Hydrus is a suite of Windows-based modeling software that can be used for analysis of water flow, heat and solute transport in variably saturated porous media (e.g., soils). HYDRUS suite of software is supported by an interactive graphics-based interface for data-preprocessing, discretization of the soil profile, and graphic presentation of the results. While HYDRUS-1D simulates water flow, solute and heat transport in one-dimension, and is a public domain software, HYDRUS 2D/3D extends the simulation capabilities to the second and third dimensions, and is distributed commercially.
History HYDRUS 1D
HYDRUS-1D traces its roots to the early work of van Genuchtenvan Genuchten, M. Th. 1978. Mass transport in saturated-unsaturated media: one-dimensional solutions. Research Rep. No. 78-WR-11. Water Resources Program. Princeton Univ. Princeton, NJ.van Genuchten, M. Th. 1987. A numerical model for water and solute movement in and below the root zone. Research Report No 121. U.S. Salinity laboratory, USDA, ARS, Riverside, California. and his SUMATRA and WORM models, as well as later work by Vogel (1987)Vogel, T. 1987. SWMII – Numerical model of two-dimensional flow in a variably saturated porous medium. Research Rep. No. 87. Dept. of Hydraulics and Catchment Hydrology. Agricultural Univ., Wageningen, The Netherlands. and Kool and van Genuchten (1989)Kool, J. B., and M. Th. van Genuchten. 1989. HYDRUS, One-dimensional variably saturated flow and transport model, including hysteresis and root water uptake. Version 3.2. User Manual. Hydrogeologic Inc., Herndon, VA, 116 p. and their SWMI and HYDRUS models, respectively. While Hermitian cubic finite element numerical schemes were used in SUMATRA and linear finite elements in WORM and the older HYDRUS code for solution of both the water flow and solute transport equations, SWMI used finite differences to solve the flow equation.
Various features of these four early models were combined first in the MS-DOS-based SWMI_ST model (Šimůnek et al., 1993), and later in the Windows-based HYDRUS-1D simulator (Šimůnek et al., 1998).Šimůnek, J., M. Šejna, and M. Th. van Genuchten. 1998. The HYDRUS-1D software package for simulating the one-dimensional movement of water, heat, and multiple solutes in variably-saturated media. Version 1.0. IGWMC – TPS – 70, International Ground Water Modeling Center, Colorado School of Mines, Golden, Colorado, 186pp. After releasing versions 1 (for 16-bit Windows 3.1) and 2 (for 32-bit Windows 95), the next two major updates (versions 3 and 4) were released in 2005 and 2008. These last two versions included additional modules applicable to more complex biogeochemical reactions than the standard HYDRUS modules.
While the standard modules of HYDRUS-1D can simulate the transport of solutes that are either fully independent or involved in the sequential first-order degradation chains, the two new modules can consider mutual interactions between multiple solutes, such as cation exchange and precipitation/dissolution.
Version 3 included the UNSATCHEM module (Suarez and Šimůnek, 1997Suarez, D. L., and J. Šimůnek. 1997. UNSATCHEM: Unsaturated water and solute transport model with equilibrium and kinetic chemistry. Soil Sci. Soc. Am. J., 61:1633–1646.) for simulating carbon dioxide transport as well as the multi-component transport of major ions. The UNSATCHEM major ion module was recently included also in version 2 of HYDRUS (2D/3D) (Šimůnek et al., 2011Šimůnek, J., M. Th. van Genuchten, and M. Šejna. 2011. The HYDRUS Software Package for Simulating Two- and Three-Dimensional Movement of Water, Heat, and Multiple Solutes in Variably-Saturated Media. Technical Manual, Version 2.0, PC Progress, Prague, Czech Republic, pp. 258.). Version 4 of HYDRUS-1D includes now not only the UNSATCHEM module, but also the HP1 program (Jacques and Šimůnek, 2005Jacques, D., and J. Šimůnek. 2005. User Manual of the Multicomponent Variably-Saturated Flow and Transport Model HP1, Description, Verification and Examples. Version 1.0. SCK•CEN-BLG-998, Waste and Disposal, SCK•CEN, Mol, Belgium, 79 pp.), which resulted from coupling HYDRUS-1D with the biogeochemical program PHREEQC.Parkhurst, D. L., and C. A. J. Appelo. 1999. User’s guide to PHREEQC (Version 2): A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. Water-Resour. Invest. Rep. 99-4259. USGS, Denver, CO.
HYDRUS 2D/3D The current HYDRUS (2D/3D) suite of software and their predecessors have a long history. The origin of these models can be traced back to the early work of Dr. Shlomo Neuman and collaborators (e.g., Neuman, 1972Neuman, S. P. 1972. Finite element computer programs for flow in saturated-unsaturated porous media. Second Annual Report, Project No. A10-SWC-77, Hydraulic Engineering Lab., Technion, Haifa, Israel.) who developed their UNSAT model at the Hydraulic Engineering Laboratory of Technion – Israel Institute of Technology, in Haifa, Israel, long before the introduction of personal computers. UNSAT was a finite element model simulating water flow in two-dimensional variably-saturated domains as described with the Richards equation. The model additionally considered root water uptake as well as a range of pertinent boundary conditions required to ensure wide applicability of the model. UNSAT was later modified by Davis and Neuman (1983)Davis, L. A., and S. P. Neuman. 1983. Documentation and user's guide: UNSAT2 – Variably saturated flow model. Final Rep., WWL/TM-1791-1, Water, Waste & Land, Inc., Ft. Collins, CO. at the University of Arizona, Tucson, such that the model could be run on personal computers.
