Simulation of moisture distribution in different soil textures under a point source using moment analysis method

Introduction

In arid and semi-arid regions, such as Iran, in order to use water optimally and prevent its wastage and evaporation, drip irrigation system is used. Factors such as soil texture, type of cultivated plant, amount of available water, distance of drippers and laterals, as well as the wetted surface and the dimensions of the moisture bulb under the soil surface are involved in the design of the drip irrigation system. Due to the variety of soil texture in the earth, the movement of water under the soil surface is different in all kinds of textures, therefore, knowing exactly how water moves in the soil and how the moisture bulb is distributed under the soil surface is of particular importance.

The purpose of this study is to know the way of movement of moisture bulb and checking its dimensions under the soil surface in different types of soil textures under different currents and evaluating the capability of Moment analysis method in simulating this process in different conditions.

Materials and Methods

In order to simulate the moisture bulb in different types of soil texture, detailed information on the physical properties of the soil, including the percentage of particles that make up the soil texture, apparent and real density of the soil, porosity, and saturated hydraulic conductivity is required. In this research, the simulation of the moisture front in the types of soil texture were defined in Rosetta software, which includes 12 types of soil textures. In these tests, the source of soil power was considered as surface and point. The total feeding volume of each type of soil texture is 24 liters, and this volume was used with different flow rates of 2, 4, 6 and 8 liters per hour. In order to numerically simulate the progress of the moisture front, Hydrus software was used. Then the analytical simulation of the moisture front was done using the equations of the Moment analysis method. In this study, the aim is to draw an ellipse corresponding to the humidity bulb simulated by Hydrus software, at different times according to the types of applied flow rates. Coefficient k is used in drawing an ellipse, the appropriate value of which is determined by minimizing the difference between the model and Hydrus results.

Results and Discussion

In order to calculate the moments, the first step is to obtain the values of 𝑀𝑜𝑜. According to the applied flow rates of 2, 4, 6 and 8 liters per hour and the amount of volume intended to feed all types of soil texture, i.e., 24 liters, the duration of irrigation is 12, 6, 4 and 3 hours, respectively. The comparison of moisture distribution in all time periods and in all soil, textures is acceptable and the distributed subsurface moisture values are similar. In the study of clay texture, with the passage of time from the start of irrigation, the difference in the total amount of distributed moisture increased, and the reason for this result is the decrease in the permeability of the clay due to the filling of fine pores. The above results state that the values of 𝜎𝑥2 change with the increase of irrigation duration. In general, with the passage of time from the beginning of the irrigation process, the values of 𝜎𝑥2 take an upward slope. Also, with the increase of the input flow rate, the range of variance changes is smaller, but the variance values increase. The highest amount of variance is related to sandy clay with an applied flow rate of 8 liters per hour with a value of 1503.3 square centimeters, and the lowest value is related to clay texture with an applied flow rate of 4 liters per hour with a value of 368.6 square centimeters. Due to the longer irrigation time in the applied flow rate of 2 liters per hour, the obtained values of variance increased and the lowest value was obtained in the application of the flow rate of 4 liters per hour. By checking the calculations, it was concluded that the values of σz2 change with the change of applied flow rate in different types of soil texture. In other words, with the increase in the amount of applied discharge, σz2 increases and the slope of this increase is different in each soil texture, according to the characteristics of that texture. Also, the effect of irrigation duration on the value of σz2 is evident. In other words, the longer the duration of irrigation, the more the amount of variance changes.

Conclusion

In the conducted research, in order to check the accuracy of the analytical method of moment calculation, in order to predict the moisture level obtained from drip irrigation, the results of Hydrus and Moment analysis was used. The results obtained from the Hydrus showed that with the passage of time from the start of the irrigation process, the amount of expansion of the moisture bulb increases in the horizontal direction as well as in depth. The increase in the flow rate of the droppers caused more expansion of the moisture bulb in the horizontal direction, and it was also stated that the time factor has a direct relationship with the expansion of moisture in the horizontal and depth direction. Hydrus data was used in the Moment method. The results showed that with the help of moment analysis method, the position of the center of mass of water distributed in the soil, the changes of the moisture front with respect to the x and z axis can be obtained. By examining and comparing the dimensions of the moisture front resulting from hydrus and ovals, it was observed that there is a suitable compatibility between the two methods. Therefore, it is possible to rely on the results obtained from the Moment analysis method to check the dimensions of the moisture bulb resulting from drip irrigation, and with the provided relationships, the duration of experiments and field researches can be carried out optimally and in the shortest time.