Water meter readings don’t add up

By From page A6 | March 25, 2014

Here’s an explanation for everyone wanting to conserve water who is bewildered by the fickle gallons per day (GPD) reporting. Math teachers could offer this to students as a real-world example of misinterpreting numbers.

Davis meter reading accuracy stops at the digit that represents 100 cubic feet. If the last three digits advance in a particular month from 200 to 799 (599 cubic feet), a consumption of only 500 cubic feet (5 CCF) is reported because the hundreds digit advanced from 2 up to 7. If the meter were to advance from 199 to 800 cubic feet (essentially the same actual consumption at 601), the water bill would show 7 CCF given the hundreds digit increase from 1 to 8.

For a 31-day month, the corresponding GPD numbers are 121 for 5 CCF and 169 for 7 CCF. The actual consumption above is 145 GPD. While the CCF numbers can be merely inaccurate, the GPD calculations are misleading because the three-digit numbers tend to imply unrealistic accuracy.

In early January, I happened to read my own water meter on the same day that the city’s contractor did. The last three digits were 690. Because the hundreds digit had not quite reached 7, my reported December consumption was unusually low by 1 CCF, while the number for January was unusually high. Accepting the numbers at face value would incorrectly suggest that my family wasted a lot of water in January.

While the inaccuracies average out over time, they are one reason to not strictly bill us “per gallon” for the measured amount each month. My suggestion for the city is to include the range of uncertainty when GPD is reported, then reconsider whether the information is useful for water conservation.

In 2008, I asked the city of Davis billing department to show the starting and ending meter readings, as we see on our bills for electricity and gas. Such an obvious customer-friendly feature remains elusive. If a local electric utility district ever replaces PG&E in Davis, will the monthly statements similarly omit key information?

John Whitehead

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