Determining the Most Important Indicators of Surface Crust Resistance and Their Effect on Wind Erosion Control in Dust Storm Sources in Khuzestan

Introduction
In most arid and semi-arid areas, the soils have a compact surface layer that is denser and less permeable than the underlying layers. These layers are referred to as surface coating when they are wet, and when they dry, they are referred to as surface crust. Surface crust can be divided into physical, chemical, and biological crusts depending on the nature of formation. Physical and biological soil crusts are the most significant types of soil crusts in arid and semi-arid regions. Two types of the physical crust are structural and depositional. Natural events, such as raindrops and the drying process, are the primary causes of the formation of surface crust. The process involves the creation of hard, thin layers on the surface of the soil. The thickness of the surface crusts is typically between 1 mm and 5 cm. Many researches have described different theoretical mechanisms for the formation of surface crusts in soil and reported their effective role in controlling wind erosion. The formation of crust in soil involves multiple stages, and the reaction of the soil surface to raindrop energy is divided into two main parts, as it was demonstrated. The first part involves the splashing of soil particles by raindrops' impact. A layer with a thickness of approximately 0. 1 mm is produced in this instance. The second component is composed of fine soil particles that penetrate the soil pores with water and cause the formation of a layer that is 2 mm thick. According to a study, soil loss control is not affected by the crust cover beyond 30%. There is a linear relationship between soil loss and surface crust coverage, as the surface crust cover develops gradually, soil loss differs between soil types.
 
Material and Methods
During field surveys of dust storm sources in Khuzestan, it was observed that there are surface crusts that are strong enough to act as a barrier against wind erosion in certain locations. The


 
suggestion was made to investigate the type and general characteristics of these surface crusts. The purpose of this study is to examine the general characteristics of these surface crusts and their effect on wind erosion control. The study was conducted by randomly obtaining 18 surface crust samples, each containing three sub-samples from a depth of 0 to 5 cm from the dust storm sources of Khuzestan province. After being air dried, they were then passed through a sieve of 2 mm. The factors of pH, EC, CaCo3, CEC, ESP, Ca, Mg, Po^3-₄ texture, initial moisture, and apparent density of the surface crusts were measured. The soil's sensitivity to crust formation was assessed by using soil stability index, Crusting Index, pressure resistance, and shear resistance. The stability of soil aggregates was assessed using the mean weight diameter and geometric mean diameter indices. Soil samples were collected and tested in a wind tunnel to examine the impact of these surface crusts on wind erosion control and the amount of soil loss and the speed of wind erosion were assessed. An orbital wind tunnel device was utilized for this purpose. Trays with soil were placed on the bottom of the tunnel in this manner. After the wind blew (speeds of 15, 10, 25 m/s) by measuring the difference in the weight of the tray before and after the wind blowing, the amount of erosion was calculated from a certain surface. Then, the threshold speed for wind erosion was determined. Statistical analysis was carried out using SPSS 26 software. The Kolmogorov-Smirnov test was used to verify data normality. Then, the data sets were entered into a step-by-step regression as independent components to determine their relationship with the index of crust formation and stability of soil aggregates, as well as the effect of crusts and stability of soil aggregates on the amount of wind erosion. The accuracy of the regression models was verified by using RMSE, RSE, MAE, and R² statistics. The morphology of the crusts was finally determined by using a scanning electron microscope.
 
Results and Discussion
The results showed that the lowest amount of RMSE, and RSE statistics, and the lowest absolute error for evaluating the sensitivity of soil to crust formation in the dust storm sources of East Ahvaz, Southeast Ahvaz and Omidieh, Mahshahr and Hendijan, respectively belong to the index of compressive strength, soil stability and shear resistance. The scanning electron microscope images taken of the crusts of three dust storm sources indicated that small foreign matter is present in the soil's pores. The features of the sedimentary crust are displayed in all the images. The external materials in the crust structure reflect the different washing processes in soil particles, which cause adhesion and connection between the soil particles and make the crusts resistant to wind erosion. Based on the given explanations, it is possible that the crusts found in Khuzestan's dust storm sources are usually sedimentary crusts. Results also showed that the wind erosion threshold speed in the dust storm sources of East Ahvaz, South East Ahvaz and the sources of Mahshahr, Omidiye and Hendijan are 1. 44, 1. 62 and 2. 1 time their powdered state, respectively. These sources experience a soil loss of 3. 55, 2. 09, and 3. 01 times the intact crusts, respectively.