In head dependant flux boundaries, the rate of flow from the boundary into or out of the groundwater varies in response to changes in head in the boundary cell.
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 Drain Return package is like the drain package except that it allows some or all of the water that exited through the drain to return to the groundwater system at some other location.
The Evapotranspiration package causes water to leave the system at rate that depends on the head in the cell with the evapotranspiration boundary. If the head is above a defined value, the rate of water leaving the system, is set to some constant rate. If the head n the cell drops low enough, no water will be removed from the groundwater system through the boundary. Between those extremes, the rate of water loss varies linearly from zero to the maximum rate.
The Evapotranspiration Segments package is like the Evapotranspiration package except that between the maximum and zero evapotranspiration rate, the relationship between head and evapotranspiration is a more complicated function of the head.
In the General Head Boundary package, there is a linear relationship between the head in the general head boundary cell and the flow of water into or out of the groundwater system at the boundary.
The Lake package in MODFLOW-2005 and related versions of MODFLOW simulates lakes whose elevations can rise or fall due to interaction with groundwater or with streams simulated using the Stream Flow Routing package (SFR).
The Lake package in MODFLOW 6 simulates lakes whose elevations can rise or fall due to interaction with groundwater
The Multi-Aquifer Well package is used to simulate wells that have open intervals in more than one cell. It is only available in MODFLOW 6.
Both version 1 (MNW1) and version 2 (MNW2) of The Multinode Well package simulate wells that have open intervals in more than one cell.
The Water Mover package in MODFLOW 6 is used to move flow exiting one boundary condition into another boundary condition.
The Reservoir package (RES) simulates leakage between a reservoir and an underlying groundwater system as the reservoir area expands and contracts in response to changes in reservoir stage. Unlike the Lake package the user specifies the heads in reservoirs rather than the heads being computed.
The Riparian Evapotranspiration package in MODFLOW-OWHM allows the user to define separate transpiration curves for different plant groups relating the head to the evapotranspiration rate. The user can define the fractional coverage of each plant group in each cell and also varying land surface elevations in each cell to determine the evapotranspiration rate in each cell.
The River package is used to boundaries in which the flow into or out of the groundwater system is a function of the head. If the head is above a threshold value the flow is a linear function of head. However, if the head in the cell falls below the threshold, the flow from the river to the model cell is set to a specified lower bound.
The Streamflow-Routing package is used to simulate streams in a model. The flow in a stream is routed instantaneously to downstream streams or lakes. Flow can be routed through lakes defined in the Lake package. Unsaturated flow beneath streams can be simulated. There are two versions of this package: One in MODFLOW-2005 and related versions of MODFLOW and the other in MODFLOW 6.
The Stream package is also used to simulate streams in a model. The flow in a stream is routed instantaneously to downstream streams. The Streamflow-Routing package was developed from the Stream package.
