The groundwater flow packages are used to define the aquifer properties such as hydraulic conductivity or transmissivity. There are three groundwater flow packages in MODFLOW: the Block-Centered Flow package, the Layer Property Flow package and the Hydrogeologic Unit Flow package. A fourth package, the Upstream Weighting package is part of MODFLOW-NWT. One and only one of these packages must be used in all models.
•In the Block-Centered Flow package, the user defines all the aquifer properties for all cells directly; parameters are not used.
•In the Layer Property Flow package, the user defines aquifer properties for all cells either directly or through the use of parameters.
•In the Hydrogeologic Unit Flow package, the user defines properties of hydrogeologic units exclusively through the use of parameters. The hydrogeologic units do not correspond directly to model layers. Instead, the user defines the top and thickness of each hydrogeologic units and MODFLOW combines data from multiple hydrogeologic units to determine the appropriate values for the aquifer properties at each cell.
•The Upstream Weighting package in MODFLOW-NWT is nearly identical in its structure to the LPF package but performs the calculations differently. The Upstream Weighting package can be helpful for models that don't converge because of problems with repeated wetting and drying.
Two additional packages provide additional options related to groundwater flow.
•The Horizontal Flow Barrier package can be used to reduce the conductances between horizontally adjacent cells.
•In the Unsaturated-Zone Flow (UZF) package, water is routed from the land surface, through the unsaturated zone to the water table. Evapotranspiration (ET) can be simulated in the UZF package and whatever is not removed by ET becomes recharge to the saturated zone. Infiltration excess, can be routed to streams (SFR) or lakes (LAK).
•The Seawater Intrusion Package (SWI2) simulates variable density flow with a sharp interface approximation.