One of major difficulties in a water distribution system modeling is water usage or demand, which continuously varies for many reasons.  In order for a transient simulation to accurately reflect system dynamics, these demand fluctuations should be incorporated into the model; but, at least over short intervals, demands are typically modeled as constant, independent of both pressure and time. Especially considering surge characteristics: the fine time step of calculation and rapid head fluctuation, the accurate representation of demand is crucially important to provide an accurate transient modeling result.  

InfoSurge has a feature of pressure sensitive demand based on the orifice relation (Q = Cv· [dP]0.5). This feature can be applied by clicking on the Pressure Sensitive Demand checkbox in the Run Manager as shown below (see Figure 1). Otherwise, demands remain constant during the transient. System demand are usually best modeled using this type of demands so the demands will response to pressure changes in the pipe system. Figure 2 presents a example of difference between pressure sensitive demand and pressure insensitive demand.