The Cobb County Water System informs us that despite 6 percent hikes in the water rates for 2011 and 2012, the increases are less than that because of the complicated and arcane system of water-rate figuring.

Somewhere within your monthly bill, there was a 6 percent increase but that was only “in the commodity portion of the water bill,” according to Cobb’s water rate expert, Kathy Nguyen, senior project manager for the system and a nice lady who painstakingly dissected the innards of what until now was considered by me to be a simple little monthly water bill.

The 6 percent increase is for water but does not apply to the “water base charge” of $7, which has not changed in years, Ms. Nguyen explained. So for a monthly use of 5,000 gallons of water in 2011 — including “water commodity tier 1” at 3,000 gallons and “water commodity tier 2” at 2,000 gallons — the water-only charge was $23.71, plus $26.50 for the “sewer commodity” charge for 5,000 gallons, bringing the total bill to $50.12.

Therefore, using those figures, the “2011 percentage increase from 2010 total bill” was 4 percent, said Ms. Nguyen, and it’s not to be confused by what the Cobb Water System website says. In answer to a frequently asked question, “How much have my rates gone up?,” the official reply is: “Water — 6 percent. Wastewater 2 percent.”

The answer continues: “This year we were able to lower our scheduled 8 percent increase for water down to 6 percent” because the water authority reduced its rate to 8 percent. But you see, the rate increase applies only to “water commodity” categories in the bill, not the total bill. I hope that’s clear.

Another example from Ms. Nguyen is a 2012 monthly bill “for 7,000 gallons of use water only and water and sewer customers.” It shows: “Water Base Charge $7, Water Commodity Tier 1 (3,000) $8.49, Water Commodity Tier 2 (4,000) $17.44” for a water-commodity-only total of $32.93, plus “Sewer Commodity (7,000) $37.87,” bringing the total bill to $70.80.

Based on those figures, says Ms. Nguyen, the “2012 percentage increase from 2010 total bill” was 6.9 percent, “cumulative increase,” and the “2012 percentage increase water only from 2010” was 7.9 percent, “cumulative increase” — and not 12 percent as stated in my Friday column. No doubt, my newfound water rate friend Kathy Nguyen is right on the money, and thus my figure was about four percentage points too high. Thanks to her for correcting the miscalculation and explaining the “water commodity” billing.

And for the record, the Cobb Water System “has among the lowest rates in the metropolitan region despite the annual rate adjustments,” our rate expert says. Good show, whether it’s based on the “water commodity” rate or the total bill.

Now can we get some of the county’s brainpower to work on fixing the problem of continuing to dip into the water system funds every year for $20 million or so to balance the county budget?

dmckee9613@aol.com