Vertical unsaturated colomn - Bottom BC

Hello,

I am working on a sand aquifer. The groundwater level is 50 m below the surface and I just want to simulate the first meters of the unsaturated zone. As the groundwater level is far below the unsaturated zone that I try to model, I don't know what kinf of bottom BC I can put : normally, I would have put a free drainage BC but it does'nt seem to exist on Feflow ...

Can you please help me ?
Best regards

You could try to use the Fluid-Flux BC (Gradient):
http://www.feflow.info/html/help71/feflow/mainpage.htm#t=09_Parameters%2FBoundary_Conditions%2FFlow%2Ffluid_flux_bc_gradient.html

Thank you for your answer !