  The goal of this paper is to identify the universal Whitham hierarchy of
genus zero with a dispersionless limit of the multi-component KP hierarchy. To
this end, the multi-component KP hierarchy is (re)formulated to depend on
several discrete variables called ``charges''. These discrete variables play
the role of lattice coordinates in underlying Toda field equations. A
multi-component version of the so called differential Fay identity are derived
from the Hirota equations of the $\tau$-function of this ``charged''
multi-component KP hierarchy. These multi-component differential Fay identities
have a well-defined dispersionless limit (the dispersionless Hirota equations).
The dispersionless Hirota equations turn out to be equivalent to the
Hamilton-Jacobi equations for the $S$-functions of the universal Whitham
hierarchy. The differential Fay identities themselves are shown to be a
generating functional expression of auxiliary linear equations for
scalar-valued wave functions of the multi-component KP hierarchy.
