Blade efficiency of a franscis turbine

The blade efficiency of a turbine is defined as the ratio of the power extracted from the fluid by the rotor to the power available at the inlet of the rotor. The power available at the inlet of the rotor is given by the sum of the power extracted from the fluid by the rotor and The power available at the outlet of the rotor. In calculating the blade efficiency of a franscis turbine, the power available at the outlet is taken as the kinetic energy outflow rate. why is the effect of vaccum or negative pressure at the outlet of the rotor ignored? Is it neglected??