Page 266 - Water Loss Control

Contr olling Appar ent Losses—Systematic Data Handling Err ors 237

How the customer billing system awards this credit has bearing on both the billing
(financial) and operational (engineering) functions of the system. While money can flow
both to and from the drinking water utility—via charges and credits, respectively—
water flows in only one direction, being supplied by the utility to the customer. If the
billing system contains only a single field for customer consumption, then the billed
consumption value for August 2004 is negative 4132 cubic feet. While a negative con-
sumption number is acceptable for use for billing (financial) reasons as it translates into
a monetary credit, a negative consumption number is unacceptable for operational
(engineering) purposes since the actual consumption for August 2004 was 825 cubic
feet (Column G), not negative 4132 cubic feet as shown in Column D.
The distortion of the consumption data is further reflected in the estimated versus
actual consumption based upon yearly periods. Water utility analysts reviewing the
account data shown in Table 14.4 for conservation or loss control purposes would be in
error by 3840 cubic feet (10,620 minus 6780) over the actual consumption in 2003. Con-
versely, the analysis would be understated for this account by 3967 cubic feet (8915
minus 4948) in 2004. Some may reason that the periods of estimation and adjustment
ultimately balance with no net difference over the long term, therefore using a single
consumption value is acceptable. However, many analytical and reporting functions
are performed over the course of a calendar or business year. If a given account has
been poorly estimated for many years, the use of a huge multiyear adjustment in the
last year will greatly distort the consumption for that final year. Additionally, in any
given drinking water utility many hundreds or thousands of accounts could utilize
estimates for varying periods of time. Reliably estimating the net impact of the aggre-
gate overestimation or underestimation of these accounts in a given year is unnecessar-
ily complex. Clearly, while a negative consumption value can be acceptable for billing
(financial) purposes, it is quite harmful to the integrity of the data for operational (engi-
neering) purposes.
For the reasons explained above, it is recommended that water utility customer
billing systems include two separate fields for customer consumption: one for regis-
tered consumption and a separate field for billed consumption. Using the same data
from the example in Table 14.4, the form of the data with separate fields is shown in
Table 14.5.
Table 14.5 includes separate columns for billed consumption (Column D) and regis-
tered consumption (Column G). When actual meter readings resumed in August 2004
the consumption adjustment of negative 4132 cubic feet appears as billed consumption
in Column D and is used to generate the monetary credit to the customer. However,
Column G reflects the revised estimate of consumption for the prior 30-day period,
which is based upon the difference between the two most recent actual meter readings
(September 2001 and August 2003). This one-time estimate is determined as:

(38345 − 23007)/23 months = 667 cubic feet

By September 2004, the second consecutive actual monthly meter reading was
obtained, estimates are no longer utilized, and billed consumption once again matches
registered consumption. The benefit to the operational integrity of data using separate
billed and registered consumption fields is shown by comparing the cumulative con-
sumption for 2004 in Column D and Column G, or 4948 and 9747 cubic feet, respectively.
If only a single field is used for consumption the billed value of 4948 greatly understates