Specified flux boundaries can be specified with

the Flow and Head Boundary package,

the Recharge package, or

the Well package.

In the Flow and Head Boundary package, the user enters a series of times and corresponding flux rates at specific cells. Linear interpolation is used to determine the flux rates at intervening times.  The times do not need to be a the beginnings or ends of stress periods.

In the Recharge package, the user enters an array of flux values covering the entire upper surface of the model.  The rates can be positive, negative or zero.  There are several options for determining to which  layer the recharge will be applied.  The flux rate is in units of L/T instead of L3/T as in the the Flow and Head Boundary or the Well packages.

In the Well package, the user enters a flux rate for particular cells in particular stress periods.

Parameters can be used with the Recharge and Well packages but not the Flow and Head Boundary package.