Free Drainage

Free drainage is simulated in terms of a unit total vertical hydraulic gradient (i.e., a zero pressure head gradient). This situation is often observed in field studies of water flow during drainage/redistribution in the vadose zone [Sisson, 1987; McCord, 1991]. McCord [1991] states that the most pertinent application of a free drainage boundary condition is its use as a bottom outflow boundary condition for situations where the water table is situated far below the domain of interest.

Rather than using a unit total vertical hydraulic gradient (zero pressure head gradient), one can specify a different gradient value using the Boundary Condition Options. Version 1.xx of HYDRUS implements gradient type boundary condition only as the Free Drainage boundary condition, or the unit gradient boundary condition. However, in many situations one needs a non-unit gradient BC. For example, it is difficult to select appropriate boundary conditions for vertical boundaries with flow in a hill slope where the side-gradient is more or less parallel with the direction of the slope. We have implemented in Version 2 of HYDRUS an option, using which users can specify a gradient other than one (unit gradient). This option, i.e., non-unit gradient boundary condition, needs to be selected from the Special Boundary Conditions tab of the Boundary Condition Options dialog window. Gradients are positive for flow against a particular axis, i.e., from right to left (in the x-direction) and from back to front (in the y-direction), and should be used only (or mainly) on sides of the transport domain. In 3D, gradient BC can be specified only in one direction (i.e., either in x- or y-direction). For the Gradient Boundary Condition to be considered, it needs to be specified

See also the "How to Edit Boundary Conditions" topic.

See available Water Flow Boundary Conditions.