In specified flux boundaries, the rate of fluid moving into or out of the groundwater is specified. The head at the specified flux cell changes in response to the flux.
The Recharge package allows the user to specify a recharge rate over an area. The total flux entering each cell will be the rate times the horizontal area of the cell. (The UZF package can also simulate recharge but in a somewhat different way.)
The Well package allows the user to specify a volumetric rate of fluid withdrawal or injection.
The Flow and Head Boundary (FHB) package allows the user to specify heads and flow rates in cells that vary in time in a manner independent of the stress periods.
