Wind-driven rain influence on interrill erosion using the simultaneous wind-rainfall-runoff simulator

Introduction: Under natural conditions, intensive and erosive storms commonly associate with high-speed winds. In fact, wind velocity affects water erosion rate through enforcing falling drops and enhancing rainfall erosivity. Therefore, knowledge of interaction between wind and rain as erosive agents on interrill erosion is of prime importance. However, no comprehensive study has been done on this topic under controlled laboratory conditions. This study was conducted to investigate interrill erosion affected by different rain intensities and wind velocities on several soils with different aggregate size distributions using the Simultaneous Wind-Rainfall-Runoff Simulator (SWRRS). For this purpose, a multisystem was constructed for the first time in Iran to investigate the simultaneous effects of wind and rain erosivity agents on soil erosion under laboratory conditions. Materials and Methods: The simulator was calibrated in two cases. First, the intensity and uniformity of the simulated rains were assessed for each nozzle, separately. Second, the calibration procedure was performed for different combinations of the selected nozzles to achieve the best performance. For each case, different water pressures were generated to introduce several water discharges and make initial raindrop velocities. Afterwards, the interrill erosion experiment was done using four constant wind speeds including 0, 6, 9 and 12 m s-1at the height of 40 cm which were applied in combination with three rain intensities of 30, 50 and 75 mm h-1 on three soil samples with different aggregate size distributions (D2mm, D4.75mm and D8mm). Each treatment was conducted at three replicates under laboratory controlled conditions. By using different wind speeds, rain intensities and soil aggregate sizes, interrill erosion rate was measured under steady state conditions. Results and Discussion: Results showed that wind velocity has a significant effect on interrill erosion rate and the interaction between wind and rain on interrill erosion was significant, as well. Although, there was no significant difference between the erosion rate at wind velocity of 0 and 6 m s-1, the wind velocity of 9 and 12 m s-1 showed significant difference with and higher erosion rates than the velocity of 6 m s-1. The mean erosion rate at wind velocities of 0, 6, 9, 12 m s-1 was 0.43 × 10-4, 0.54 × 10-4, 0.97 × 10-4 and 1.46 × 10-4 kg m-2 s-1, respectively. With increasing rain intensity from 30 to 75 mm h-1, the erosion rate increased from 0.52 × 10-4 to 1.16 × 10-4 kg m-2 s-1. On average, the erosion rate of the soil containing larges aggregates i.e. D8mm (0.73 × 10-4 kg m-2 s-1) was less than that with the finest aggregates i.e. D2mm (0.99 × 10-4 kg m-2 s-1). The findings of this study highlighted the importance and necessity of more attention to wind speed particularly those velocities faster than a threshold velocity in the study of interrill erosion. Conclusion: In arid and semi-arid regions such as most parts of Iran, rainstorms are usually accompanied by strong winds. Despite the undeniable influence of wind on the erosive power of rain, a host of research has investigated water and wind erosion processes, separately. Therefore, this study was done to investigate the simultaneous effect of wind velocity and rain intensity on interrill erosion rate in three soil samples. The results indicated that wind velocity has a remarkable influence on interrill erosion rate due to wind-driven rain. Wind velocities faster than 6 m s-1 increased interrill soil erosion rate, particularly those combined with higher rain intensities. This is due to an increase in the velocity of falling raindrops on the soil surface which results in greater kinetic energy. Also, the findings showed that the soil containing coarser aggregates due to greater random roughness exhibited less sensitivity and interrill erosion rates as compared with the soil having finer aggregates, especially at faster wind velocities. The rate of interrill erosion in soil D2mm was 1.35 times higher than soil D8mm indicating the importance of random roughness. In addition, there was no significant difference between the measured erosion rates at wind speeds of 0 and 6 m s-1, in all cases. However, with increasing wind speed from 6 to 9 and also to 12 m s-1, significant increases in soil erosion rates were observed. Accordingly, a threshold wind velocity can be considered in wind-driven interrill erosion. The findings of the present study can be applied for better understanding and modeling of water and wind erosion mechanisms and dominant processes.