The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 25

On the scale of an individual living roof, when rainfall begins, a small amount
that strikes the foliage is intercepted. During very small events (e.g., less than a
couple of mm), all rainfall may be intercepted (Berretta et al. 2014). As rain con-
tinues, water percolates into and begins to wet the growing medium. In theory,
signiicant quantities of water should not begin to discharge from the living roof
until the ield capacity of the growing medium is illed. According to soil scien-
tists and geotechnical engineers, ield capacity measures the quantity of water
stored by a medium against gravity drainage ( Figure 2.2 ). During small rainfall
events, negligible (if any) runoff occurs and most of the precipitation captured by
the living roof eventually returns to the atmosphere by ET, slowly, once rain
ceases. For larger storms (in theory) rainfall in excess of the ield capacity follows
a tortuous low path through the pore space of the growing medium to eventu-
ally discharge through the drainage layer, leading to the roof's gutters and
downspouts.
The premise that a living roof provides a threshold minimum water storage
capacity, and discharges any water inputs larger than this threshold provides a
static interpretation of what is really a hydraulically complex, dynamic system. It is
thought that mechanisms of water retention and detention in a living roof
change according to climate and rainfall patterns (for example, intensity in a
given event and frequency of events), states of plant vigor and root development,
roof slope, and composition of the growing medium. Runoff may be initiated at
moisture contents below the ield capacity due to heterogeneity in the growing
media (Stovin et al. 2013). Researchers worldwide are currently studying these
phenomena, but universal rules or models to quantify these important inluences
have not yet been established.
For any given rainfall event, the ability of a living roof to retain rainfall
depends on the available water storage capacity. That is, how dry is the growing
media compared to its total potential water storage capacity? Available storage is
dictated primarily by ET rates, and the elapsed time, atmospheric and plant con-
ditions since the last rain, as well as by water-holding characteristics of the media
itself. In turn, ET is inluenced by water availability (see Section 2.6 ), and how
easily plants can physically access the stored water. Agronomists and horticultur-
alists measure plant available water ( PAW ) as the amount of moisture stored in a
growing media between the ield capacity and the permanent wilting point
( Figure 2.2 ). Some moisture is retained by soils below the wilting point; this
hydroscopic water is tightly bound to the soil particles. It is unable to be extracted
by plant roots, and thus not available for transpiration.
Regardless of the nuances behind day-to-day variation, the net volume of
runoff is primarily reduced by rainfall captured within the pore space of the
growing medium. A growing medium's water storage capacity is inite, yet is the
most critical characteristic to promote stormwater retention and sustain plant life
(Fassman and Simcock 2012). It is derived from characteristics of the media itself
and its installed depth, and is inluenced by the underlying drainage layer (or its
absence). The water storage capacity may be manipulated or “engineered” by