  In the fireball model, it is more physically realistic that gamma-ray burst
(GRB) ejecta have a range of bulk Lorentz factors (assuming $M\propto
\Gamma^{-s}$). The low Lorentz factor part of the ejecta will catch up with the
high Lorentz factor part when the latter is decelerated by the surrounding
medium to a comparable Lorentz factor. Such a process will develop a
long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile,
the forward shocked materials are gradually supplied with energy from the
ejecta that are catching-up, and thus the temporal decay of the forward shock
emission will be slower than that without an energy supply. However, the
reverse shock may be strong. Here, we extend the standard reverse-forward shock
model to the case of radially nonuniform ejecta. We show that this process can
be classified into two cases: the thick shell case and the thin shell case. In
the thin shell case, the reverse shock is weak and the temporal scaling law of
the afterglow is the same as that in Sari & Mesz (2000). However, in the thick
shell case, the reverse shock is strong and thus its emission dominates the
afterglow in the high energy band. Our results also show slower decaying
behavior of the afterglow due to the energy supply by low Lorentz factor
materials, which may help the understanding of the plateau observed in the
early optical and X-ray afterglows.
