HYDRUS

Criterion	Explanation
General Description	Simulates water, heat, and solute movement in one-dimensional variably saturated media.
Model Domain	Wetlands
Developer	United States Department of Agriculture (USDA)
Hardware computing requirements	No hardware requirements specified for running the model.
Code language	FORTRAN
Original application	Originally used to simulates water, heat, and solute movement in one-dimensional variably saturated media. There are now 2-D and 3-D versions. Originally used fro extrapolating information from a limited number of field experiments to different soil, crop and climatic conditions, as well as to different tillage and water management schemes. Used by HydroFocus to simulate unsaturated-zone water movement in the Delta.
Public/proprietary and cost	Publicly available at no cost.
Physically or empirically based	Physically based
Mathematical methods used	Numerically solves the Richard's equation for saturated-unsaturated water flow. A sink term accounts for water uptake by plant roots. Fickian-based advection-dispersion equations are used for heat and solute transport. For heat transport, conduction as well as convection are considered with flowing water. For solute transport, the equations consider advective-dispersive transport in the liquid phase, and diffusion in the gaseous phase. Solute transport can be simulated in both the liquid and gaseous phases simultaneously. Convection-dispersion solute transport equations account for non-linear non-equilibrium reactions between the solid and liquid phases, and linear equilibrium reactions between the liquid and gaseous phases. This allows for both absorbed and volatile solutes such as pesticides to be considered. For nonequilibrium solute transport, the liquid phase is partitioned into mobile and immobile regions. Governing flow and transport equations are solved using standard Galerkin-type linear finite element schemes. Transport of viruses, colloids, or bacteria may be simulated when transport equations include provisions for kinetic attachment/detachment of solutes to the solid phase. First and third-type boundary conditions can be implemented in both the solute and heat transport parts of the model. Boundary conditions dealt with by the water flow part of the model – Prescribed head and flux boundaries, boundaries controlled by atmospheric conditions, and free drainage boundaries. Inverse estimation of soil hydraulic and/or solute transport and reaction parameters from measured transient or steady-state flow and/or transport data. HYDRUS also has modules for simulating CO2 and major ion solute movement. CO2 transport mechanisms are – diffusion (in both liquid and gas phases), and convection (in the liquid phase). The model accounts for equilibrium chemical reactions between the major variables of the chemical system – Ca, Mg, Na, K, SO4, Cl, NO3, H4SiO4, alkalinity, and CO2.
Input data requirements	Input data are given in four separate input files, each consisting on one or more input blocks. Input files are placed into a common subdirectory. All data are read in using list-directed formatting. HYDRUS1D interactive graphics-based user-friendly interface for MS Windows environments greatly assists with management of relatively complex input data files for flow problems. Input files may be created manually or with HYDRIS1D. Additional input files are required for the major ion chemistry module.
Outputs	The program consists of 9+(ns-1) output files (when major ion chemistry is not considered), where ns is the number of solutes considered in the first-order decay chain. When major ion chemistry is considered the program, output consists of 13 output files. The output is organized into 3 groups: T-level – files with info printed at the end of each time step P-level – files printed at prescribed print times A-level – files printed each time a time-dependent boundary conditions is specified. Output files are printed to the same subdirectory as input files.
Pre-processing and post-processing tools	The HYDRUS packages use a Microsoft Windows based graphical user interface (GUI) to manage the input data required to run the program, as well as for nodal discretization and editing, parameter allocation, problem execution, and visualization of results.
Representation of uncertainty	Uncertainty can be assessed using sensitivity analysis.
Prevalence	Both HYDRUS-1D and HYDRUS (2D/3D) has been used in hundreds, if not thousands of applications referenced in peer-reviewed journal articles and many technical reports. Both software packages are also used in classrooms of many universities in courses covering Soil Physics, Processes in the Vadose Zone, or Vadose Zone Hydrology. A selected list of hundreds of applications of both HYDRUS software packages are given at: http://www.pc-progress.com/en/Default.aspx?h3d-references http://www.pc-progress.com/en/Default.aspx?h1d-references The website also provides many specific applications in the libraries of HYDRUS projects at: http://www.pc-progress.com/en/Default.aspx?h1d-library http://www.pc-progress.com/en/Default.aspx?h3d-applications HYDRUS software also provides capabilities for simulating water flow and solute transport for specialized domains.
Ease of use for public entities	The HYDRUS model is readily available at https://www.ars.usda.gov/pacific-west-area/riverside-ca/us-salinity-laboratory/docs/hydrus-1d-model/
Ease of obtaining information and availability of technical support	Substantial information is available at the USDA website: https://www.ars.usda.gov/pacific-west-area/riverside-ca/us-salinity-laboratory/docs/hydrus-1d-model/ See https://www.pc-progress.com/forum/viewforum.php?f=4 for users forum
Source code availability	Source code is not publicly accessible.
Status of model development	Fully developed and ready for use
Challenges for integration	Primary challenge is for integration with groundwater flow model due to time-step frequency needed to simulate solute transport in the unsaturated zone.

References

Šimůnek, J., M. Th. Van Genuchten, and M. Šejna. "The HYDRUS-1D Software Package for Simulating the One-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media." 3 (April 2005).

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