  We present a new scheme for modeling the broad line region in active galactic
nuclei. It involves photoionization calculations applied to a number of
variable emission lines at {\it all times}. We demonstrate how fitting all
lines simultaneously provide strong constraints on several of the more
important parameters, such as the density and column density, and the radial
distribution of the emission line clouds.
  When applying the model to the Seyfert 1 galaxy NGC 5548, we are able to
reconstruct the light curves of four emission-lines, in time and in absolute
flux. We argue that the Balmer line light curves, and possibly also the MgII
light curve, do not fit this scheme because of the limitations of present-day
photoionization codes. We rule out models where the particle density scales as
r^{-2} and favor models where it scales as r^{-(1-1.5)}. We can place lower
limits on the column density at a distance of 10 ld, of N_{col}(r=10)>10^{22}
cm^{-2}, and limit the particle density to be in the range of
10^{11}>N(r=10)>10^{9.5} cm^{-3}.
