Clouds as a result of
transpiration of the
The water cycle, also known as the hydrologic cycle or the H2O
cycle, describes the continuous movement of water on, above and
below the surface of the Earth.
The mass of water on Earth remains fairly constant over time but
the partitioning of the water into the major reservoirs of ice, fresh
water, saline water and atmospheric water is variable depending on
a wide range of climatic variables. The water moves from one
reservoir to another, such as from river to ocean, or from the ocean
to the atmosphere, by the physical processes of evaporation,
condensation, precipitation, infiltration, runoff, and subsurface
flow. In so doing, the water goes through different phases: liquid,
solid (ice), and gas (vapor).
The water cycle involves the exchange of energy, which leads to
temperature changes. For instance, when water evaporates, it
takes up energy from its surroundings and cools the environment.
When it condenses, it releases energy and warms the environment.
These heat exchanges influence climate.
The Sun, which drives the water cycle, heats water in
oceans and seas. Water evaporates as water vapor into
the air. Evapotranspiration is water transpired from
plants and evaporated from the soil. Rising air currents
take the vapor up into the atmosphere where cooler
temperatures cause it to condense into clouds. Air
currents move water vapor around the globe, cloud
particles collide, grow, and fall out of the upper
atmospheric layers as precipitation. Most water falls
back into the oceans or onto land as rain, where the
water flows over the ground as surface runoff. A portion of
runoff enters rivers in valleys in the landscape, with
streamflow moving water towards the oceans. Runoff and
water emerging from the ground (groundwater) may be
stored as freshwater in lakes. Not all runoff flows into
rivers, much of it soaks into the ground as infiltration.
Some water infiltrates deep into the ground and
replenishes aquifers, which can store freshwater for long
periods of time. Some infiltration stays close to the land
surface and can seep back into surface-water bodies
(and the ocean) as groundwater discharge. Some
groundwater finds openings in the land surface and
comes out as freshwater springs. In river valleys and
flood-plains there is often continuous water exchange
between surface water and ground water in the hyporheic
zone. Over time, the water returns to the ocean, to
continue the water cycle.
Transpiration is the process of water movement through
a plant and its evaporation from aerial parts, such as from
leaves but also from stems and flowers. Leaf surfaces are
dotted with pores called stomata, and in most plants they
are more numerous on the undersides of the foliage. The
stomata are bordered by guard cells and their stomatal
accessory cells (together known as stomatal complex)
that open and close the pore. Transpiration occurs
through the stomatal apertures, and can be thought of as
a necessary "cost" associated with the opening of the
stomata to allow the diffusion of carbon dioxide gas from
the air for photosynthesis. Transpiration also cools plants,
changes osmotic pressure of cells, and enables mass flow
of mineral nutrients and water from roots to shoots.
What is Green Infrastructure?
Green Infrastructure can be broadly defined as a
strategically planned network of high quality natural
and semi-natural areas with other environmental
features, which is designed and managed to deliver a
wide range of ecosystem services and protect
biodiversity in both rural and urban settings.
Building a Green Infrastructure for Europe
Green Infrastructure is a concept originating in the United States in the mid-
1990s. Also known as “blue-green infrastructure” or “green-blue urban grids” the
terms are used by many design, conservation and planning related disciplines
and commonly feature stormwater management, climate adaptation and
multifunctional green space.
The United States Environmental Protection Agency (EPA) has extended the
concept of “green infrastructure” to apply to the management of stormwater
runoff at the local level through the use of natural systems, or engineered
systems that mimic natural systems, to treat polluted runoff.
Today, many cities are facing severe water uncertainties, such as floods,
droughts and upstream activities on trans-boundary rivers. The increasing
pressure, intensity and speed of urbanisation has led to the disappearance
of any visible form of water infrastructure in most cities.
One fifth of the world’s population live in areas of water scarcity. Climate change
and water-related disasters will place increasing demands on urban systems and
will result in increased migration to urban areas. Cities require a very large input
of freshwater and in turn have a huge impact on freshwater systems.
At the scale of a city or county, green
infrastructure refers to the patchwork of
natural areas that provides habitat, flood
protection, cleaner air, and cleaner water.
At the scale of a neighborhood or site,
green infrastructure refers to stormwater
management systems that mimic nature by
soaking up and storing water.
When it rains, some water is absorbed through pervious surfaces—such as vegetated areas with
uncompacted soil, sand, or gravel that allow the passage of water. Other water, called
stormwater, flows over impervious surfaces—such as rooftops, sidewalks, and streets that
obstruct natural infiltration.
