If you want the CONSTRAINT to be variably active, you can define the maximum (in) flow constraint value as a time series (0 m3/d when seepage face active, 100000 m3/d [for example] otherwise)
If you want the BC to be variably active, but always a seepage face when it is active, you can add a minimum (out) flow constraint and set at -100000 m3/d when active and 0 m3/d when inactive. Hence, at times when you want no BC activity, it will have a maximum and minimum flow constraint of 0 m3/d. At other times it will have a maximum flow constraint of 0 m3/d (no inflow) and minimum flow constraint of -100000 m3/d (unrestricted outflow).
Tip: select a very large constraint value when constraint inactive - otherwise the BC can behave in an undesirable manner due to a bug
