how to define hydrostatic pressure?

Dear all,

I am using COMSOL to simulate dam behavior due to hydrostatic pressure at upstream face, to do that, I have to define the load depending on Y coordinates of nodes at upstream face, (Pressure = Yrho_watergravity) where rho_water is density of water and gravity acceleration(9.81m/s2), How can I do that with COMSOL multiphysics ? Thank for your help Cordially