This can be expressed in general terms as the cost required to provide an equivalent environmental service if one were to use conventional infrastructure.
Protection and enhancement of existing ecosystems is a cost-effective approach to stormwater management. Rainfall capture in a tree's canopy can equate to 100 to 600 litres a year, depending on the tree species and its maturity (McPherson et al., 2006). Based on construction costs to provide equivalent infrastructure, trees have been valued at billions of dollars for stormwater management in US cities (MacMullan & Reich, 2007).
Generally WSD delivers better stormwater runoff quality than conventional stormwater management approaches. The cumulative impact of implementing WSD throughout a city would have a significant effect on the health of freshwater and marine receiving environments. This has a potential benefit to freshwater and marine fish species and recreation. In addition this is likely to have a direct effect on the prices of properties adjacent to enhanced streams and wetlands (Dornbusch & Faleke, 1974; Bicknell & Gan, 1997).
Stormwater is also a resource in itself, with raintanks and underground detention systems collecting stormwater runoff from rooftops and impervious surfaces, which can be used on-site for non-potable water, landscape irrigation, water features and passive heating and cooling. The potential cost saving from using stormwater on-site is through reducing potable water use and purchase.
Presently Auckland water is affordable and savings are equivalent to the cost to install, maintain and renew stormwater collection systems (Vesely et al., 2005). Cost savings assume there is sufficient rainfall to accommodate water demand, as transporting water to a site during droughts may add significantly to costs. From a council perspective, on-site water storage provides a distributed network, giving a more resilient water supply for communities in the event of a systems failure.
WSD is associated with enhanced riparian buffers, re-vegetation, and vegetated WSD practices, such as raingardens and swales. Important ecosystem services provided by these natural and green infrastructure components include carbon uptake (or sequestration) and reducing localised 'heat island' effects. Studies have indicated that urban forestry and associated soils can sequester over ten metric tonnes of CO2 per hectare per year (US Environmental Protection Agency, 2010). Trees transpire to reduce ambient air temperatures in the summer, and moderate temperatures in cold seasons by reducing winds speeds and heat transfer. The combined effect can significantly reduce energy consumption (McPherson et al., 2006; Akbari et al., 1992).
Living roofs have a particularly significant effect on ambient air temperatures within and around buildings. This is based on their insulative properties and evapotranspiration around rooftop air conditioning ducts. A living roof on an office building in Chicago resulted in a 2% reduction in total building electricity consumption (Sailor, 2008). A simulation for Toronto found that 50% coverage of buildings with living roofs would be expected to reduce the city's ambient air temperature by 2 degrees Celsius (Bass et al., 2002).
Vegetation plays a key role in Auckland's urban environments in moderating the effects of localised air pollution by absorbing contaminants such as nitrous dioxide and sulphur dioxide, and intercepting fine particulate matter that can harm human respiratory systems. Trees, landscaped areas and living roofs take up gaseous contaminants in their leaf stomata and filter fine particulate matter. The reduction in ambient air temperatures in a city also slows reaction rates of smog, which can form volatile organic compounds at high temperatures (Currie & Bass, 2008). Wider, long-term economic benefits are achieved through the consenquent reduction of negative health effects (e.g. asthma and bronchitis).
In general, vegetation by itself is largely ineffective at moderating noise. However, recent studies have demonstrated that the use of porous concrete can reduce roadway noise pollution by as much as 10 dB (Olek et al., 2003; Gerharz, 1999). The dampening effect of pervious pavement is put down to the porosity of the material and the reduced traffic speed. Field tests of living roofs in British Columbia found a relative transmission loss of 5-13 dB in low to midrange frequencies, and 2-8 dB in the high frequency range (Connelly & Hodgson, 2008). Living walls may be expected to offer similar noise insulation properties depending on structural components and soil texture.
Optional value relates to preserving an option to utilise a resource at a later time when the economic, social or political climate may have changed. At a minimum, the protection of ecosystems by WSD approaches allows for their restoration/enhancement to achieve direct use values in the future, in effect preventing an opportunity cost. Irreversible decisions should be avoided wherever practicable such as habitat fragmentation or filling floodplains (OECD, 2000).
One of the reasons WSD and other green infrastructure approaches are not attributed their true value is the difficulty in calculating the economic value of non-market ecosystem services (Smith & Desvousges, 1986). Willingness to pay is often used to determine non-market good and service preferences. If one assumes the willingness of the public to pay for an increase in natural ecosystem values in the future, then it is reasonable to assume one avoids an opportunity cost by retaining these systems in the present (Lewis, 2003).