A new quasi-3-D model with a dual iterative coupling scheme for simulating unsaturated-saturated water flow and solute transport at a regional scale

Abstract

Soil water in an unsaturated zone is the major source of groundwater recharge, and likewise soil contaminants are the major source of groundwater contamination. It is thus essential to jointly consider soil water movement and solute transport together with groundwater movement and solute transport, which is particularly important when addressing regional-scale problems. This study developed a new quasi-3-D model for simulating regional-scale saturated–unsaturated water movement and solute transport in arid and semi-arid agricultural areas. The unsaturated soil water movement and solute transport processes are simulated by a 1-D soil water and solute transport model referred to as UBMOD, and the saturated groundwater movement and solute transport are simulated by MODFLOW and MT3DMS. In the quasi-3-D model, the soil water movement and groundwater movement modules are iteratively coupled and the soil solute transport and groundwater solute transport modules are iteratively couple. This is referred to as a dual iterative coupling scheme. Strength and weakness of the quasi-3-D model and the coupling scheme were evaluated with two synthetic cases and a real-world application. Results indicate satisfactory performance of the model, which demonstrates the reliability of the dual iterative coupling scheme. Besides, the impacts of spatial and temporal discretization and stress period on model stability and accuracy has been evaluated. And the model is more computationally efficient than the fully numerical methods based on Richards’ equation when the problem is a 2D/3D problem with a scale larger than 10 m. Moreover, the application to the Hetao Irrigation District indicates that the developed model is effective and efficient for the real-world regional-scale agricultural problems. Above all, it is concluded that the new model is more suitable than existing models for simulating regional-scale unsaturated–saturated soil water movement and solute transport in arid and semi-arid agricultural areas.