Impervious cover exacerbates the problem of stormwater when runoff flows directly into the
nearest storm drain without being mitigated. If untreated before entering our waterways, this
contaminated water can have a detrimental effect on water quality.
The more impervious surfaces there are in the city, the more polluted stormwater enters the
sewer system, increasing the total volume of water the city's infrastructure network must handle.
Below ground lies a vast network of
underground pipes. In Philadelphia, we have
two types of sewer systems.
In areas with combined sewers, a single pipe
carries both stormwater from streets, houses,
and businesses as well as waste water from
houses and businesses to a water treatment
In areas with separate sewers, one pipe
carries stormwater to the city's streams while
another carries wastewater to a water
When it rains and the volume of combined
stormwater and wastewater is larger than the
combined sewer system's capacity, the
mixed stormwater and wastewater is
discharged into the city's streams at a
combined sewer outfall (CSO) before it is
n the separate sewer system, stormwater is
not routed to a treatment plant and is
discharged directly to a stream. Pollutants
picked up as the stormwater ran off the city's
impervious surfaces are discharged into the
How long is the infrastructure expected to last?
Green stormwater infrastructure includes a range of soil-water-plant systems that intercept
stormwater, infiltrate a portion of it into the ground, evaporate a portion of it into the air, and in
some cases release a portion of it slowly back into the sewer system.
Our vision is to protect and enhance our watersheds by managing stormwater runoff with
innovative green stormwater infrastructure throughout our City, maximizing economic, social,
and environmental benefits for Philadelphia.
A green roof is a roof or section of roof that is vegetated. A green roof system is composed of
multiple layers including waterproofing, a drainage layer, an engineered planting media, and
specially selected plants. Green roofs can be installed on many types of roofs, from small
slanting roofs to large commercial flat roofs. Two basic types of green roofs have been
developed: extensive and intensive. An extensive green roof system is a thin, lighter-weight
system planted predominantly with drought-tolerant succulent plants and grasses. An intensive
green roof is a deeper, heavier system designed to sustain more complex landscapes. A green
roof is effective in reducing the volume and velocity of stormwater runoff from roofs by
temporarily storing stormwater, slowing excess stormwater release into the combined sewer
system, and promoting evapotranspiration.
Stormwater Tree Trench
A stormwater tree trench is a system of trees that are connected by an underground
infiltration structure. On the surface, a stormwater tree trench looks just like a series of street
tree pits. However, under the sidewalk, there is an engineered system to manage the incoming
runoff. This system is composed of a trench dug along the sidewalk, lined with a permeable
geotextile fabric, filled with stone or gravel, and topped off with soil and trees. Stormwater
runoff flows through a special inlet (storm drain) leading to the stormwater tree trench. The
runoff is stored in the empty spaces between the stones, watering the trees and slowly
infiltrating through the bottom. If the capacity of this system is exceeded, stormwater runoff can
bypass it entirely and flow into an existing street inlet.
A stormwater bump-out is a vegetated curb extension that protrudes into the street either mid-
block or at an intersection, creating a new curb some distance from the existing curb. A bump-
out is composed of a layer of stone that is topped with soil and plants. An inlet or curb-cut
directs runoff into the bump-out structure where it can be stored, infiltrated, and taken up
by the plants (evapotranspiration). Excess runoff is permitted to leave the system and flow to an
existing inlet. The vegetation of the bump-out will be short enough to allow for open sight lines
of traffic. Aside from managing stormwater, bump-outs also help with traffic-calming, and when
located at crosswalks, they provide a pedestrian safety benefit by reducing the street crossing
A stormwater planter is a specialized planter installed in the sidewalk area that is designed to
manage street and sidewalk runoff. It is normally rectangular, with four concrete sides providing
structure and curbs for the planter. The planter is lined with a permeable fabric, filled with gravel
or stone, and topped off with soil, plants, and sometimes trees. The top of the soil in the planter
is lower in elevation than the sidewalk, allowing for runoff to flow into the planter through an inlet
at street level. These planters manage stormwater by providing storage, infiltration, and
evapotranspiration of runoff. Excess runoff is directed into an overflow pipe connected to the
existing combined sewer pipe.
Rain Barrel or Cistern
A rain barrel or cistern is a structure that collects and stores stormwater runoff from rooftops.
The collected rain water can be used for irrigation to water lawns, gardens, window boxes or
street trees. By temporarily holding the stormwater runoff during a rain event, more capacity can
be added to the city’s sewer system. However, rain barrels and cisterns only serve an effective
stormwater control function if the stored water is used or emptied between most storms so that
there is free storage volume for the next storm. Rain barrels are designed to overflow into the
sewer system through the existing downspout connection in large storm events. Although these
systems store only a small volume of stormwater, collectively they can be effective at preventing
large volumes of runoff from entering the sewer system.
