  In this paper a conservation equation is derived for the radially dependent
entropy in toroidal geometry using the local approximation of the gyro-kinetic
framework. This equation naturally leads to an operative definition for the
turbulence intensity. It is shown that the conservation equation can be split
in two separate conservation equations, one describing the dynamics of the
zonal modes and one for the non-zonal modes. In essence the paper provides an
operative tool for both analytic as well as numeric studies of the radial
propagation of turbulence in tokamak plasmas.
