  We study inclusive electroproduction on the proton at low $x$ and low $Q^2$
using a soft and a hard Pomeron. The contribution of the soft Pomeron is based
on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross
section can be calculated by means of a gauge invariant gluon field strength
correlator. To model the hard Pomeron exchange we phenomenologically extend the
leading order evolution of a power-behaved structure function, $F_2 \propto
x^{- \lambda}$, proposed by L\'opez and Yndur\'ain. This extension allows to
consider both the case $Q^2 = 0$ and the region of higher $Q^2$ on the basis of
the same parametrization. A good simultaneous fit to the data on $F_2$ and on
the cross section $\sigma_{\gamma p}$ of real photoproduction is obtained for
$\lambda=0.37$. With four parameters we achieve a $\chi^2/\textrm{d.o.f.} =
0.98$ for 222 data points. In addition, we use our model of the inclusive
$\gamma^{\ast} p$ interaction to compute the longitudinal structure function
$F_L$.
