Standing at the entrance to the new West Side office of Southwest Hazard Control, an environmental-cleanup firm, are 11 corrugated metal tanks, all 8 feet tall and 6 feet in diameter.

After the West Grant Road site's native mesquites and yuccas and the South American jacaranda trees become established in a year or two, every drop of their irrigation water will come from the tanks.

On Oct. 14, the Tucson City Council will vote on an ordinance to require new commercial developments to harvest rainwater. If it's approved, most of those affected likely won't go that far. The proposal, supported by the city's staff and an advisory committee, mandates that 50 percent of a development's landscaping water must be from rainfall.

That's down from a proposed 75 percent that the committee had endorsed last spring and from some environmentalists' original hope for 100 percent.

The change was made in the name of flexibility after developers said 75 percent would be too costly and otherwise would be difficult to meet.

Not everyone is happy about it. The committee voted 8-4 last month to recommend the lower figure. Environmentalist dissenters said the group was moving without adequate discussion. Even now, not all developers are on board with the new proposal, but they're clearly happier.

But all factions involved agree that a 50 percent ordinance still would put the city in the vanguard. It would bring water conservation one more step toward the foreground during a drought that many scientists say threatens the city's long-term supply of Colorado River water, backers say.

Most committee members who voted against the latest change still support the current proposal. It's the only one of its kind in the nation, as far as Tucson officials know.

"We have to address the issue of drought, and we need to provide for as much creativity as possible when it comes to water conservation," said Ward 2 Councilman Rodney Glassman, who pushed the water-harvesting proposal. "Fifty percent is not the end goal. It is the minimum standard. It is just the beginning."

City staffers and advisory committee members said the key factor making 50 percent more flexible is that according to staff calculations, that wouldn't force developers to install expensive cistern tanks.

Instead, they could use "earthworks," a buzzword for the practice of building berms or contouring slopes on a site to guide rainwater to trees and shrubs. Or developers could depress curbs near landscape areas so water would flow into them.

"When people typically think of rainwater harvesting, they think of corrugated metal tanks next to the house. This will be something different," said Kevin Barber, an architect on the water-harvesting advisory committee. "It was found that there is sort of a breaking point — somewhere around 50 percent — where it would require both tanks and earthworks."

Rob Paulus, the architect who designed the Southwest Hazard landscaping system, agreed that a 75 percent rule isn't desirable, even though his site will use 100 percent.

The water-harvesting system there costs a little over $1 per gallon of stored water, he said, compared with an estimate from the business-backed Metropolitan Pima Alliance of $2 to $4 per gallon for a cistern system. But Paulus said that every site needs to be considered separately, and the city should give incentives such as permit-fee waivers to encourage harvesting.

The alliance, representing a wide range of business interests and also including some government officials, voted last week to endorse the 50 percent standard. A Tucson Chamber of Commerce official said that 50 percent is much more reasonable than 75 percent.

"There could be scenarios where cisterns are the least expensive option, but now at least the developer has different options to choose," alliance Director Michael Guymon said.

On the environmentalist side, Sierra Club committee representative Matt Hogel said the 50 percent standard is too low, because he believes Tucson is 15 years away from a water crisis.

The Tucson Audubon Society and the Sonoran Institute were mainly concerned that after hearing the city staff analysis on Sept. 18, they needed more time to digest what they felt were complex calculations.

City staffer Ann Audrey said her studies found that roughly a 50 percent standard gives developers the flexibility to handle year-to-year fluctuations in rainfall. Another desert reality that must be considered is that some areas get more rain than others during a storm, she said.

Advisory committee member Brad Lancaster, a harvesting expert who has written two books on the subject, said he believes that if developers plant low-water-use plants — which city codes already require — they can meet a 75 percent goal once plants are established. Lancaster was out of town for the committee vote, didn't otherwise take sides on the 50-75 percent issue and now supports the proposal.

But conditions for plants at a shopping center are far harsher than in the natural desert, replied Audrey, of the city's Office of Conservation and Sustainable Development.

A critical point that wasn't discussed is the balancing act between dealing with a continuing drought that could be aggravated by global warming, while the business community faces a serious economic crunch, said Amy McCoy of the Sonoran Institute.

"We don't want to create an ordinance that puts an onerous financial burden upon people," McCoy said. "We're trying to find that spot where we can use this untapped resource of rainwater in an economical and efficient way."

Councilman Glassman defended the decision to move quickly, saying that all sides originally had agreed to an Oct. 14 vote. As for the committee membership, he said he wanted balance.

He noted that a few months ago, the Southern Arizona Home Builders Association had criticized as unfair another committee recommendation to make gray-water plumbing mandatory in new homes, because existing homes use far more water. The recommendation became law last month after SAHBA withdrew its objection.

"Perhaps there's balance in everyone feeling it's unbalanced," Glassman said.

What: City Council water-harvesting discussion.

Where: In Council Chambers, 250 W. Alameda St.

When: Meeting starts at 7 p.m. Oct. 14.

On StarNet: Find environment and wildlife resources including a special report on global warming and a database of Southern Arizona critters at azstarnet.com/environment