Page 66 - Water and wastewater engineering

GENERAL WATER SUPPLY DESIGN CONSIDERATIONS 2-9

Population water source System supply source
Surface
water
8%

Groundwater
39% Surface
water
61% Groundwater
92%

(a) (b)
Number of systems (thousands) Population served (millions)
Large Small
Medium 3.6 25
4.3
Medium
25.1

Small Large
202.4
45.5

(c) (d)
FIGURE 2-2
(a) Percentage of the population served by drinking-water system source. (b) Percentage of drinking-water systems by supply
source. (c) Number of drinking-water systems (in thousands) by size. (d) Population served (in millions of people) by drinking-
water system size.
Source: 1997 National Public Water Systems Compliance Report. U.S. EPA, Office of Water. Washington, D.C. 20460.
(EPA-305-R-99-002).
(Note: Small systems serve 25-3,300 people; medium systems serve 3301–10,000 people; large systems serve 10,000 people.)

Complete Series. A complete series analysis is used to construct a flow-duration curve. This
curve is used to determine whether or not the long-term average flow exceeds the long-term aver-
age demand. All of the observed data are used in a complete series analysis. This analysis is usu-
ally presented in one of two forms: as a yield curve (also known as a duration curve, Figure 2-3 )
or as a cumulative probability distribution function (CDF). In either form the analysis shows the
percent of time that a given flow will be equaled or exceeded. The percent of time is interpreted
as the probability that a watershed will yield a given flow over a long period of time. Thus, it is
sometimes called a yield analysis.
To perform a yield analysis, discharge data are typed into a spreadsheet in the order of their
occurrence. Using the spreadsheet “sort” function, the data are arranged in descending order of
flow rate. The percent of time each value is equaled or exceeded is calculated. The spreadsheet
is then used to create the duration curve: a plot of the discharge versus the percent of time the

discharge is exceeded. This is demonstrated in Example 2-1 using the data in Table 2-7 .