Simulation of Water and Salinity fluxes in Tape Irrigation in Maize Cultivation Using HYDRUS-2D Software

Due to water resources shortage, better agricultural water using is one of the most important challenges facing the agricultural water sector. Today, the use of pressurized irrigation systems such as tape irrigation system is one of the best ways to improve agricultural water use. Due to increasing use of tape irrigation systems in row crops and the importance of understanding how to distribute moisture and salts in these irrigation methods, the purpose of this study is to use HYDRUS -2D software for simulation. The geometry of the flow and movement of water and solutes in the soil was tape irrigation method. For this purpose, while modeling the flow geometry by two-dimensional HYDRUS -2D software using observational data obtained from field experiments, hydraulic parameters and soil solute transfer were optimized by reverse solution method.

The experiments were performed on corn plants in the Research Farm of the Department of Water Science and Engineering, Imam Khomeini International University, located in Qazvin. The experiment started in August 2020 and ended in mid-November after 105 days of corn growth period. Corn was cultivated in plots with an area of ​​9 square meters with dimensions of 3.3 meter. The distance between planting rows was 75 cm and the distance between corn seeds on the ridges was 30 cm. The irrigation tape used in the experiment was of the plate type tape and the discharge of each plate at the operating pressure of the experiment was measured as 1.3 liters per hour. The distance between the plates on the irrigation strip was 25 cm. Profile probe PR2 was used to measure soil moisture to determine the time and duration of irrigation. To do this, in the middle of each plot and to a depth of one meter, the tubes of the device were placed by using the auger. Due to the high price of the device and for economical use, high pressure polyvinyl pipes with an internal diameter of 26 mm, were used which had already been calibrated. To save and reduce the volume of operations, assuming the soil is homogeneous, harvests were made on one side of the ridge. The HYDRUS -2D software package uses the numerical solution of the Richards equation to analyze the motion of water in a porous medium in the saturated and unsaturated states. The HYDRUS -2D uses various models to estimate soil hydraulic parameters. In the mentioned model, the initial guess values ​​of soil hydraulic parameters are estimated using a neural network paired in a model called Rosetta and soil information such as texture and percentage of its components and some moisture points in the soil characteristic curve.

The results showed that although HYDRUS -2D model has a high ability to simulate the movement of water in porous media, however, this issue is directly related to the accuracy of the flow geometry and the limitations of the HYDRUS -2D model regarding the definition of irrigation systems should be considered. In addition, to evaluate the accuracy of data estimates by the model, it is not enough to just check the appropriateness of statistical indicators. The estimation of the model from the soil moisture characteristic curve as well as the water balance given to the model should be controlled. In the conditions of this research, the best result was obtained when the length of nodes with variable flow boundary condition was equal to 3.14 cm. For optimizing soil hydraulic parameters, NRMSE statistical indices of 9.3%, RMSE of 0.025 (cm3 cm-3) and MSE of 0.00066 (cm3 cm-3) show excellent model simulation. In addition, the water balance given to the model by the software was calculated to be equal to 285 liters, which was close to the actual amount of water given to the ground, i.e. 253 liters. The values ​​of the above indices for optimizing the parameters of solute transfer in soil were 22.2%, 0.17 (mg cm-3) and 0.028 (mg cm-3), respectively.

For evaluating the accuracy of the results presented by the model, it is not enough to just place the statistical indicators in the appropriate range and judge the results based on it, but also to control the initial conditions, soil moisture limits and also compare the volume of water. The data given by the model with the actual volume of water given are among the items that should be checked. Based on the results of this study, the accuracy of the water balance presented by the model is directly related to the length of nodes with variable flow conditions. Therefore, it seems necessary to study and determine the optimal length of nodes with variable flow boundary conditions before performing any simulation operation.