Rain Garden and Swale
A rain garden is a garden designed to collect runoff from impervious surfaces such as roofs,
walkways, and parking lots, allowing water to infiltrate the ground. The garden is normally
moderately depressed (lower than the surrounding ground level), with the bottom layer filled
with stone so runoff can collect and pond within it. The site is graded appropriately to cause
stormwater to flow into the rain garden area from the nearby impervious area. The water ponds
on the surface, is used by the vegetation in evapotranspiration, and infiltrates into the
subsurface stone storage and soil. Rain gardens can be connected to sewer systems through
an overflow structure, but usually they are sized to infiltrate the collected stormwater runoff
within 72 hours. Flexible and easy to incorporate into landscaped areas, rain gardens are
suitable for many types and sizes of development and retrofits. Rain gardens are effective at
removing pollutants and reducing stormwater runoff volume.
A flow-through planter is a structure that is designed to allow stormwater from roof gutters to
flow through and be used by the plants. Flow-through planters are filled with gravel, soil,
vegetation and a connection to the roof downspout to let water flow in. They temporarily store
stormwater runoff on top of the soil and filter sediment and pollutants as water infiltrates down
through the planter. They are typically waterproofed, and the bottom of the planter is normally
impervious. As a result, planters do not infiltrate runoff into the ground; they rely on
evapotranspiration and short-term storage to manage stormwater. Excess water can overflow
into the existing downspout connection. Flow-through planters can be constructed in many sizes
and shapes and with various materials, including concrete, brick, plastic lumber, or wood.
Pervious pavement is a specially designed pavement system that allows water to infiltrate
through the pavement and never become runoff. This system provides the structural support of
conventional pavement but is made up of a porous surface and an underground stone reservoir.
The stone reservoir provides temporary storage before the water infiltrates the soil. There are
many different types of porous surfaces, including pervious asphalt, pervious concrete, and
interlocking pavers. Interlocking pavers function slightly differently than pervious concrete and
asphalt. Rather than allowing the water to penetrate through the paving, pavers are spaced
apart with gravel or grass in between to allow for infiltration.
Green streets and alleys integrate green infrastructure elements into the street and/or alley
design design to store, infiltrate, and evapotranspire stormwater. Permeable pavement,
bioswales, planter boxes, and trees are among the many green infrastructure features that
may be woven into street or alley design.
La raccolta e il riuso delle acque di pioggia
Le acque meteoriche rappresentano una fonte rinnovabile e
locale e richiedono trattamenti semplici ed economici per un
utilizzo limitato a certe applicazioni. In generale, gli impieghi
che si prestano al riutilizzo delle acque meteoriche riguardano
usi esterni, come:
- l'irrigazione di aree a verde, prati, giardini, orti;
- il lavaggio di aree esterne (strade, piazzali, parcheggi,
balconi) e automobili;
- usi tecnologici (ad esempio acque di raffreddamento);
- alimentazione delle reti antincendio;
e usi interni agli edifici, come:
- l'alimentazione delle cassette di risciacquo dei WC;
- l'alimentazione di lavatrici;
- usi tecnologici relativi, come ad esempio sistemi di
Schema di un sistema di raccolta della pioggia
Immagini di G.Conte,
Le tecniche di depurazione naturale
Un sistema di fitodepurazione è un ecosistema umido
artificiale, in cui le varie componenti (piante, animali,
microrganismi, terreno, radiazioni solari) contribuiscono alla
rimozione degli inquinanti presenti nelle acque di scarico.
Nei fitodepuratori si riproducono le caratteristiche di un
ecosistema naturale, dove la materia viene continuamente
riciclata, così la biomassa batterica viene utilizzata da
organismi superiori ed entra nella catena alimentare.
Le piante utilizzano i nutrienti presenti nel sistema (prodotti
della biodegradazione) per il loro accrescimento, rimuovendoli
dalle acque. Alla fine del processo tutti gli inquinanti vengono
trasformati in anidride carbonica, acqua o “incorporati” nella
Nell’ambito delle tecniche di depurazione naturale esistono diverse soluzioni
FWS: i sistemi a flusso libero riproducono quanto più fedelmente una zona palustre
naturale, l’acqua è a diretto contatto con l’atmosfera e generalmente poco
profonda, le specie vegetali che vi vengono inserite possono essere di moltissime
specie e con diverse caratteristiche (galleggianti, radicate sommerse, radicate
emergenti), purché acquatiche. Questa tecnica – certamente la più interessante dal
punto di vista paesaggistico – deve essere applicata su acque poco inquinate, ad
esempio acque di prima pioggia o acque nere/grigie che hanno già subito un
trattamento precedente con sistemi a flusso sommerso.
