عنوان مقاله [English]
Background and Objectives: Rill erosion is a major factor of soil loss in the marl formations. The marl formations are very susceptible to water erosion processes and cover a wide area in some watersheds in arid and semi-arid regions. Rill erosion is active water erosion in these areas which temporally varies during year, from each event to other or during each rainfall event. Temporal variation of rill erosion during a given rainfall event can occur due to the change of soil properties and its effect on the characteristics of concentrated flow. Knowledge of temporal variation of rill erosion and effect of during a rainfall event can provide information on the mechanism of rill erosion in the hillslopes. The rate of temporal variation of flow characteristics and rill erosion can be affected by the rainfall intensity. Therefore, this study was conducted to the study of temporal variation of flow characteristics and rill erosion in a marl soil under different simulated rainfalls.
Materials and Methods: A laboratory experiment was carried out using six simulated rainfall intensities ranging from 10 mm h-1 to 60 mm h-1 with three replications. Soil samples were collected from the marl formations in west of Zanjan and separately purred to a flume with 4m in length and 0.94 m in width putted on 10% slope. Rill erosion and flow characteristics (discharge and concentration) were measured at 5-min from starting flow/ runoff in each rainfall intensity. Rate of rill erosion and flow characteristics versus rainfall duration was obtained and differences among the different rainfall intensities were computed using the variance analysis method. The dependency of rill erosion on the flow characteristics (discharge and concentration) was determined for all rainfall intensities. All data analysis was performed using SPSS version 21.
Results: Results indicated that there are substantial differences in the flow starting time, flow concentration and rill erosion among the rainfall intensities (P< 0.001). Rill flow and erosion rapidly occurred with increasing rainfall intensity. Rill erosion increased speedily during rainfall and reached to approximately constant value in the last times (about 45 min). The flow concentration appeared also a similar trend with the rill erosion, while flow discharge showed an increasing trend in the last times. Most of erodible particles were eroded during 45 min from rainfall and after this time, large flows containing lower concentration/sediment were observed in the rills. A strong relationship was found between rill erosion and flow discharge in different rainfall intensities.
Conclusion: The study revealed that the threshold and pick time of rill erosion were strongly varied during rainfall. The variation trend of rill erosion during rainfall increases with increasing the rainfall intensity. Rill erosion temporally varies during rainfall. Rill erosion increases during rainfall due to increases in flow discharge as well as flow concentration. Rill erosion is strongly dependent on the flow discharge in initial times of rainfall, while in the last times its trend is very different from flow discharge