Effect of soil water repellency on runoff and erosion under different rainfall intensities

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, thirty minute rainfall intensity (I30) is 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 hydrophobicity (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 sediment or more sediment in hydrophobic soils. Considering Iran's location in arid and semi-arid climates and the importance of studying hydrological phenomena in this climate, simulating the effect of different probability of precipitation intensity on 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 done in 5 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 on five soil treatments under five rainfall intensities with a probability of occurrence of 0 (probability of occurrence), ±10% and ±20% relative to the 30-minute rainfall with a return period of 1000 year of Shahr-e-Kord rain gauge station (base rainfall intensity: 1.41 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 equal to 0.55 KW. There are eight nozzles that provide sprinkler irrigation with square spraying pattern. The spraying flow is adjusted up to max. 1500 l. /hr., by a rotameter underneath the equipment. The required water for spraying is stored in a 220-Litre tank. Working with the equipment is easy. The equipment consists of a soil tray with 2 m length, 1.2 m width, and 20 cm depth. Due to the 20-cm depth of the tray, the upper 5 cm is considered as the free board space. Five cm of the bottom of the tray was filled by sand in order to facilitate and accelerate the drainage of water. A galvanized mesh is placed on the sand. The upper 5 cm of the 10-cm of soil was considered as the surface soil. 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 control treatment, the rainfall duration depends only on the rainfall intensity and the maximum rainfall time is 53 minutes (related to the lowest rainfall level (-20% Probability of the base rainfall) and the minimum rainfall time is also 47 minutes (related to the highest rainfall level (+ 20% Probability of the base rainfall). In the hydrophilic soil treatments in addition to rainfall intensity, the degree of soil hydrophobicity affects also the rainfall time. Since the water absorption is negligible, the rainfall duration is shorter than the control sample. The higher the water hydrophobicity, the less rainfall duration. The less hydrophobicity degree, the less runoff generated. In hydrophobic treatment, due to water repellency, the runoff occurs rapidly, and (hydrophilic soil). The highest observed runoff was 7000cc in severe repellency treatment and the rainfall with +20% Probability, means 120lph. However, the minimum volume of runoff observed in control sample in probability level of -20%, 720 cc. On the other hand, the higher hydrophobicity level, the higher runoff coefficient was observed. Runoff coefficient has been observed in hydrophilic soil from 0.000 to 0.0011 and in hydrophobic treatments 0.05 to 0.94. Severe and extremely severe hydrophobic treatments worked quite imperviously. The rate of sedimentation exited along with runoff is decreases by increasing the hydrophobic level. The highest observed sedimentation was 12gr in hydrophilic treatment and the rainfall with +20% Probability. However, the minimum volume of sedimentation is 2 gr which is observed in extreme soil repellency treatment in probability level of -20%. The observed deep percolation in the control treatment (hydrophilic soil) is 17.5 to 23.8 liters. The deep percolation amount was observed only in low and moderate hydrophilic levels (380 to 950 cubic centimeters). No significant deep percolation, was observed in severe and extreme severe hydrophobic treatments.