عنوان مقاله [English]
نویسندگان [English]چکیده [English]
Many factors affect runoff and erosion. However, rainfall and soil characteristics are the two main factors affecting runoff and soil erosion. Among the rainfall characteristics, intensity and duration of precipitation are the two dominant factors that control the hydrological responses. Changes in rainfall intensity have major effects on soil erosion. In this study, the rainfall intensity of ?? minutes(I??) was considered as a rain erodible index based on the kinetic energy of rainfall intensity, which is a well-known indicator in different parts of Iran. Soil Water Repellency (SWR) is a soil feature that affects the hydraulics and hydrologic features of soils. When soil particles are coated with hydrophobic materials, water penetration is severely delayed, which reduces soil moisture capability. Studies have shown that under conditions of soil hydrophobicity, infiltration and runoff production time will be shorter. In the case of high rainfall, the effect of SWR will also be intensified. According to the studies, it cannot be concluded that there is less or more sediment in hydrophobic soils. Iran is located in arid and semi-arid climates and studying the hydrological phenomena in this climate is important. Therefore, the aim of this study was to simulate the effect of different probabilities of precipitation intensity on the volume and coefficient of runoff and sedimentation directly using physical model and also effect. These probabilities on hydrophilic agricultural soils and different degrees of hydrophobic soils are essential both in research and practical projects.
To create the homogeneous hydrophobic conditions in the soil with definite physical specifications, the soil is made hydrophobic artificially by the use of stearic acid. The texture of the tested soil was sandy loam.
The soil hydrophobicity is categorized in ? degrees (hydrophilic as the control sample, slight repellency, strong repellency, severe repellency, and extreme repellency). The amount of the required stearic acid estimated for different hydrophobic levels was determined by WPDT empirical test, together with trial and error considerations. Using a physical model, the amount of sediment and runoff was simulated in five soil treatments under five rainfall intensities with a occurrence probability of ? (probability of occurrence), ± ??% and ± ??% relative to the ??-minute rainfall with a return period of ???? year of Shahr-e-Kord rain gauge station (base rainfall intensity: ?.?? mm/min). The physical model of a special equipment is named “Advanced Hydrological Investigations”. The driving force of the equipment includes an electric pump with the power of ?.?? KW. There are eight nozzles that provide sprinkler irrigation with square spraying pattern. The spraying flow was adjusted up to maximum of ???? L/h, by a rotameter underneath the equipment. The required water for spraying was stored in a ???-Litre tank. The equipment was consisted of a soil tray with ? m length, ?.? m width, and ?? cm depth. Due to the ??-cm depth of the tray, the upper ? cm was considered as the free board space. Five cm at the bottom of the tray was filled by sand in order to facilitate and accelerate the water drainage. A galvanized mesh was placed on the sand. The upper ? cm of the ?? cm soil was considered as the surface soil. The runoff was volumetrically measured and sampled continuously for sediment concentration. The sediment concentration was determined as the ratio of the dry sediment mass to the sampled runoff volume.
In the control treatment, the rainfall duration depends only on the rainfall intensity, and the maximum and minimum rainfall times were ?? (related to the lowest rainfall level, -??% probability of the base rainfall) and ?? minutes (related to the highest rainfall level, +??% probability of the base rainfall), respectively. In the hydrophilic soil treatments in addition to rainfall intensity, the soil hydrophobicity degree also affects the rainfall time. Since the water absorption is negligible, the rainfall duration is shorter than the control sample. The higher the water hydrophobicity was, the less the rainfall duration became.
The less the hydrophobicity degree was, the less runoff was generated. In hydrophobic treatment, due to the water repellency, the runoff occurs rapidly. The highest observed runoff was ???? cc in severe repellency treatment and the rainfall with +??% probability, means ??? L/h. However, the minimum volume of runoff observed in the control sample in probability level of -??%, was ??? cc.
On the other hand, with the higher hydrophobicity level, the higher runoff coefficient was observed. Runoff coefficient has been observed in hydrophilic and hydrophobic soil treatments from ?.??? to ?.????, and ?.?? to ?.??, respectively. Severe and extreme hydrophobic treatments worked quite imperviously.
The rate of sedimentation exited along with runoff was decreased by increasing the hydrophobic level. The highest observed sedimentation was ?? gr in the hydrophilic treatment and the rainfall with +??% probability. However, the minimum volume of sedimentation was ? gr which was observed in the extreme soil repellency treatment with probability level of -??%.
The observed deep percolation in the control treatment (hydrophilic soil) was ??.? to ??.? L. The deep percolation amount was observed only in low and moderate hydrophilic levels (??? to ??? cm?). No significant deep percolation was observed in severe and extreme severe hydrophobic treatments.