the Far East economic region. The main driver of the post-reform dynamics of agricultural erosion was the countrywide decrease of cropland area and the change in the soil-protecting capacity of agrocenoses.

The erosive capacity of slope water flows, a key parameter in the quantitative assessment of soil erosion, is defined as the difference between the transporting capacity of the flow and the total content of its suspended load and bedload. Therefore, it is necessary to assess the factors and intensity of soil aggregates attrition in the water flow that determine the shares of suspended and dragged particles in the sediment load. The earlier simulation of the attrition of river sediments (H. Sternberg) and soil aggregates (G.I. Shvebs) fail to fully reflect the condition of interaction between soils and slope flows. The further attempts to describe the attrition process using empirical dependences have not given any significant improvements. A fundamentally different model of particle attrition based on the laws of mechanics allows us to describe the attrition of soil aggregates broken away by water flow differentiating the total load between the bedload and suspended load. The experimental verification of the model calculations appears to be satisfactory.

It has been experimentally shown that there are two ranges of water flow velocities, at which the erodibility of a monofractional soil (of aggregates 1–2 mm) sharply differs. In the low-velocity range, the erodibility varies from 171.53 to 3.17 m –2 s 2 at an increase in the soil density from 1.2 to 1.5 g/cm 3 . In the range of high velocities, it varies from 36.88 to 0.88 m –2 s 2 . The simultaneous solution of equations for the two velocity ranges enables us to obtain the boundary values of the flow velocity. Above them, other erodibility values should be taken into account at calculations. The boundary velocities for the model soil are within 1.6–1.7 m/s. This is explained by the fact that at a slow flow, water removes aggregates, which have lost the contact with the main soil as a result of its peptization by water. At high-velocity water flow, aggregates are detached under the effect of hydrodynamic forces.

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/(m 2 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.

It has been shown in experiments in a hydraulic flume with a knee-shaped bend that the rate of soil erosion more than doubles at the flow impact angles to the channel side from 0° to 50°. At higher channel bends, the experiment could not be performed because of backwater. Results of erosion by water stream approaching the sample surface at angles between 2° and 90° are reported. It has been found that the maximum erosion rate is observed at flow impact angles of about 45°, and the minimum rate at 90°. The minimum soil erosion rate is five times lower than the maximum erosion rate. This is due to the difference in the rate of free water penetration into the upper soil layer, and the impact of the hydrodynamic pressure, which is maximum at the impact angle of 90°. The penetration of water into the interaggregate space results in the breaking of bonds between aggregates, which is the main condition for the capture of particles by the flow.

The effect of density of monofraction samples from the plow horizon of leached chernozem on the rupture rate of interaggregate bonds in water has been studied. The rupture rate of bonds has been determined in a hydraulic flume by alternating passive phases of 1–5 min in duration, during which the sample occurs under a nonmoving water layer, with short (15-s long) active phases with a water flow in the flume. Samples have also been tested for tensile strength and water infiltration rate. It has been shown that the rupture rate of interaggregate bonds is related by a hyperbolic law to the soil density and by an exponential law to the rate of water infiltration to the soil. The latter relationship varies within a year and, hence, can be used as reliable parameter for predicting the seasonal dynamics of soil erodibility.