The Drain package causes water the leave the groundwater system when the head in a cell with a drain boundary rises above the elevation of the drain boundary. The pane for the Drain package is on the MODFLOW Packages and Programs dialog box under Head Dependant Flux.
On the DRN: Drain package pane, the user can define parameters for the drain boundaries. These parameters will be multiplied by user-specified multipliers to determine the drain conductances defined by parameters. The user can also set up drain boundaries that do not involve parameters. Objects must be used to define the locations of the drain boundaries.
In MODFLOW 6 models, "multipliers" can be used with drain boundaries. Multipliers will be used if the "Use Multiplier (MODFLOW 6)" checkbox is checked.. Ordinarily, multipliers are not needed. If multipliers are used, the drain conductance is multiplied by the value of the multiplier. One use for multipliers might be when the drain conductances are defined using a time-series and it is desired to multiply the drain conductance values in the time series by a different amount at different locations.
Parameters can also be edited in the Manage Parameters dialog box.
The Drain package is similar to the River package but water can leave the groundwater system through drains but can not enter the groundwater system through drains whereas the River package allows water to either leave or enter the groundwater system.
The Drain package is closely related to the Drain-Return package. The Drain-Return package allows some of the water that leaves through a drain to re-enter the groundwater system at a different location.
