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Water Distribution System Hydraulic Model Calibration and System Evaluation Report Chapter 4 – Water Demand Allocation December 2019 P05049-2019-000 Page 12 4.2 Diurnal Demand Pattern Water usage for the City of Arden Hills is unsteady due to continuous fluctuations in the water demand throughout the day. The temporal changes in water usage typically follow a similar cycle over a 24-hour period. This cycle is referred to as a diurnal demand pattern. It should be noted that in addition to fluctuations at varying times throughout the day, diurnal patterns are also impacted by seasonal and climatic conditions (winter vs. summer, precipitation events, etc.). Large water users such as industrial or commercial businesses can also have an impact on diurnal demand patterns. In June 2017, AE2S developed a water distribution hydraulic model for the City of Roseville. Since the Roseville and Arden Hills water distribution systems are linked together, the diurnal demand pattern that was developed analyzed Roseville and Arden Hills as one system. The process for determining the diurnal demand pattern is described in the excerpt below from section 4.2 of the Roseville Water Distribution System Hydraulic Model Calibration and System Evaluation Report. Rather than reiterate this process, the same diurnal demand pattern from the Roseville model was projected onto the Arden Hills model using updated demand data. The system-wide diurnal demand curves for Roseville’s water distribution system were constructed by using the flow balance technique. For a water distribution system, a flow balance simply indicates that the water that enters the distribution system must be equal to the water that exits the distribution system. The demand for Roseville’s water distribution system is equal to the Booster Station flow rate plus or minus the flow into or out of the water storage facilities. A flow balance was performed from Booster Station pumping and water storage level readings from the SCADA system collected in five-minute increments. The data was then averaged into hourly increments to define the diurnal pattern over the entire day for the entire distribution system. Diurnal demand patterns were prepared for each day during the fire flow testing period and the extended pressure testing period from June 16th through June 22nd, 2017, as will be described later. From these patterns, an average day diurnal demand pattern was developed and is illustrated in Figure 4-1. The time step for the diurnal pattern used in the model was one hour. Although a smaller time step could be used, it is not recommended because small errors in water tower levels on time steps shorter than one hour can lead to large errors in water use calculations. The diurnal demand patterns were incorporated into the model and used in conjunction with the field data to assist in the calibration of the hydraulic model. The average day and maximum day diurnal demand curves are presented in Figure 4-2.