The role of living roofs in holistic stormwater management systems - Living Roofs in Integrated Urban Water Systems - page 35

The rate of ET at any given moment is a function of the plants' metabolism
(which usually differs amongst species), air and growing medium temperature,
relative humidity, solar radiation, wind speed and water availability. Amongst
these factors, water availability (speciically the amount of water stored in the
growing medium in this case) has the most pronounced effect on living roof ET.
Several studies measuring living roof ET have demonstrated that it is highest
when the growing medium is wettest. In several laboratory experiments begin-
ning with well watered conditions, the rate of ET was shown to decrease expo-
nentially as the system dries ( Figure 2.4 ) to rates similar to that of evaporation
from unplanted growing media (Rezaei 2005; Starry 2013; Voyde 2011), while
ield experiments conirm the general decreasing trend between natural storm
events (DiGiovanni 2013; Wadzuk 2013; Yio et al. 2014).
Several succulents (sedums are a variety of succulents) have been shown to
transpire at moisture contents below the nominal permanent wilting point in
living roof experiments (Starry 2013; Voyde 2011). The moisture is harvested
from water stored in plant stems and leaves (a deining characteristic of succu-
lents), enabling survival during extended dry periods (Voyde et al. 2010). This
additional water storage potential is more important in terms of plant resilience
and structural loading than day-to-day stormwater management.
Altogether, studies conirm that transpiration is a signiicant component of
overall ET; when water is available, and climatic conditions are favourable (namely
low relative humidity and adequate solar radiation) ET from planted systems

Figure 2.4
Plants decrease ET rate
exponentially as the
amount of water stored
in the growing medium
depletes (data based on
laboratory experiments
beginning with healthy
turgid plants by Voyde
[2011]; igure adapted
with permission from
Afoa [2014])

Nominal permanent
wilting point

Day 9

4.5

4.0

3.5

3.0

2.5

2.0

Start of
measurement
(day 1)

FLL ield
capacity

1.5

1.0

0.5

0.0

0 10 20 30 40

Growing media moisture content (%)

D. australe
S. mexicanum

Best it regression ( S. mexicanum )
Best it regression ( D. australe )