Cross correlation of demands in water distribution network design

The aim of water distribution design is to size and configure a system so that it meets existing and future demands while providing pressures above a minimum level for service and fire protection. Extended period simulation (EPS) is used in design to determine network pressures under varied diurnal demand patterns. In EPS, diurnal demands are almost invariably assumed to change in unison, or in statistical terms, to be strongly correlated in space. This paper first tests this common assumption by investigating the extent to which cross correlation in demand affects the mean and standard deviation of pressure heads in water networks, and then investigates how cross correlated demands can influence capital costs in network design. Preliminary findings from two examples indicate that the standard deviation of pressure head and capital costs can be sensitive to the level of cross correlation between nodal demands. Thus a realistic assessment of cross correlation in demand can lead to a more economical design.