Study of Water Flow and Nitrate Transport under Center Pivot Irrigation Using the HYDRUS-3D Model

Document Type : Research Article

Authors

• S. Etminan ¹
• V. Jalali ²

¹ , Faculty of Agriculture, Shahid Bahonar University of Kerman, Iran

² Department of Nature Engineering, Shirvan Faculty of Agriculture, University of Bojnord, Iran

10.22067/jsw.2025.93128.1480

Abstract

Introduction 
Climate change and drought events over the past decades have led to a decrease in surface and groundwater resources, particularly fresh water sources. On the other hand, the global population growth rate is increasing, which has resulted in a rising demand for food. One of the essential pillars of food security is providing adequate water resources for agricultural use. In recent years, water resource management aimed at improving efficiency has emerged as both a management and research challenge. One proposed strategy is blending saline and fresh water with the application of modern irrigation techniques at the farm scale. This study examines the effects of center-pivot irrigation using saline water sources on alfalfa farm performance over four consecutive years. The effect of this management approach was analyzed using the HYDRUS-3D model, focusing on plant growth and yield, soil moisture variations, leaching rates, and nitrate accumulation within the farm.
 
Materials and Methods
In this study, a four-year-old alfalfa farm at Birjand University, covering an area of 3 hectares and irrigated using the center-pivot method, was selected. A one-meter-deep soil profile was excavated to examine changes in the soil's physical and chemical properties over time. Soil moisture was measured using Time Domain Reflectometry (TDR) at different depths and various irrigation intervals to assess root-zone moisture dynamics. Since the soil in the study area was deficient in organic matter, soil samples were collected before planting alfalfa. To ensure adequate phosphorus levels, diammonium phosphate fertilizer was applied during the tillering stage. Ammonium and nitrate concentrations were also analyzed by collecting soil samples at different depths over various periods and measuring them using a spectrophotometer. The Levenberg-Marquardt optimization algorithm was employed to estimate hydraulic parameters and solute transport characteristics. To model changes in ammonium and nitrate levels in the soil, the Freundlich adsorption coefficient was applied. For simulating variations in soil moisture, ammonium, nitrate, and plant growth trends in the second and fourth years, the HYDRUS-3D hydraulic model was utilized.
 
Results and Discussion 
The accuracy and efficiency of the HYDRUS-3D model in analyzing soil variations in terms of water flow and solutes transport were assessed using two statistical indices: RMSE (Root Mean Square Error) and NSE (Nash-Sutcliffe Efficiency). The RMSE values of calibration for soil moisture variations at three studied depths (0-40 cm, 40-60 cm, and 60-100 cm) were 0.0057, 0.0049, and 0.0044 cm³.cm^-3, respectively. The NSE values at these depths during the calibration phase were 0.91, 0.98, and 0.99, respectively. For the validation phase, the RMSE and NSE values at 0-40 cm were 0.0021 and 0.97 cm³.cm^-3, at 40-60 cm were 0.0038 and 0.99, and at 60-100 cm were 0.0029 and 0.99, respectively. Based on the results, the efficiency of the Levenberg-Marquardt optimization algorithm and the capability of the HYDRUS-3D model in simulating soil moisture dynamics in the plant root zone under center-pivot irrigation were verified. The RMSE values for ammonium simulation at the three depths during validation were 0.0055, 0.0003, and 0.0008 mg.l, while the NSE values were 0.97, 0.99, and 0.99, respectively. For nitrate concentration analysis at the same depths, the RMSE values were 0.009, 0.009, and 0.008 mg.l, while the NSE values were 0.99, 0.98, and 0.99, respectively. These findings confirm the effectiveness of the HYDRUS-3D model in estimating solute variations in soil. However, accuracy decreased with depth due to soil heterogeneity and unsaturated conditions, as the model assumes a homogeneous environment. Nitrate accumulation in plants showed an increasing trend as the plant growth period increased. The measured nitrate concentration in two-year-old alfalfa was significantly lower than that in four-year-old alfalfa. Additionally, nitrate accumulation in the root zone of four-year-old plants was higher than the younger ones. This process is influenced by fertilization practices and the expansion of the root system in the fourth year, which enhances nutrient uptake efficiency.
 
Conclusion 
Based on the statistical indices obtained from the simulation of soil variations using the HYDRUS-3D model compared to measured values, it can be concluded that the Levenberg-Marquardt optimization algorithm had provided an accurate and practical estimation of soil hydraulic parameters under the applied management conditions. Furthermore, the HYDRUS-3D model had effectively simulated long-term variations over a four-year period within this management framework. Therefore, both the optimization algorithm and the HYDRUS-3D model demonstrated sufficient capability for assessing soil moisture dynamics and solute variations under modern irrigation management techniques at the farm scale. These methods can serve as powerful tools for formulating management strategies and evaluating the outcomes of different irrigation practices.

Keywords

• Hydrous model
• Modern irrigation
• Reverse solution
• Soil salts

Main Subjects

• Soil science

©2025 The author(s). This is an open access article distributed under Creative Commons Attribution 4.0 International License (CC BY 4.0).

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