8  Model Description    8.2.1  Supply System    8.2.2  Reservoir Operations    8.2.3  Moraine Seepage    8.2.4  Instream Flow Requirements    8.2.6  Conservation    8.2.7  Future Sources    8.2.8  Hydrologic Data    8.3.7  TSI/SSP

The annual base demand forecasts for the Seattle service area (direct and wholesale customers) were obtained from SPU in 1999 and are based on the population growth estimates of the Puget Sound Regional Council.  Weekly demands are calculated by multiplying base demand values by a scaling factor. The scaling factors can either be conventional or variable, determined by the user.  Conventional demand multiplies the base demand by a monthly factor to determine water usage for a specific month.  For instance, winter month demands are 0.85 the base demand, while the peak summer month (July) is 1.4 times the base demand. Variable demands consider historical temperature and precipitation records and adjust demand levels for climate.

In addition to the conservation program SPU is implementing (see Conservation and Curtailments), the city of Seattle recently agreed to a 1% conservation savings per year for 10 years; however, the purveyors that constitute over 50% of Seattle’s demand chose not to adopt this target as a group, but may choose to implement similar programs on an individual basis.  Historical demand, projected demand, and 1% conservation demands for the Seattle service area are depicted below (Figure 8.17).  SPU has created demand projections up through the year 2020.  The Crystal model uses linearly extrapolated demands from these values for the years 2021-2040.

The demand forecasts already account for price elasticity of water demands to water and wastewater rates.  In the future, price elasticity will be incorporated into the model to evaluate the impacts of hypothetical pricing changes on regional demand.

Table of Contents    8.1.1  Everett Supply System    8.3.1  Tacoma Supply System