The physically based-approaches can conceptually be compared with a drainage system (see Figure 2.6). Water flow to a drainage pipe occurs when the simulated groundwater table, zsat, is above the bottom level of the pipe, i.e. flow occurs horizontally from a layer to drainage pipes when the soil is saturated. Three different options are available for this equation (see switch PhysicalDrainEq) .
In addition, a source flow from a water-filled ditch or stream to the soil profile will be simulated based on straightforward use of the Darcy equation (see switch ReturnFlow) when the drainage depth is above the groundwater level in the simulated profile. In this case, the different radial and vertical resistances are neglected and only the horizontal resistance from eq. (2.46)is applied.
The simulated ground water level may optionally be forced to match a certain variation if the drainage level is allowed to change with time (see switch DriveDrainLevel) i.e. a changing zp (see below).