The Participation of hill slopes sediment delivery contribution in Rainfalls different patterns by determine of the degraded Rills Volume

Background and objectives: Soil erosion is a complex phenomenon involving the detachment and transport of soil particles, storage and runoff of rainfall. Generally the rill and surface erosions are among the main sub-processes of water erosion. Obviously, determination of the rill and surface erosion proportions in a rainfall event has an intrinsic value in order to develop of physically based model of erosion. The rainfall intensities and frequency are the effectible factors important on magnitude of the soil loss. The purpose of this study is for the assessment effect of the rainfall patterns on sediment delivery contribution in the steep-slope on the Marl soils.

Materials and methods: For this purpose, was used from liquid paraffin technique for determine rills volumes, on two the slope length, two slope gradient and three rainfall intensity and they were carried out in three repetitions. At the initial stage of each experiment, the flume was set at the desired slope length, slope gradient and rainfall intensity. In each test, the starting time of the simulated rainfall, the time when runoff reached the outlet of the plot, and the time when rill initiation occurred were recorded. In each experiment, sediment samples together with runoff were taken every minute for about 30 minutes after the start of the runoff at 1,2,3,5,8,10,15 and 30 minute's. The runoff discharge, runoff volume, and sediment concentration and rill numbers were measured at the outlet of the test plot for different rainfall intensities and slope lengths and gradients.
Results: The result showed that the rainfall intensity of 10mm/h and slope of 22% do not create rill formation during 30min rainfall in the Marl soil and all output sediment at the bottom of the flume is caused by surface erosion events. The results showed that the rill and surface erosion on the both increase exponentially with increasing slope length and gradient and rainfall intensity levels, simultaneously. Also, with continuing rainfall, however surface erosion contribution is reduced but the rill erosion contribution is increased. In the conditions, average of the surface erosion increased from 21.2 to 527×10-6 Kg m-2 s-1 and rill erosion from 19.4 to 3172 ×10-6 Kg m-2 s-1. With the increase in the rainfall intensity, soil loss changes to rainfall changes ratio (loss/ precipitation), to the low intensity (10 mm/h) 1.5 times and for high intensity (110 mm/h) about five times has shown growth.

Conclusion: Finally the results showed, doubling of the plot length not only hasn't affected on the amount of surface erosion contribution but also has increased contribution of rill erosion about 2.1 times. So with increasing the mentioned parameters, remained constant the ratio the surface erosion contribution, but added the ratio of rill erosion contribution. In upper rainfall intensities, the rill erosion and surface erosion is contributed to 90 and 10 percent of amount sediment yield of plot, respectively. But in lower rainfall intensities surface erosion is dominant.