Ann Arbor resident Barb Gilbert waters flowers in her yard at her Sixth Street home back in June. Water usage went up to 20.5 million gallons per day in Ann Arbor in June, compared to 16.4 million gallons per day in June 2011 — a 25 percent increase.

Jeffrey Smith | AnnArbor.com

Ann Arbor city water customers used nearly 337 million more gallons of water from May through August this year compared with 2011 — a 16.3 percent increase, according to an AnnArbor.com analysis of data obtained from the city's water department.

Molly Robinson, the city's water treatment plant manager, said there's no doubt the drought and heat are to blame for the increased levels.

"We definitely attribute it to the hot weather and the lack of rain we had over the summer — definitely more people in general watering outside," she said. "We can watch that very easily at the plant. As soon as temperatures drop or it rains, the demand drops off."

Sprinklers water the front lawn of the new City Place student apartments on a hot day in mid-July. Molly Robinson, the city's water treatment plant manager, said she's not sure how much new developments like this have contributed to the city's increased water consumption.

Ryan J. Stanton | AnnArbor.com

In June, water usage went up to 20.5 million gallons per day, compared with 16.4 million gallons per day in June 2011 — a 25 percent increase.

In July, it went up again to 22.9 million gallons per day, compared with 21.1 million gallons per day in July 2011 — an 8.5 percent increase.

And in August, it slowed to 19.8 million gallons per day, compared with 17.3 million gallons per day in August 2011 — a 14.5 percent increase.

There was one instance in July when city water customers used 28.1 million gallons in a single day, and another day in August when consumption peaked at 25.2 million gallons.

The most recent report for September shows water usage tapered off to 17.1 million gallons per day, compared with 15.7 million gallons per day in September 2011.

That's still 1.4 million gallons more per day — or a 9 percent increase.

"Use was high at the beginning of the month, but then dropped off quite a bit," Robinson said in reference to the September report.

Robinson said she's not sure how much new student housing has contributed to the city's increased water consumption. The projects that have been finished — Zaragon West, City Place and Landmark — only just opened within the last month or two.

"Development is ticking up in the city, but I can't say we can attribute it to that," Robinson said of the increased water consumption. "If you look at the different months, you can see the increases and it correlates to the increases in temperature we saw."

The city's drinking water rates for most residential customers are tiered based on usage. The city measures usage in units equal to 100 cubic feet of water or 748 gallons.

A typical single family bill is calculated with 21 units per quarter, is in the second tier for stormwater and receives a 10 percent discount for payment on or before the due date.

Even at the highest rate, the city's charges for drinking water are still significantly less than a penny a gallon.

Robinson said she thinks Ann Arbor residents are conservative in their water use, and the city's climbing rates — where the more you use, the more you pay — help with that.

"In general, the residents in the city are conscious of conservation," she said. "Overall, our per capita use is quite low compared to other municipalities, so I don't think the jump is a negative for us."

In total, city water customers used nearly 5.2 billion gallons of water last year. Usage for the first three quarters of this year totals nearly 4.4 billion gallons.

AnnArbor.com requested reports showing total billed water charges for the past two quarters of this year compared with last year.

Lisa Wondrash, a spokeswoman for the city, said the city isn't able to easily provide that data because there are 12 water districts, all of which are billed quarterly and at different intervals.

Ryan J. Stanton covers government and politics for AnnArbor.com. Reach him at ryanstanton@annarbor.com or 734-623-2529. You also can follow him on Twitter or subscribe to AnnArbor.com's email newsletters.