Adaptation and validation of the ParSWMS numerical code for simulation of water flow and solute transport in soilless greenhouse substrates

Numerical simulation of three-dimensional water flow and solute transport in containerized variably saturated soilless substrates with complex hydraulic properties and boundary conditions necessitates high-resolution discretization of the spatial and temporal domains, which commonly leads to several million nodes requiring numerical evaluation. Even today's computing prowess of workstations is not adequate to tackle such problems within a reasonable timeframe, especially when numerous realizations are required to optimize the geometry, substrate properties, and irrigation and fertigation management of soilless plant growth modules. Hence, the parallelization of the numerical code and utilization of high performance computing (HPC) are essential. Here, we adapted and applied the ParSWMS parallelized code that is amenable to solving the 3D Richards equation for water flow and the convection-dispersion equation for solute transport subject to linear solute adsorption. The code was modified to allow for nonlinear equilibrium solute adsorption with new boundary conditions and applied to simulate water flow and nitrogen and phosphorus transport in containerized soilless substrates. Multi-solute transport simulations with the modified Linux ParSWMS code were first performed on a workstation and referenced to the Windows-based HYDRUS (2D/3D) numerical code. After confirming the agreement between the modified ParSWMS code and HYDRUS (2D/3D), various preconditioners and iterative solvers were evaluated to find the computationally most efficient combinations. The performance of the modified ParSWMS code and its stability were compared to HYDRUS (2D/3D) simulations for three soilless substrates consisting of horticultural perlite, volcanic tuff, and a volcanic tuff/coconut coir mixture. Considering the solute mass balance error as a stability measure, ParSWMS outperformed HYDRUS (2D/3D). Moreover, simulations with the modified ParSWMS code were about 22% faster than simulations with HYDRUS (2D/3D) on the workstation. Tests of the modified ParSWMS on two HPC clusters with 28 and 94 cores revealed a potential computational speedup of 94% relative to the HYDRUS (2D/3D) simulations performed on the workstation.