Gradient Boundary

Version 1 of HYDRUS implemented 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 for flow in a hillslope where the side-gradient is more or less parallel with the direction of the slope. We have thus implemented in Version 2.0 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 Conditions 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 BCs can be specified only in one direction (i.e., in either x- or y-direction).

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