The Effect of Raindrop Impact on Runoff and Soil Loss from Rills under Different Rainfall Intensities

Introduction  Rill erosion is one of the main factors of soil degradation, especially in rainfed lands in semi-arid regions. These soils have relatively lower organic matter content with weakly-aggregated units, which increases their susceptibility to water erosion processes. Conventional tillage systems are adversely affect on soil structure and surface soil cover in rainfed lands. Raindrop energy and flow shear stress are the main erosive factors in the slope lands. The raindrop impact destroys soil structure and changes it to erodible unites; micro-aggregates and single particles, and so makes them to more detachment. A few studies have been done on the role of raindrop impact to soil erosion by water. Nevertheless, there is no sufficient information on the effect of raindrop impact on soil loss in the rills particularly in semi-arid regions. Therefore, this study was conducted to investigate the role of raindrop impact on soil loss from rills in various soil textures under different rainfall intensities.   Materials and Methods  A laboratory experiment was performed on two soil textures (clay loam and sandy loam) under four rainfall intensities (30, 50, 72 and 83 mm.h-1) in two rainfall conditions (under raindrops impact and without raindrops impact). Soil samples (0-30 cm) were taken from a semi-arid region in Zanjan province in 2020. The experiments were set up in an erosion flume with 100 cm long and 60 cm width and 15 cm depth which were exposed to simulated rainfalls for 30 min duration. Runoff and soil loss were measured at three rills under slope gradient 10% in the two rainfall conditions for each rainfall intensity. Soil loss from rills was determined as the mass of sediment collected from rill outlet per rill surface area (g.m-2). Under raindrop impact, the soil was exposed directly to raindrop impact and under without raindrop impact, a metal mesh sheet was used to eliminate raindrops impact to soil surface. The role of raindrops impact to runoff and soil loss was computed from the difference of runoff and soil loss under raindrops impact and without raindrops impacts. A t-test was used to assess the role of raindrops impact between the two rainfall conditions for the soils and rainfall intensities.     Results and Discussion  Results indicated that runoff production and soil loss were significantly affected by the soil texture and rainfall intensity. Runoff and soil loss under raindrops impact increased in the soils with increasing rainfall intensity. Clay loam showed more runoff production and soil loss than sandy loam which was associated to lower aggregate stability and hydraulic conductivity. Runoff and soil loss in the two soils and four rainfall intensities were significantly affected by raindrops impact. Runoff production and soil loss except to 72 mm.h-1 rainfall intensity were very higher under raindrop impact than without raindrop impact. It seems under 72 mm.h-1 rainfall intensity, raindrops impact varied the rill’s morphology and prevent more runoff production. Runoff production in clay loam and sandy loam under raindrop impact were increased by 44 and 36 percent, respectively (p< 0.01). Soil loss resulted by raindrop impact in clay loam and sandy loam increased by 53 and 62 percent, respectively (p< 0.01). Raindrops impact had more importance in soil loss rather than runoff production. This result is related to the role of raindrops impact in destroying aggregates and producing more erodible soil particles and closing soil macrospores and declining water infiltration. The role of raindrop impact in runoff production and soil loss varied among the rainfall intensities. A slight reduction in the role of raindrop impact in runoff and soil loss was occurred with increasing rainfall intensity, especially in sandy loam.   Conclusion  The role of raindrop impact in runoff production and soil loss was significantly affected by soil type and rainfall intensity. Raindrops impact has more important in runoff and soil loss in the soils having higher aggregate stability and more hydraulic conductivity. The role of raindrop impact in runoff and soil loss in these soils declines with increasing rainfall intensity. In general, maintain soil surface cover is essential to control raindrops impact and decrease runoff and soil loss in semi-arid areas. The importance of soil surface cover is most obvious under different rainfalls in weakly-aggregated soils which are dominant in many slope lands. Also, soil surface cover has important role in controlling runoff and soil loss under heavy rainfalls in soils with more water-stable aggregates. Prevention from intensive tillage and using conservation tillage systems such as minimum tillage are effective strategies in controlling raindrop impact in rainfed lands in semi-arid regions.