Impact of Rainfall Intensity, Slope Inclination, and Soil Wetness on Erosion of Agrochernozems: The Case Study of a Model Experiment

Water erosion is one of the main types of soil degradation in the Southern Cis-Urals and in the Republic of Bashkortostan (RB), with rainfall erosion causing the greatest negative impact. However, current data on rainfall erosion rates and mechanisms of its development are insufficiently studied in the region. The use of rainfall simulators significantly accelerates the acquisition of up-to-date data on the development of erosion processes compared with in situ studies. In this research study, the influence of different rainfall types (light rain (25 mm·h−1), heavy rain (50 mm·h−1), and storm (100 mm·h−1), as the most common in the region and potentially erosion-hazardous), slope gradient (1–15°, as frequently encountered), and soil wetness (dry and wet) on the erosion of bare agrochernozems (as the most dominant soils in the RB and Ufimsky District in particular) was studied using a rainfall simulator under laboratory conditions. It was found that the onset of surface runoff and, consequently, soil loss were not recorded on dry soil after an hour of light rain at any of the studied slopes and up to 6 degrees under heavy rain; starting from this inclination, the soil losses gradually increased with the increase in slope but were generally insignificant (0.6–2 t·ha−1). Doubling the intensity (from heavy rain to storm) resulted in losses increasing tenfold (up to 21.3 t·ha−1). Runoff on wet soil appeared 31–44 min earlier than on dry variants during heavy rain and 7–19 min earlier during a storm, where it was recorded to be faster on steeper slopes. Compared with dry soil, sediment yield on wet soil was 14 times greater under heavy rain (4–25.3 t·ha−1) and 5 times greater under storm conditions (6–110.6 t·ha−1). On both dry and wet soils, sediment yield rose with the increase in slope, and on slightly inclined plains and gentle slopes, it was insignificant, but starting from moderate slopes, soil losses increased sharply. The average turbidity of runoff was somewhat higher for dry soil variants than wet ones, but in both cases, it was at its maximum at the beginning of runoff or at its 5th min; then, it gradually decreased and stabilized, graphically represented by the shape of a deformed bell. All sediments formed after rainfall simulations with different variants contained more fine particles and less sand compared with the initial test soil. Moreover, as the intensity of rain and the slope gradient increased, the proportion of clay and silt in them decreased, and the share of sand increased; i.e., with the acceleration of the flow/runoff rate, the sand particles became more available for their detachment and transportation. To sum up, rainfall intensity, slope gradient, and soil wetness are the leading factors in the development of rainfall erosion, and the results of the present study can be useful for taking appropriate soil conservation and soil management measures, as well as calculating potential soil losses or performing erosion modeling for the studied region or areas with similar natural geographical conditions.