  (Abridged) Our deep multiwavelength campaign on NGC 5548 revealed an
unusually strong X-ray obscuration. The resulting dramatic decrease in incident
ionizing flux allowed us to construct a comprehensive physical, spatial and
temporal picture for the long-studied AGN wind in this object. Here we analyze
the UV spectra of the outflow acquired during the campaign as well as from four
previous epochs. We find that a simple model based on a fixed total
column-density absorber, reacting to changes in ionizing illumination, matches
the very different ionization states seen in five spectroscopic epochs spanning
16 years. Absorption troughs from C III* appeared for the first time during our
campaign. From these troughs, we infer that the main outflow component is
situated at 3.5+-1 pc from the central source. Three other components are
situated between 5-70 pc and two are further than 100 pc. The wealth of
observational constraints and the disparate relationship of the observed X-ray
and UV flux between different epochs make our physical model a leading
contender for interpreting trough variability data of quasar outflows.