This last version of UNSAT formed the basis of the SWMII model developed by Vogel (1987) during his stay at Wageningen University, the Netherlands. SWMII significantly extended the capabilities and ease of use of UNSAT. The code simulated variably-saturated water flow in two-dimensional transport domains, implemented the van Genuchten soil hydraulic functions (van Genuchten, 1980van Genuchten, M. Th. 1980. A closedform equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J., 44:892898.) and modifications thereof, considered root water uptake by taking advantage of some of the features of the SWATRE model (Feddes et al., 1978Feddes, R. A., P. J. Kowalik, and H. Zaradny. 1978. Simulation of Field Water Use and Crop Yield. John Wiley & Sons, New York, NY.), and included scaling factors to enable simulations of flow in heterogeneous soils. The code also allowed the flow region to be composed of nonuniform soils having an arbitrary degree of local anisotropy. SWMII was a direct predecessor of the SWMS_2D model (Šimůnek et al., 1992Šimůnek, J., T. Vogel and M. Th. van Genuchten. 1992. The SWMS_2D code for simulating water flow and solute transport in two-dimensional variably saturated media, Version 1.1, Research Report No. 126, U. S. Salinity Laboratory, USDA, ARS, Riverside, CA.) developed later at US Salinity Laboratory.
The SWMS_2D model (Šimůnek et al., 1992Šimůnek, J., T. Vogel and M. Th. van Genuchten. 1992. The SWMS_2D code for simulating water flow and solute transport in two-dimensional variably saturated media, Version 1.1, Research Report No. 126, U. S. Salinity Laboratory, USDA, ARS, Riverside, CA.) considerably extended the capabilities of SWMII by including provisions for solute transport. Solute transport was described using the standard advection-dispersion equation that included linear sorption, first-order degradation in both the liquid and solid phases, and zero-order production in both phases. Several other numerical improvements were at the time also implemented in SWMS_2D. These included solution of the mixed form of the Richards equation as suggested by Celia et al. (1990),Celia, M. A., and E. T. Bououtas, R. L. Zarba. 1990. A general mass-conservative numerical solution for the unsaturated flow equation. Water Resour. Res., 26:1483-1496. thus providing excellent mass balances in the water flow calculations. While SWMII could simulate water flow in either two-dimensional vertical or horizontal planes, SWMS_2D extended the range of applications also to three-dimensional axisymmetrical flow domains around a vertical axis of symmetry. Examples are flow to a well, infiltration from a surface ring or tension disc infiltrometer, and infiltration from a surface or subsurface dripper.
The first major upgrade of SWMS_2D was released under the name CHAIN_2D (Šimůnek et al., 1994bŠimůnek, J., and M. Th. van Genuchten. 1994. The CHAIN_2D code for simulating two-dimensional movement of water flow, heat, and multiple solutes in variably-saturated porous media. Version 1.1, Research Report No 136, U.S. Salinity laboratory, USDA, ARS, Riverside, California.). This model greatly expanded upon the capabilities of SWMS_2D by including, among other things, sequential first-order solute decay chains and heat transport. The temperature dependence of the soil hydraulic properties was included by considering the effects of temperature on surface tension, dynamic viscosity and the density of water. The heat transport equation in CHAIN_2D considered transport due to conduction and advection with flowing water. The solute transport equations considered advective-dispersive transport in the liquid phase, as well as diffusion in the gaseous phase. The transport equations also included provisions for nonlinear nonequilibrium reactions between the solid and liquid phases, linear equilibrium reactions between the liquid and gaseous phase, zero-order production and two first-order degradation reactions: one which was independent of other solutes, and one which provided the coupling between solutes involved in the sequential first-order decay reactions.
The SWMS_2D and CHAIN_2D models formed the bases of versions 1.0 (for 16-bit Windows 3.1) and 2.0 (for 32-bit Windows 95) of HYDRUS-2D (Šimůnek et al., 1999Šimůnek, J., M. Šejna, and M. Th. van Genuchten. 1999. The HYDRUS-2D software package for simulating two-dimensional movement of water, heat, and multiple solutes in variably saturated media. Version 2.0, IGWMC – TPS – 53, International Ground Water Modeling Center, Colorado School of Mines, Golden, Colorado, 251pp.). A unique feature of HYDRUS-2D was that it used 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. It could handle flow regions delineated by irregular boundaries, as well as three-dimensional regions exhibiting radial symmetry about the vertical axis. The code includes the MeshGen2D mesh generator, which was specifically designed for variably-saturated subsurface flow and transport problems. The mesh generator may be used for defining very general domain geometries, and for discretizing the transport domain into an unstructured finite element mesh. HYDRUS-2D has been recently fully replaced with HYDRUS (2D/3D) as described below.