Saving for a rainy day.

"Instinctively, we knew that 'paving paradise and putting up a parking lot' was a bad idea," says Gary Moll of AMERICAN FORESTS, "but now, with scientific and engineering data, we can prove it." Moll works with dozens of city and suburban officials who have witnessed what songwriter Joni Mitchell warned of - block after block of impervious surfaces replacing spongy, leafy ones. Replacing tree cover with a "hard-scape" creates a permanent problem that contributes to, but does not help mitigate, heat waves, polluted air, and surges of stormwater.

"Cities find themselves with less and less greenscape at a time when they could really use the help natural systems provide," says Moll. As clean air and water regulations - federal, state, and local - move into their more stringent phases, the costs of complying also start to climb.

Phase II of the federal Clean Water Act, for instance, means cities with fewer than 100,000 people must now come up with plans to manage and monitor stormwater discharges into streams and lakes, plans that may include expensive retrofits of their existing containment structures or treatment plants.

Meanwhile, at the growing edge and the densifying middle of cities, trees that could help us meet environmental rules are still being lost or removed because of a lack of understanding. As Moll puts it, "We're scratching our heads and draining our budgets to come up with mitigation technologies that paradise could have provided for free."

Natural Allies

Luckily, many cities still have some organic infrastructure - trees, vegetation and healthy soil - in place, and they are beginning, for the first time, to take stock of these allies. Some cities, like Bellevue, Washington, use AMERICAN FORESTS' analysis and mapping software, CITY green, to superimpose a green matrix of organic infrastructure onto city, maps. And since most communities put their official data into some sort of computer database, this layer of green can show up on computer screens throughout all city departments. And what's on their computer screens, contends Moll, will be on their radar screens as well.

When it comes time to widen a road, for instance, the natural vegetation shaved away will be visible on the GIS maps, and therefore part of the decision-making process. Even more useful is the fact that the loss will be recorded not just in acreage of greenscape lost, but in dollars and cents squandered.

"When we have to replace the services that trees provide - filtering and cooling air, cleansing water, and reducing runoff - we usually turn to structural engineering solutions that have hefty price tags," Moll says. By quantifying those costs, CITYgreen shows how much trees are worth, and this enables city leaders to compare them head-to-head with other public goods, like curbs, gutters, and traffic signals, that are competing for space in the new road plan.

In Bellevue they came up with a unique way to demonstrate that trees count. At an Arbor Day tree-planting event, oversized "tags" tied to trees detailed their worth: $4,055 benefit: stormwater storage $183, air quality improvements $241, and aesthetics (property value increase) $3,631.

"Our goal is to make natural vegetation one of the many tools that engineers can reach for," Moll says. Nowhere is the case for trees-as-tools easier to make than in the issue of stormwater abatement.

A Slippery Slope to a Scoured Stream

Stormwater containment costs are a major item in developers' budgets and one of the many downsides of paving paradise. When you add impervious surfaces - asphalt, rooftops - you give rainwater a sure, fast slide to the street. Without a complex of tree branches to slowly drip down, a spongy layer to soak into, raindrops land on the side of a building or on a parking lot and stream straight to the gutter. This deluge of drops decreases the time it takes to reach a stream or lake and increases peak flow - sometimes to the point of overload.

The more impervious surfaces you have, the more damage storms can do. What would be a mild storm in a city. of spongy natural areas becomes a flood in a city of sloping storm drains. When all that water, laden with the chemicals that ooze from our crankcases and our fertilized lawns, finally reaches a natural river or stream, it's one large dose of poison.

Even without the chemicals, a torrent of sediment-laden stormwater can wreak havoc with natural watercourses.

For all these reasons stormwater control and treatment is a big infrastructure cost for cities. Those costs are rising as clean water laws shift into more-vigilant phases. Traditionally, developers have used hardscape solutions - containment ponds, underground vaults, or recessed parking lots. CITYgreen reminds engineers that greenscape provides some of this containment naturally.

Don Woodward, national hydraulic engineer at the Natural Resources Conservation Service, is one of the authors of TR-55, the stormwater model embedded in CITYgreen. When introduced back in 1986, the model's genius was its runoff index, which designated "runoff curve numbers" to various land types according to characteristics that would affect rate of runoff: soil type, land-use practice, and condition of practice.

