Water bills in Larchmont are slated to rise later this year due to rising costs from the village's water source -- the New York City water supply system.

The village has kept residential water rates constant over the past three years while absorbing year-to-year increases set by the New York City Water Board, a five-member council appointed by New York Mayor Michael Bloomberg.

Those increases have been as high as 11 percent annually, said Denis Brucciani, the village treasurer. Larchmont pays New York City about $900 per million gallons of water, and like other municipalities dependent on New York City's water supply system, the village is limited by a cap on water usage. If residents use more than the allotted amount -- and officials say they often do during the warmer months -- the per-million-gallons cost for additional water rises to $2,567.

The consumption cap is set on a monthly basis by New York City by analyzing the per-capita water usage for city residents. That means residents of communities like Larchmont are allowed to use the same amount of water as their New York City counterparts. But needs in the leafy suburbs are often very different: The rates make no allowances for water used for lawns, private pools and gardens, features common to suburban homes but exceedingly rare in the city, outside of multi-million-dollar luxury penthouses that might have rooftop gardens.

Because the water usage cap for suburban locations varies from month-to-month based on how much water the average New York City resident uses, there's no such thing as a typical month -- but officials can be specific with some numbers, such as the 24.4 million gallons of water Larchmont was allotted for March of this year. That works out to about 121 gallons per person, per day, a figure that may seem high without considering things like lawns and pools.

Mayor Joshua Mandell said the move to pass on the increased costs to residents "is probably going to happen some time before October." Officials say they don't have numbers yet for the average cost per household, but they do have an overall estimate.

"The increase that you've seen elsewhere, like in the village of Mamaroneck, of 18 to 20 percent, are probably within the ballpark," Mandell said at Monday night's board meeting.

Michael Gottfried, a Larchmont resident who sits on the village's finance and budget committee, said fellow residents should keep in mind the last increase within the village was three years ago.

"Even with the rate increase, it's still going to be a very competitive water price relative to the various jurisdictions around here," Gottfried said.

The village plans public hearings on the issue before action is taken, and one option open for discussion is smaller annual adjustments that could average out to about six or seven percent, rather than larger increases every three years.

The New York City water supply system provides more than 1.2 billion gallons of water per day to about one million people who live north of the city, in addition to the estimated 8.3 million city residents, according to the New York City Municipal Water Finance Authority. The penalty for consuming more than the allotted amount of water is known as an "excess consumption charge," and Larchmont's trustees say they'd like increased cooperation on the part of residents for water conservation efforts.

"Water conservation is not only important for your personal finances, [but also] for the village's excess charges, which are enormously high," said Trustee Marlene Kolbert.

So far this fiscal year, which will end May 31, Larchmont has paid $434,000 to New York City for water, Brucciani said. The annual excess consumption charge has climbed by an average of thirteen percent over the past three years.

Larchmont's elected officials can't do anything about how much New York City charges for water -- or how much the city charges in penalties -- but with the cooperation of residents they can cut down on the costs of using too much water. That's why village officials think it's important to let the public know conserving water really does help save money in the long run, as those gallons add up.

"While you can't control the rates," Brucciani said, "you can certainly control consumption."