How to define a BC that only effective for a period of time ? ? ?

Say that I need these nodes on fixed head (1st kind) for 1 year and cut them loose after then, How may I define that in ONE model run?

Thanks

You cannot totally remove a BC during the model run without an IFM.

Your alternative option is to control BC activity with constraints.  At times when the BC needs to be inactive, both inflow and outflow constraints must be 0.  These constraints can be assigned with time series.

Thanks Blair, I tried the constraints way, ant it worked! For those who are also interested on this, it is really simple, here is a picture I made for the definition of this trick.