I am doing a dam seepage modeling and feel confused about how to set the hydraulic head boundary for the upstream face of the dam.
I have reservoir level data varies with time as a function of h(t) which can be used as 1st type (Dirichlet) boundary. However, as reservoir level varies a lot in a year, can be 7m differences, this will make some nodes at the upstream boundary exposed to the air some time.
For eg. if the reservoir level varies from elevation 30'-37', and I set the nodes which have elevation from 0-37' as the upstream boundary. In this case, for the nodes between 30'-37', like 35', it may exposed  to the air. So I just wondering, for the nodes between 30'-37', do I need to add some constraints or do some other treatments? or only a 1st type boundary is enough for this case.

Because this make me thinking about the seepage face which should be used for the downstream face boundary, it has a constraint Qmax=0 to prevent inflow. The free surface also varies at the boundary. But I am not sure what I can do with the upstream boundary.

I will be appreciated for any help.