3.3. Drain Boundary Conditions

Drain boundary conditions define parts of the groundwater system that receive water from the groundwater system but cannot supply water to it, such as seeps, springs, and wetlands. Besides natural groundwater system components, underground excavations, sub-horizontal drainholes, and drainage galleries can be simulated using drains.

The equation for calculating discharge from the drain nodes is defined as follows:

(3.3)\[\begin{split}q_{d} = \sum_{i=1}^{n} \begin{cases} C_{di}\,\bigl(H_{di} - H_i\bigr), & \text{for } H_{di} < H_i,\\ 0, & \text{for } H_{di} \ge H_i . \end{cases}\end{split}\]

Where:

  • \(q_d\) = functional representation of groundwater discharge due to the drain boundary condition [L³T⁻¹],

  • \(C_{di}\) = leakance factor at node i [L²T⁻¹],

  • \(H_{di}\) = specified head for the node i [L],

  • \(H_i\) = computed hydraulic head at node i [L], and

  • \(n\) = number of nodes [L⁰].

The \(H_{di}\) value must not exceed the elevation at node i. The leakance factor (\(C_{di}\)) for drain nodes relates to the hydraulic conductivity at the drain node. By default, drain boundary conditions apply to all ground-surface nodes lacking other user-specified boundary conditions. The boundary head for each ground-surface drain node equals the ground-surface elevation at that node. Thus, the drain boundary condition prevents computed hydraulic heads from exceeding ground-surface elevation, preventing ponded water simulation unless explicitly specified by the user. Water discharging from ground-surface drain nodes appears in the water budget as surface runoff.