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The optical spectra obtained during five years of monitoring of the active galactic nucleus NGC 5548 by the International AGN Watch consortium are used in a detailed study of the variability of broad H$\beta$ emission line. It is shown that profile changes occur over longer time scales than those associated with reverberation effects. Furthermore, the profile changes are not due to the variations in the ionizing continuum, rather, they appear to be due to emissivity redistributions within the observed phase space of the broad-line region on a time scale of several months to years. This has important consequences for understanding reverberation-mapping results for sources such as NGC 5548 where the system is not stationary over the duration of the experiment. Two possible scenarios for the profile changes are discussed: one in which the broad-line region is anisotropically radiating and one in which the continuum source is anisotropically radiating. Both scenarios require a ``clumpy'' broad-line region. The geometry and kinematics of the broad-line region can be obtained by studying the evolution of the broad-line region rather than by reverberation mapping.
We are grateful support of this work through NSF grant AST-9117086 and NASA grants NAG5-2477 and NAGW-3315.
