When it comes to using water, local towns are damned if they do, damned if they don't.

At a time when other towns are preaching water conservation to avoid overconsumption charges, the Village of Mamaroneck is facing the certain prospect of a 75 percent water rate increase (or an added $300 to the average water bill per year), all because its residents didn't use enough water last year.

As part of the Westchester Joint Water Works -- a cooperative of local towns that split the cost of buying water from New York City -- the village is locked in when it comes to water rates and usage, and if residents don't use the water, the village can't bill them for it.

That's what happened last year, when Mamaroneck village estimated more than $11.7 million in revenue from water consumption, and ended up $1.3 million shy of that mark. The reason? It rained too much.

That shortfall came despite a 17 percent increase in the water rate for 2009, said Dan Sarnoff, the village's assistant manager.

"So what that tells you," Sarnoff said, "is consumption was down by a tremendous amount."

More rain means less water used to irrigate golf courses, less water used by lawn sprinklers, and fewer people doing things like washing their cars in the summer months. The unpredictable nature of the weather makes estimating water use a difficult game for public officials, who look to past years and trends to set future rates. But as Town of Mamaroneck Supervisor Valerie O'Keeffe noted, it's not an exact science, and the best estimates can be ruined by the whims of clouds and air currents.

"Nature affects the amount of water people will use," O'Keeffe said. "We try to use norms and look at averages over a period of time, and that's how we try to guess...but you can't always tell."

The town just passed its own water rate increase, its third over a span of about six years. With a vote on May 5, the town council decided on an 18 percent increase over the previous rates set a year and a half ago. The town was also hurt by last year's light water consumption, but its revenue shortfall wasn't as devastating as the gap village leaders are dealing with.

The village is expected to vote on its water rate increase at Monday's board meeting, when residents will have a chance to voice their opinions on the hike during a public comment period.

Membership in the Joint Water Works gives towns and villages advantages in the economies of scale, or, as Sarnoff called it, "The Walmart Principle": The combined towns have greater buying power and lower operational costs as a cooperative, each avoiding the massive expense of running individual water utilities.

Aside from buying and delivering the water, towns cover other costs, like the full-time technicians and water treatment chemicals needed to operate the water works.

But ultimately, the water rates are set by the New York City Water Board, a five-member council appointed by Mayor Michael Bloomberg. New York City provides water to about a million people who live north of the city, as well as 8.2 million people who live in the five boroughs, and elected officials in lower Westchester bemoan the fact that the city's water board sets rates based on the average city resident's water consumption.

That means Westchester residents with lawns, backyards, gardens and pools are allotted the same amount of water as urban apartment-dwellers. If residents in a town consume more water than New York City allocates for them, their town is hit with "excess consumption charges," and the per-gallon rate is more than doubled.

In Westchester, using more water in the summer months is not uncommon. That's why elected officials have to estimate, and it's also why Mamaroneck village was predicting $11.7 million in revenue -- in a normal year, many lawn sprinklers would be spinning full blast, and golf course greenkeepers would be pumping out thousands of gallons to keep their fairways and roughs ready for divots.

But with rising costs associated with buying water from Westchester's big neighbor to the south, public officials say they don't have much control over the cost of water.

"We're passing on what New York City is doing to us, to our wholesale customers," O'Keeffe said. "The costs to deliver the water from the water works to the customer are costing more and more, and the water rates from New York for the wholesale water are higher and higher."

The village board will meet on Monday, May 10, at 7:30 p.m. in the courtroom at Village Hall. For more information, visit the Village of Mamaroneck's Web site.