A demand site is best defined as a set of water users that share a physical distribution system, that are all within a defined region, or that share an important withdrawal supply point. You also must decide whether to lump demands together into aggregate demand sites (e.g., counties) or to separate key water uses into individual demand sites. The level of aggregation generally is determined by the level of detail of water use data available. Demand data may not be available for individual sites, but may only be available for a larger unit such as a city or county. In addition to data, your definition of demand sites may also depend on the level of detail desired for your analysis.
When defining demand sites, it is useful to inventory the actual physical infrastructure, such as pumping stations, withdrawal facilities, wastewater treatment plants and well fields. You should think carefully about the configuration of the entire demand and supply system, including the links between supplies and demands. You should also take into consideration the details of the water accounting picture you wish to present, any key water uses, and any key supply sources and river points that need to be tracked, described and evaluated. You might want to define demand sites according to the following groupings:
major cities or counties
individual user which manages a surface or groundwater withdrawal point, such as an industrial facility
irrigation districts
demands which return to a unique wastewater treatment plant
water utilities
Each demand site needs a transmission link from its source, and where applicable, a return link either directly to a river, wastewater treatment plant or other location. The demand site cannot be placed directly on the river. The user-defined priority system determines the order of allocations to demand sites.