Renton has plenty of water – for now – unless everyone decides to use it all at once.

That peak use happens in the summer, when lawns turn brown and plants wilt in the heat. Too often, the water is wasted as runoff, doing little to quench a plant’s or a lawn’s thirst.

For several years the peak use on some days has crept close to the regulatory limit the city is allowed to draw from its water supply, a large underground aquifer, at any one time.

Two years ago, the city instituted a rate structure based on the use of a block of water. Those who use less water pay less. Those who use more water – say those with a big lawn – pay more for their water.

“The issue is to try to change behavior using a cost signal,” Lys Hornsby, the city’s utility systems director told the Renton City Council Utilities Committee recently.

But behavior didn’t change enough and now the city is trying again, with potentially even higher rates for the biggest water users or irrigators.

Any rate changes would go into effect in summer 2011.

Complicating efforts to curb peak use is the likelihood that the city will impose double-digit, water-rate increases to make up for several years when rates remained relatively steady.

Some council members worry about the political ramifications – and the impact of extra fees on residents’ budgets – of adding to the cost of summer water at a time when a general rate hike looms.

The Utilities Committee was briefed recently on the proposal for the new blocks of water and asked for additional information. There’s a lot more work to be done, they told public works director Gregg Zimmerman and Hornsby.

One committee member, King Parker, said it was “a revelation” that the city was pushing up against the limit for water use at a particular time.

Parker and the other two committee members, Terri Briere and Rich Zwicker, strongly suggested the city should consider educating the public about conserving water.

“I think that’s far more beneficial than raising rates,” Parker said.

Hornsby pointed out that money for such education was reduced in this year’s city budget. And, she said, the city may not have time “to get the education out there” to make a difference.

The city is not trying to raise revenue through higher rates, just control use, Hornsby said.

It’s likely a new rate structure would have little impact on the water bill of those in the lower blocks. That impact would increase, even more than it does now, as consumption grows.

Because of its parks and golf course, the City of Renton is the biggest water user in the city. It has instituted a number of practices and programs to limit its water use and to help residents do so, too.

Rate discussions come at a time when the city is preparing for an update of its comprehensive water plan, something it’s required to do at regular intervals. Proposals will go to the City Council later this year.

How much water the city can use annually and at any one time is controlled by longstanding water rights issued by the state Department of Ecology.

One water right, known as Qa (the a stands for annual), allows the city to draw up to 4.8 billion gallons of water annually from its underground aquifer. The aquifer is a stream of water flowing constantly under much of Renton from the Cascades.

The second water right is known as Qi (the i stands for instantaneous) that allows the city to draw up to 13,950 gallons per minute from its aquifer at one time. It’s the Qi limit that has nearly been breached in recent summers.

The city is breaking the law if it exceeds the Qi, but residents would still get water. Sanctions include a moratorium on growth.

Although it seems Western Washington is awash in water, it’s not always available where it’s needed the most or overuse can rob other users of their rights. Through that Qi right, the ecology department ensures that one user at a particular point in time isn’t taking too much water to the detriment of others, including fish in the Cedar River.

The increased demand on city water only comes from infill and from redevelopment within the city, Zimmerman said in an interview. Annexations don’t stress the city’s water supply because the water districts serving those unincorporated areas continue to do so after an annexation, he said.

Still a time is coming, in roughly 20 years, when the city could exceed its annual water right and need to add a new water supply, Zimmerman said.

Some possibilities exist already, including buying water from the Cascade Water Alliance or Seattle Public Utilities. Conservation is another source of water.

For now, the planning is routine and the city is not facing a problem in terms of supply.

“We are definitely not in a crisis,” Zimmerman said, adding “I want to get the Qi under control.”