  A simple function for the spectral power $P(\epsilon,t) \equiv \nu L(\nu) $
is proposed to model, with 9 parameters, the spectral and temporal evolution of
the observed nonthermal synchrotron power flux from GRBs in the blast wave
model. Here $\epsilon = h\nu/$m$_e$c$^2$ is the observed dimensionless photon
energy and $t$ is the observing time. Assumptions and an issue of lack of
self-consistency are spelled out. The spectra are found to be most sensitive to
the baryon loading, expressed in terms of the initial bulk Lorentz factor
$\Gamma_0$, and an equipartition term $q$ which is assumed to be constant in
time and independent of $\Gamma_0$. Expressions are given for the peak spectral
power $P_p(t) = P(\epsilon_p,t)$ at the photon energy $\epsilon =
\epsilon_p(t)$ of the spectral power peak. A general rule is that the total
fireball particle kinetic energy $E_0 \sim \Pi_0 t_d$, where $t_d \propto
\Gamma_0^{-8/3}$ is the deceleration time scale and $\Pi_0 \equiv
P(\epsilon_p,t_d) \propto \Gamma_0^{8/3}$ is the maximum measured bolometric
power output in radiation, during which it is carried primarily by photons with
energy ${\cal E}_0 = \epsilon_p(t_d) \propto q\Gamma_0^4$.
