I am happy to be speaking at the Green Cities Florida conference in May (see link on home page) on the topic of “vegetated water management on site and structure.” Increasingly, I am convinced that vegetation is the key to a “green” city for more reasons than color, and that water management is inextricably linked. How is vegetation more important than, say, energy or health care? Let’s take a quick look at the role of vegetated systems.

What we are learning is that vegetation has the ability to temper the impacts of most urban problems. With health care as an example, any guesses as to the leading cause of death by natural disaster? Earthquake or flooding perhaps? In fact it is heat waves. That’s right, heat waves kill more Americans than any other natural disaster…and there is some question as to whether or not this disaster is natural. Studies indicate large cities create “urban heat islands,” in which peak temperatures increase 8º, 10º, or even as much as 14ºF due to the thermal mass created primarily by concrete, asphalt, and roofing systems. It is easy to speculate that dropping peak temperatures by 8-10º would dramatically reduce associated deaths. This is a known problem, and “high-albido” surfaces and roofs — those that reflect heat back into the atmosphere — earn credits in the LEED rating system. There is some question as to the wisdom of reflecting heat in a warming climate, but let’s stay on track.

In other studies, from Florida to Canada, it has been shown that exterior roof temperatures can be reduced 90º+ by applying green roof systems. Not only do the plants protect the surface from heat, but foliage also evapotranspires moisture into surrounding air, creating a cooling effect. Using such a strategy at scale, one can deduce that overall city temperatures can be reduced by the simple application of vegetation to our rooftops and increasing planting areas and tree cover, which many cities are beginning to understand.

But it doesn’t stop there; our energy infrastructure is designed for “peak loads,” or the moment of highest energy demand. At other times, we have plenty of generation, but it is that moment of spike that the system capacity is designed to handle. In the west our most severe energy demands are during these heat waves, and those who live in the west will recall that we suffer from power outages on those hottest days of the year when air conditioners are working their hardest. Not only can vegetation help cool cities overall, but green roofs can also reduce the cooling needs of a specific building by as much as 30% according to recent studies. So not only are we solving a health crisis, but we are also removing the peak from our energy capacity woes.

The obvious question is, “but doesn’t it take a lot of water to keep that vegetation alive?” The answer is yes, but let’s consider what water it takes, and agree that we should not redirect drinking water to rooftops. 35% of household water use is for shower and bath. Another 20% is for laundry. These minimally tarnished waters are known as “graywater,” and have been legal for irrigation in the State of California since 1992 (under a code so draconian that only 200 of an estimated 1.7 million California systems have permits, according to the Soap Industry). This 55% of household water use is sufficient to irrigate urban vegetation, with the added benefit of reduced sewage volume (and associated reductions in energy for pumping). Very simple disinfection systems can eliminate all risk of pathogens at site scale inexpensively.

To recap, we are proposing the use of vegetation and wastewater to reduce urban temperatures and associated deaths while reducing energy associated with air conditioning and sewage pumping. Sounds like a good deal, but does it pencil out for the land owner? In some cases it does pencil out, primarily due to reduced maintenance and energy costs (green roof systems protect the membrane from UV and direct damage, as much as doubling the life of a roof). Sometimes it does not pencil, but to get to the real cost/benefit analysis it is necessary to look at all of the benefits: urban vegetation also improves stormwater quality, reduces stormwater peak volumes, removes contaminants from the air, provides habitat, can produce food, is a marketing opportunity for many companies, provides habitat, and in my opinion is a dramatic improvement in city character, creating civic pride and stewardship. All of this added value comes without additional cost — it’s still the same roof that was just saving lives and driving down energy demands — and in fact there are some buried treasures in there, such as brand value and commodity food products.

How do we quantify, incentivize, and realize those benefits? Well that’s a spreadsheet we’re working on, one that includes natural capital, social equity, and a number of “intangible” benefits such as habitat and urban character…one that speaks clearly to the value of removing obstacles in the development code and creating incentives for reuniting human development patterns and natural processes.

Filed under: Events, graywater, green city, green infrastructure, green roofs, landscape, living architecture, living walls, urban heat island