Analysis from
the measurements revealed that the roof plants
condition and development over the growing
season play a major part in evapotranspiration
rates. The thickness of the rooted soil layer
was important for the duration of drought stress
but made not much difference for the maximum
evapotranspiration rate between the two used
vegetation types between the two pot types.
Further it can be concluded that support
irrigation is highly recommended for keeping the
roof plants alive and thus maintaining a high
evaporation potential over the season. As an
alternative a drought-resistant mix of plants
may be used which allows a fast recovery after
strong drought also without support irrigation.
Overall, the driest conditions were found at the
sites with higher wind loads and low shading,
indicating higher evapotranspiration and cooling
potential in case of enough available water and
an active growing plant canopy.
An increase
of 3.76 K for a 5-day heat wave is expected for
the climate period centered around 2050 in the
RCP8.5 scenario.
High traffic sites can cause an
increase in air temperature by 1-2 K due to the
wasted heat of combustion engines.
Electrifying vehicles could however reduce
anthropogenic heat emissions by up to 75%.
For medium insulated buildings,
irrigated green roofs or street trees (without
irrigation) can bring more than 1 K reduction in
building indoor temperature. Additional cooling
can be achieved by window shading, ventilation
during outdoor minimum temperatures or bright
roofs. Although air conditioning can bring fast
cooling, each air conditioning system has its
limits of capacity. Additionally, air
conditioning increases urban outdoor
temperatures and electricity demand. On
the contrary, increase in reflectivity
reduces roof temperatures, radiation
temperatures, sensible heat flux and improves
thermal comfort in the street canyon. The
greatest reduction of the thermal comfort index
°UTCI is achieved by the addition of trees at
street level (-4 °UTCI) or unsealing and
irrigation (-3 °UTCI). The highest levels of
evapotranspiration can be achieved by irrigation
of green roofs – ranging up to 8 mm/d – thus
effectively cooling roof temperatures.
Evapotranspiration of green roofs and low
vegetation at street level can exceed
evapotranspiration levels of street trees.
Using maximum
evapotranspiration in the greater Vienna region
including irrigation of Marchfeld, unsealing in
Vienna and exploiting the full green roof
potential (irrigated), temperature can be
reduced by 0.5 – 1.5 K. This amount of water is
in critical competition with the drinking water
resources of the city.
·Outlook
and summary
To address
urban heat and water management in Vienna,
several strategies are recommended:
Green Roofs
and Urban Vegetation: Use lighter
substrates like dolomite chippings for green
roofs to increase albedo and water retention,
rather than focus on cooling the city. Urban
vegetation should provide shade without
obstructing airflow, especially in wide street
canyons. Technical shading elements are also
recommended. When purchasing substrates,
attention should be paid to environmental
compatibility.
Traffic and
Electrification: Electrifying
vehicles can reduce anthropogenic heat emissions
by up to 75%. Public awareness and promotion of
car-sharing models are essential. Financial
barriers include the high cost of electric cars,
though leasing and shared mobility options can
mitigate this. Legal barriers involve the need
for public space for charging stations and
lengthy approval processes.
Water
Management in Agriculture: Groundwater
is a limiting factor for irrigation in
Marchfeld. Water retention measures, optimized
irrigation methods, and crop rotations are
recommended to manage water resources
effectively. Transitioning to sustainable
agriculture with minimal irrigation is advised
to prevent heating Vienna.
Building
Temperature Control: Traditional
(shading and nighttime ventilation) and
alternative (thermal component activation) air
conditioning systems and insulation can help to
manage building temperatures and maintain indoor
thermal comfort. Photovoltaic cells can help
cool buildings by converting solar energy into
electricity. Public awareness should be raised
to prevent the installation of air conditioning
systems without alternative energy sources.
City
Greening: Planting
trees, to provide shade at pedestrian level is
recommended. Legal and financial barriers
include the need for resources, maintenance, and
lengthy approval processes. The choice of
species should consider allergic potential and
climate projections.
These
strategies aim to balance cooling, water
retention, and sustainable urban and
agricultural practices in Vienna.