In the formula to calculate Kd , the pressure drop and velocity are involved (Kd= dP/dx * 1/u^2). Which velocity is this one?? Ustream velocity, velocity through the pourous media or dawnstream velocity (right after comming out from the pourous media)?

Since the equations are discretized in a code, this should be the velocity at the node where you compute/store your solution, i.e. the local velocity through the porous media.

cheers