Effects of surface roughness on sediment heterogeneity under different slopes and rainfall intensities using rainfall simulator

Extended Abstract
Introduction
The efficient management of vital soil and water resources requires a deep and precise understanding of the complex mechanisms of sedimentation and runoff formation in various ecosystems. This understanding must encompass the variable conditions of topography, land slope, and surface cover, as effective erosion control, especially on steep slopes prone to degradation, is considered the cornerstone of sustainable development in environmental and agricultural sectors. The intensity of soil erosion is a function of the complex interaction of numerous factors such as regional climate, inherent soil characteristics, topographic features, land use type; These factors collectively determine the final fate of eroded sediments, which may lead to their drainage from the system or storage in lower points of the watershed. Meanwhile, surface runoff acts as the primary driver for soil particle detachment. Key hydrological processes, including runoff generation, water infiltration, and ultimately sediment transport, are strongly influenced by the physical characteristics of the soil surface. Specifically, surface roughness, or microtopography—which involves small elevation changes (on the scale of 2 to 25 cm)—plays a pivotal role. This roughness, influenced by agricultural activities and vegetation type, directly affects the intensity of erosion process and the overall sediment transport rate by creating resistance to or guiding the flow.
Materials and Methods
This study investigated the effect of surface roughness on sediment heterogeneity using an artificial rainfall simulator. This system, operating at a height of 2.5 meters with a droplet spray mechanism, was installed over a plot measuring approximately 0.9 m×0.5 m to replicate natural rainfall conditions at a small scale. The experimental variables included several key components for system operation and control: an electric motor to supply the necessary power, a computerized control unit for precise nozzle management, a water reservoir, a pump, and a pressure gauge for regulating water flow and pressure. Rainfall intensities (45,60, and 70 mm/h and slopes10%, 20%, and 30%), selected based on the erosivity limits of the study area. Experiments were conducted on both bare and vegetated soil conditions. Each treatment was replicated three times, with each test run lasted for 60 minutes and was divided into six 10‑minute intervals. to allow for the collection and measurement of runoff and sediment yield. The influence of surface roughness on sediment heterogeneity was assessed using indices for intra-cluster and inter-cluster heterogeneity. Total heterogeneity was defined as the algebraic sum of these two components: intra-cluster heterogeneity indicating internal variation within blocks of a cluster, and inter-cluster heterogeneity representing the differences between neighboring clusters (based on rainfall intensity). Furthermore, a two-way Analysis of Variance (ANOVA) was employed to evaluate the main and interactive effects of rainfall intensity and slope on the resulting sediment yield.
Results and Discussion
The highest sedimentation without roughness at an intensity of 45 mm/h was related to a 30% slope, which increased with increasing slope due to increased shear energy and runoff. At an intensity of 60 mm/h, the highest sedimentation without roughness was related to a 20% slope, which indicates the complex effect of slope angle on surface layer protection and the strong role of runoff volume in this rainfall intensity. At an intensity of 70 mm/h, the highest sedimentation without roughness was observed at a 20% slope and the lowest at a 30% slope. In all intensities, the presence of surface roughness generally affected the sedimentation rate. The results also showed that surface roughness is an effective and acceptable factor for adjusting the volume of surface runoff and weakens the effect of runoff washing on different slopes. In general, surface roughness plays an effective role in reducing sedimentation and increases water and soil protection. Vegetation acts as a barrier to runoff, increases infiltration time, and also reduces the kinetic energy of raindrops before they directly hit the soil surface. According to two-way analysis of variance, rainfall intensity and slope are factors affecting sediment production. Increasing flow intensity and kinetic energy of rain make soil particles more likely to be transported. The results also showed that rainfall intensity of 60 mm/h and steep slopes create the greatest heterogeneity (variability) in sedimentation, which is due to the complex interaction of dynamic and physical environmental factors.
Conclusion
This study investigated the effect of surface roughness heterogeneity on sediment yield under different slope gradients and rainfall intensities using a rainfall simulator. The results indicated that at a rainfall intensity of 45 mm h⁻¹, surface roughness significantly reduced sediment yield across all slopes. On smooth surfaces, the highest sediment yield occurred at a 30% slope, while on rough surfaces it appeared at 20%. Roughness reduced flow velocity and enhanced infiltration, thereby limiting sediment transport and controlling erosion. At 60 mm h⁻¹, the maximum sediment yield was recorded on a 20% slope (smooth surface) and the minimum on a 10% slope, which may be attributed to thinner surface soil layers and changes in infiltration capacity. Under a rainfall intensity of 70 mm h⁻¹, the lowest sediment yield occurred on the steepest slope (30%), likely due to increased infiltration resulting from exposure of subsurface pores after the surface layer was eroded by raindrop impact. Two-way ANOVA confirmed that both rainfall intensity and slope gradient had significant effects on sediment yield, with the highest variability observed at 60 mm h⁻¹ and on steep slopes. Overall, findings highlight the critical role of surface roughness in reducing runoff and mitigating soil erosion. Despite experimental limitations such as calibration precision of the rainfall simulator, wind effects, and difficulties in instrument setup on different slopes, the study provides valuable insight into the interactions among rainfall intensity, slope gradient, surface roughness, and sediment generation processes.