How is surface temperature computed ?

In dealing with surface pool evaporation, I need to understand how is the surface temperature computed by CS.
When I force it through a user formula Ts(t) in the IHM, it seems to be recomputed and doesn't correspond to the imposed one.
When I try to force a Dirichlet value for the variable itempb (ground temperature) in a rough Wall boundary condition, CS refuses to run.
Thanks for clarifying this point !
Stephane Burkhart