  Be stars possess gaseous circumstellar disks that modify in many ways the
spectrum of the central B star. Furthermore, they exhibit variability at
several timescales and for a large number of observables. Putting the pieces
together of this dynamical behavior is not an easy task and requires a detailed
understanding of the physical processes that control the temporal evolution of
the observables. There is an increasing body of evidence that suggests that Be
disks are well described by standard $\alpha$-disk theory. This paper is the
first of a series that aims at studying the possibility of inferring several
disk and stellar parameters through the follow-up of various observables. Here
we study the temporal evolution of the disk density for different dynamical
scenarios, including the disk build-up as a result of a long and steady mass
injection from the star, the disk dissipation that occurs after mass injection
is turned off, as well as scenarios in which active periods are followed by
periods of quiescence. For those scenarios, we investigate the temporal
evolution of continuum photometric observables using a 3-D non-LTE radiative
transfer code. We show that lightcurves for different wavelengths are specific
of a mass loss history, inclination angle and $\alpha$ viscosity parameter. The
diagnostic potential of those lightcurves is also discussed.
