Agriculture Reference
In-Depth Information
and plant transpiration demands. Research has shown that the media's water-
holding capacity is the most critical characteristic to promote stormwater reten-
tion and sustain plant life in the absence of irrigation (Fassman and Simcock
2012; Thuring et al. 2010).
As discussed in Chapter 2 , local regulatory agencies usually prescribe
minimum requirements for stormwater control, by deining a storm or range of
storms to be controlled. The smallest storm depth in these requirements is often
associated with objectives for GI, CSO control or water quality treatment, and in
many cases is a design storm that delivers around 25 mm of rainfall. Coinciden-
tally, empirical evidence from a range of extensive living roofs across the United
States suggests that 20-25 mm is about the maximum amount of rainfall that
any of the living roofs could actually store, regardless of coniguration (growing
media type, depth, roof slope, or location from New York to North Carolina to
Michigan to Oregon to Auckland [Fassman-Beck et al. in preparation]) or mois-
ture content at the onset of rain.
For planning and design purposes, an extensive living roof's rainfall retention
properties can be estimated by the amount of moisture stored between a
growing medium's nominal ield capacity and nominal permanent wilting point
(Fassman and Simcock 2012). A common method to quantify these parameters
in the laboratory is to measure the amount of water (the moisture content) held
in the growing medium's pores between 10 and 1,500 kPa tension, which are the
nominal ield capacity and permanent wilting point, respectively [Mclaren and
Cameron 1996]), although other test methods are found amongst the relevant
reference texts. This moisture content is known in agronomic or horticultural
terms as plant available water ( PAW , Figure 2.2 ) (Mclaren and Cameron 1996).
Laboratory measures of moisture storage potential are considered nominal meas-
ures, as it is impossible to replicate all of the variability or inluencing factors
encountered in the ield.
When dry, appropriate growing media for extensive living roof applications
should be capable of storing about 25-40 percent by volume PAW , or greater (as
was shown in Table 4.2 for a number of living roofs with proven stormwater per-
formance). In theory, a 100 mm cover with 30 percent PAW effectively controls up
to 30 mm of rainfall, if the media is dry at the onset of rain. As a planning tool
using a design storm approach (Section 3.8.1), the designer may assume that
during larger storms, the media will retain rainfall up to PAW , and then slowly
release the rainfall in excess of the PAW . In practice, the maximum storage capacity
(the PAW ) of the living roofs is not fully utilized on a day-to-day basis, only when
larger rainfall events occur. It is also acknowledged that the actual amount of rain-
fall retained by a growing media for any given storm will depend on moisture
storage availability and the depth of the rain event. In other words, there is a rela-
tionship between how wet (or dry) the growing media is at the onset of rain, the
maximum water-holding capacity (ield capacity) and how much rain occurs.
Living roof media does not have to be fully dry to provide excellent long-term
retention. Studies on living roofs in Auckland showed substantial rainfall retention
 
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