Water supply networks are generally modelled for a day or longer time period. Under most
circumstances, it is normal to omit the dispersive movement of chlorine and to assume that the
convective transport dominates over dispersion. However, during night time operations the velocities
are signi®cantly decreased. This radical change in the velocity may therefore lead to the following
question: can the available convection based quality models predict the chlorine concentrations
accurately in time and space? An improved water quality model implies accurate estimation of
concentrations at nodes in time and space at all levels of branching. To enhance the utility of water
quality models dispersion may also be taken into account. In this paper, an eort that led to the
development of an unsteady 2-D convective±dispersive model is presented, and the model output is
compared with the experimental results.