SFPUC Wholesale Customer Water Demand Projections ... - BAWSCA

SECTIONTHREE
Establishing Base - Year Conditions
use breakdown in customer-billing categories is accurate for the base-year conditions and
appropriate for forecasting future water demand.
In Section 3.3, a “top-down” approach to total water use is applied by taking total water
consumption by customer-billing category and breaking it down by indoor and outdoor use and
then by end use. Initial assumptions regarding the percentage of total water use consumed by
each end use were made based on AWWARF’s research on residential, commercial and
institutional end uses of water. These initial percentages, presented in Table 3-3, provide a
starting point for estimating how water is used in a customer-billing account.
In Section 3.4, total water use in a customer-billing account is considered from a “bottom-up”
approach using fixture models. The fixture models look at water use on a plumbing fixture basis,
computing total water use in a customer-billing account by the total water use associated with
particular end uses.
Both approaches are used as way to provide a checks-and-balance system for ensuring that
separating total water use into end uses has been performed accurately. The results of both
approaches are compared and adjusted through a calibration process to match one another.
The calibration process generally involves three steps:
• Establishing the initial conditions of the fixture or appliance being modeled (Table 3-10)
• Comparing the fixture model end-use amounts with the initial percentages of water use by
end use (Table 3-3)
• Adjusting the frequency of uses per day per user (person or employee in an account)
(Table 3-11), or adjusting the initial percentage breakdown of indoor water use by end uses
(Table 3-3) until the end use in the fixture model is the same as the end use derived by
applying a percentage to the total indoor use
An example of how the actual calibration process occurs follows. The example first reviews how
water use by end use is computed in the fixture model then outlines the process of calibration.
The calibration process is performed until the amount of water allocated to each specific end use
(expressed in terms of a percentage of the total indoor water use for that category) equals the
amount of water derived in the fixture model for that end use. As a rule, the number of uses was
not allowed to vary outside the ranges in Table 3-11 and adjustments to the initial proportions in
Table 3-3 were minor.
The calibration creates a revised breakdown of water usage by end use. Figures 3-6 and 3-7 show
the average breakdown of indoor single-family and multi-family water use for SFPUC wholesale
customers for the 2001 base year.
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