SFS-h o HF: i sistemi a flusso sommerso orizzontale sono bacini riempiti con
materiale inerte (ghiaia) in cui i reflui scorrono in senso orizzontale in condizioni di
saturazione continua, le specie vegetali utilizzate sono sempre macrofite radicate
emergenti, generalmente la cannuccia di palude (Phragmites australis). E’ il sistema
più semplice e più adatto a contesti dove si prevede di fare scarsa manutenzione (ad
esempio impianti condominiali per il trattamento delle acque grigie).
SFS-v o VF: i sistemi a flusso sommerso verticale sono vassoi riempiti con
materiale inerte in cui i reflui scorrono in senso verticale in condizioni di saturazione
alternata (reattori “batch”), le specie vegetali utilizzate sono sempre macrofite radicate
ed anche in questo caso generalmente la cannuccia di palude. Sono gli impianti più
adatti per acque con valori elevati di carico organico o di ammoniaca. Richiedono
però l’utilizzo di pompe o di specifici meccanismi per l’alimentazione.
Impianto di fitodepurazione del Comune di Dicomano, progetto IRIDRA, firenze 2003
Impianto di post trattamento con fitodepurazione del depuratore delComune di Jesi, progetto
IRIDRA, Ancona 2000
Impianto di fitodepurazione dell'Hotel Certosa, progetto IRIDRA, Firenze 2002
Impianto di fitodepurazione dell'Ostello dell'Isola Polvese sul lago Trasimeno, progetto IRIDRA,
La fitodepurazione è una tecnica “estensiva” e richiede quindi una
certa disponibilità di spazio per garantire una buona efficacia.
Il dimensionamento di un impianto viene fatto in funzione dei limiti
allo scarico (che possono variare anche considerevolmente in
funzione del corpo recettore) e delle caratteristiche chimiche delle
acque di scarico.
Una stima di massima (cautelativa) dell’area necessaria per il
trattamento con la tecnologia più diffusa (SFS-h) prevede circa 4
m2 per abitante per il trattamento di un’acqua mista e circa 2 m2
per abitante per il trattamento delle sole acque grigie. I costi di
realizzazione degli impianti sono stimabili in:
FW 40 € m2
SFS-h 100 € m2
SFS-v 120 € m2
Cheonggye Stream in downtown Seoul
An elevated highway had been built through Seoul in 1976 as a way to boost economic
prospects in a low-lying area which had become a slum. In 2003, the city's mayor proposed to
remove the freeway and and turn the site into green space, which also required naturalizing the
creek that once ran there.
Not only has the greenway become a well-loved part of the city, it has proven to benefit the city in
many different ways. The temperature of the inner city has dropped several degrees, and birds,
fish and other wildlife have returned to the urban core. Also, since the freeways were removed,
fewer people are driving into the city, choosing to take public transit or other options.
Life beneath the asphalt
What was built in Seoul, Korea, in the 1960s as the road to efficiency, became a
health and safety hazard for the people of the city. The decision to dismantle an
elevated highway and a buried polluted sewer was based on the expense of
adding reinforcement and ongoing maintenance. What began as a question of
cost evolved into a question of value. Cheonggyecheon River has been
transformed from traffic corridor into people-friendly destination. Seoul has been
reunited with its past history, its present culture and its future market.
Flussbad, Realities:United, Bronze Global Holcim Award 2012, Gold European Holcim
The Spree River splits in two in the heart of Berlin and flows
around the touristy museum island. After the Berlin Wall came
down, there was a huge influx of residents and open space
has decreased significantly. This new pool would provide
space for leisure and recreation with an even greater
bonus of cleaning up the city’s water through natural
methods. The beginning portion of the canal would be filled
with reed beds and subsurface sand bed filters. After 780
meters of filtration, the water would be sufficiently clean for
people to be in it, and the sides of the canal would be
converted into stairs to provide access to the water. Incoming
drainages would be diverted to the river to maintain the water
quality of the pool. Bridges, landings, and decks would offer
space to relax and watch the activity. The new pool would be
745 meters long, with specific areas cordoned off for laps, free
swim, and even boating. Next to the pool, a new locker and
shower facility would be built to support the water activities.