Page 111 - Water Loss Control

Evaluating W ater Losses 93

0.4 f(x)

0.3 (x – m) 2
– 2s 2
f(x) e 1 e
0.2 s 2p

0.1

0 1 2 3 4 5 6
+
m− s m m s
FIGURE 7.6 Normal distribution curve. (Source: WSO).
Annual Volume 95% Confidence Variance (gal 2
12
Component (Million gal) Limits × 10 )
Source #1 7,512.80 2.6% 9,553.7
Source #2 10,519.84 2.6% 18,732.1
Source#3 6,580.71 2.6% 7,330.2
Source#4 4,411.61 2.6% 3,294.3
Source#4 7.60 2.6% 0.0
Total system input volume (a) 29,032.56 1.3% 38,910.3
Billed metered authorized 24,778.64 1.1% 20,237.7
consumption
Billed un-metered authorized 0.0 NA NA
consumption
Total billed authorized 24,778.64 1.1% 20,237.7
consumption (b)
Nonrevenue water [( ) − (b) ] 4,253.92 11.2% 59,148.0

Source: SFPUC

TABLE 7.5 Calculation of Confidence Limits for Nonrevenue Water
The standard approach to calculate the variance related to a certain volume of the
water balance, based on its 95% confidence limit, is as follows:

Variance = (volume in million gal × 95% confi dence limit/1.96) 2
The aggregated confidence limit related to a calculated volume of the water balance
is based on accumulation of error on derived values. Following these principles the
95% confidence limit related to the calculated volume of nonrevenue water is calcu-
lated as follows:
95% confidence limit for nonrevenue water

.
= 196 − (variance a + variance b )/(annual volumenonrevenuewater)