The HYDRUS  software package (including the much anticipated three-dimensional capability) is a major upgrade and extension of the HYDRUS-2D/MESHGEN-2D software package (originally developed and released by U.S. Salinity Laboratory, PC-Progress and the International Groundwater Modeling Center). HYDRUS is a Microsoft Windows based modeling environment for analysis of water flow, solute, and heat transport in variably saturated porous media.

For flow, the code solves the mixed form ofthe Richards’ equation with many functions for simulating hydraulic conductivity versus water content (or pressure head) relationships, includinghysteresis. It allows root water uptake with compensation; spatial root distribution functions; and includes new soil hydraulic property models.  The new code allows for dynamic, system-dependent boundary conditions (e.g., switching between pressure heads, seepage face, zero flux or atmospheric boundary depending on the position of the water level).

For solute transport, the code solves the advection dispersion equation, but with many processes not usually include in unsaturated zone codes. For example, in addition to the typical linear partitioning between soil, water, and gas phases, the code also simulates the following processes: non-linear and non-equilibrium partitioning between phases; diffusion in the gas phase; zero- and first-order degradation kinetics; including decay chains (e.g., for nitrates and radionuclides); adjective flow in a dual-porosity system allowing for preferential flow in fractures or macropores while storing water and dissolved chemicals in the matrix; transport of viruses, colloids, and/or bacteria using an attachment/detachment model; filtration theory; and blocking functions; and flowing particles in two-dimensional applications. Hydrus also includes a new constructed wetland module (only in 2D).  

HYDRUS still allows optimization in 1D and 2D.    Unfortunately, optimization is not provided for 3D applications.  Another useful new feature is better print management (printing at regular time intervals or after a constant number of time steps).

The GUI is much improved.  It includes many new functions improving the user-friendliness, such as drag-and-drop, context sensitive pop-up menus after clicking on objects, selection and editing of multiple objects in the same dialog window, and allowing  multiple projects and views to be opened at the same time in the HYDRUS main window.  One of the best new features is that time-varying and cumulative fluxes can be calculated and displayed across internal meshlines.

The only negatives associated with the program are that the User’s Guide could be more complete.  Not all features are explained well enough for a modeler without HYDRUS experience to easily follow.  However, the on-line discussion forum is very helpful for those who take advantage of it.  The forum can be found at http://www.pc-progress.cz/_Forum/default.asp