The amount of runoff reduction gained by leaving trees on a site is easy to calculate. The costs of building a containment structure such as a pond to effect that same amount of reduction can also be calculated. By inputting these costs into the program, planners can make comparisons and see exactly how much trees in their market are worth in terms of stormwater abatement benefits. They also can see what other benefits the trees are providing - energy savings to homeowners, property value enhancement, air pollution mitigation through carbon storage. "Suddenly the light bulb goes off," says Woodward. "And you have developers saying 'Maybe we can save these trees after all.'"

Plant a Tree, Save a Salmon

Dan DeWald, natural resource manager for Bellevue parks and community service department, is intent on sparking similar epiphanies in the citizens of his fair city. He and his colleagues were early adopters of CITYgreen, and they immediately began to use it to deepen people's appreciation for natural landscapes. The tree tags were one way that worked for both education and marketing.

"We've depended for too long on representing the intrinsic value of natural areas; but as competition for funds gets stiffer, we have to present the value of trees - to citizens, city employees, and elected officials - in terms of economics. At the parks department, we recognized that CITYgreen could help us do that."

Bellevue's parks department is known for this kind of innovation. Years ago, they began to build what landscape giant Frederick Olmstead called a "green necklace" of parks. Now a Bellevue squirrel can tree-hop through 1,800 acres of interconnected parks on its way to Snoqualmie Pass.

Informal green spaces are also a part of the city's ecosystem. To capture these in the database, DeWald is conducting on-the-ground inventories with citizens from the volunteer group Advance Bellevue. As this "green layer" is incorporated into the GIS database, all city departments will be able to consider natural landscapes in their planning discussions from day one.

"Bellevue is luckier than most smaller municipalities in the region because it has had a stormwater utility for 24 years and has most, if not all, of the required stormwater programs in place to protect its streams and lakes," says Phyllis Varner, surface water quality manager for the city's utilities department.

Bellevue is also lucky, says Varner, to have preserved its natural infrastructure to a large extent: "Although there is little undeveloped land left in the city limits, Bellevue has managed to maintain a high percentage of its canopy cover." This is due in part to an early stormwater utility policy that preserved Bellevue's streams (rather than piping them) and a 1987 sensitive area ordinance that further protected the city's streams and wetlands.

Today, the pressure to develop has moved to the larger Puget Sound Basin in which Bellevue lies. As Varner notes, "Many of the cities and counties in this basin are experiencing significant pressure to build in areas that were formerly considered unbuildable for a number of reasons such as accessibility, commuting distance from downtown areas, development costs, etc."

It's in this regional arena that Varner sees the potential for the desktop GIS software to help regional resource policymakers consider the tradeoffs between developing or preserving lands in the Puget Sound Basin. "The biggest issue in stormwater, to my way of thinking, is land use," says Varner. "We have to look at the big picture - consider the whole watershed and consider land use on a regional basis. Once you get above a certain percentage of impervious surfaces within a watershed, there are enormous impacts."

Already, fast-growing cities like Atlanta have lost enough of their natural infrastructure to rack up big costs. Twenty years of boom has removed 60 percent of the natural forest cover (undeveloped areas) in the Atlanta metro region. A recent AMERICAN FORESTS analysis calculated that losing the last 20 percent produced a 1 billion cubic-foot increase in stormwater runoff, equivalent to a $2 billion increase in stormwater management costs. Looking at the existing benefits of tree cover in a smaller study area, Austin, Texas' urban forest is worth $1.4 billion in stormwater management services.

Once lands are covered with streets and buildings, says Varner, there's not much room for additional stormwater storage. And retrofits are inherently expensive. "When you take away undeveloped land, you take away options," says Varner. But that message has been hard to sell without dollar values to back it up. "If CITYgreen could attach values to the benefits provided by regional natural areas, it could perhaps serve as a "voice" for these lands in the upcoming debates over development." Without this voice, Varner fears, Puget Sound Basin's undeveloped lands might be appreciated too little and too late.

Janine Benyus is a freelance writer in Stevensville, Montana.