The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 39

runoff depth, Q R (mm or inches) (or appropriate term), by the roof area (e.g., m 2
or ft 2 ), with appropriate unit conversions.
Predicting runoff using a water balance lends itself well to continuous simula-
tion, but can also be simpliied for a design storm approach. In SCM design, his-
toric records are assumed accurate predictors of future precipitation. These data
are usually available through public records and often freely accessed online. Irri-
gation (if applicable) is (or should be) controlled so that it does not exceed
storage capacity. The storage capacity represents the potential for precipitation
retention (volume control) at any time. For living roofs, it is often estimated as
the difference between the maximum water storage capacity (e.g., the PAW ) and
the amount of storage actually occupied (i.e., the initial moisture condition).
ET between rainfall events is the process which restores storage capacity and
thus plays an integral role in living roof stormwater retention eficiency. Quantify-
ing ET remains the most signiicant challenge in applying Equation 2.1 to predict
living roof runoff. ET from a living roof varies widely amongst plant species,
climate conditions and water availability, as mentioned in Section 2.6 . Climate
data has become relatively easy to obtain online, and will often include ET. These
ET data should be applied with caution to living roofs, as the calculations may or
may not appropriately account for the living roof growing environment, meta-
bolic adaptations of living roof plants, or the timescale of the intended runoff
simulation (Berretta et al. 2014; DiGiovanni et al. 2013; Starry et al. 2014; Voyde
2011). As discussed in Section 2.6 , living roof ET is the subject of signiicant aca-
demic research in recent years.
2.7.2 Operationalizing the water balance
Engineers often apply some version of a water balance to model SCMs using the
concept of a bowl that ills and slowly drains, or overlows when capacity is
exceeded. In living roof applications, several published models represent the
“bowl” as the growing media whose water-holding capacity can be measured by a
laboratory test of moisture storage (or ield capacity). When there is more rain than
the “bowl” can capture, the excess water percolates through the growing media
to become runoff in the drainage layer and through to the roof's downspouts. A
minimum media water content (because of gravity drainage or drying between
storm events) is often set as the media's nominal permanent wilting point.
While a relatively simple water balance ignores the physics of water movement
through a living roof, its ability to model long-term and event-based retention
from one year's observations of the ield performance of a UK living roof test plot
has been successfully demonstrated (Stovin et al. 2013). In this model, the
system's water balance was calculated using an hourly time step, the growing
media was considered as a single model element, and the only parameter
requiring calibration was the media's maximum water-holding capacity, which
was set to 20 mm. A living roof-speciic ET model was incorporated, which
included a SMEF function to reduce actual ET compared to potential ET, as the
moisture stored in the media became depleted during dry weather periods.