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Urban temperature has been escalating less greenery and high built up areas. More than half of the total population is residing in urban area so that the urban environment is highly vulnerable. Restoration of urban forestry is very expensive and almost not possible as they have already used the massive land for development purpose. In this context, green roofs technology has rapidly emerged. It has multifaceted environment, aesthetic and social benefits. It absorbs atmospheric carbon, mitigate urban heat and cool the surface temperature which reduces the energy demand and cost. Because of these several environmental benefits, green roofs can be practiced in Kathmandu. The Kathmandu, one of the fastest growing city in south Asia faces several and catastrophic environmental problems related to urban heat. In this study, we reviewed the urban heat island, green roofs techniques and use of remote sensing and GIS extension model to study urban greenery and temperature. The MODIS NDVI, CRU climate and monthly temperature data from 8 meteorological stations during 2000-2016 were used. Mann Kendell test statistic and Sen's slope were used for the analysis of temperature changes in Kathmandu valley. Beside it, the land use land cover data of 2010 were used and the previous literatures related on Kathmandu valley were reviewed. The air surface temperature has significantly increased at the rate of 0.04°C yr-1 with a maximum temperature trend of 0.06°C. The average annual air surface temperature of Kathmandu valley is 18.06°C with maximum 24.15°C during 2000-2016. In contrary, average annual land surface temperature has ranges from 15.84°C-39.17°C in 2000 and 16°C to 33.98°C in 2014. At the same time, urban area has dramatically increased; averaged LST has more than air surface temperature, Normalized Difference Vegetation Index values has low but normalized difference built up index has high in core urban area. These all environmental consequences are the main factors of urban heat Island in Kathmandu valley. Therefore, green roofs could be an effective mitigation tool to combat environment problems and urban heat island. Though, detail investigation is required to practice green roofs in Kathmandu valley.
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Applied Ecology and Environmental Sciences, 2018, Vol. 6, No. 4, 137-152
Available online at http://pubs.sciepub.com/aees/6/4/5
©Science and Education Publishing
DOI:10.12691/aees-6-4-5
A Review of Green Roofs to Mitigate Urban Heat
Island and Kathmandu Valley in Nepal
Binod Baniya1,*, Kua-anan Techato2, Sharvan Kumar Ghimire3, Gyan Chhipi-Shrestha4
1Department of Environmental Science, Patan Multiple Campus, Tribhuvan University, Nepal
2Environmental Assessment and Technology for Hazardous Waste Management Research Center,
Faculty of Environmental Management, Princes of Songkla University, Hatyai, Thailand
3Siddhartha Environmental Services, Kathmandu, Nepal
4École Supérieure d’Amenagement du Territoire, Université Laval, 1628 Pavillon Savard,
Université Laval, Québec City, QC., Canada G1K7P4
*Corresponding author: binodbaniya1609@gmail.com
Received September 01, 2018; Revised October 10, 2018; Accepted November 21, 2018
Abstract Urban temperature has been escalating less greenery and high built up areas. More than half of the total
population is residing in urban area so that the urban environment is highly vulnerable. Restoration of urban forestry
is very expensive and almost not possible as they have already used the massive land for development purpose. In
this context, green roofs technology has rapidly emerged. It has multifaceted environment, aesthetic and social
benefits. It absorbs atmospheric carbon, mitigate urban heat and cool the surface temperature which reduces the
energy demand and cost. Because of these several environmental benefits, green roofs can be practiced in
Kathmandu. The Kathmandu, one of the fastest growing city in south Asia faces several and catastrophic
environmental problems related to urban heat. In this study, we reviewed the urban heat island, green roofs
techniques and use of remote sensing and GIS extension model to study urban greenery and temperature. The
MODIS NDVI, CRU climate and monthly temperature data from 8 meteorological stations during 2000-2016 were
used. Mann Kendell test statistic and Sen’s slope were used for the analysis of temperature changes in Kathmandu
valley. Beside it, the land use land cover data of 2010 were used and the previous literatures related on Kathmandu
valley were reviewed. The air surface temperature has significantly increased at the rate of 0.04°C yr-1 with a
maximum temperature trend of 0.06°C. The average annual air surface temperature of Kathmandu valley is 18.06°C
with maximum 24.15°C during 2000-2016. In contrary, average annual land surface temperature has ranges from
15.84°C-39.17°C in 2000 and 16°C to 33.98°C in 2014. At the same time, urban area has dramatically increased;
averaged LST has more than air surface temperature, Normalized Difference Vegetation Index values has low but
normalized difference built up index has high in core urban area. These all environmental consequences are the main
factors of urban heat Island in Kathmandu valley. Therefore, green roofs could be an effective mitigation tool to
combat environment problems and urban heat island. Though, detail investigation is required to practice green roofs
in Kathmandu valley.
Keywords: green roofs, urban heat island, environment, Kathmandu valley, Nepal
Cite This Article: Binod Baniya, Kua-anan Techato, Sharvan Kumar Ghimire, and Gyan Chhipi-Shrestha,
A Review of Green Roofs to Mitigate Urban Heat Island and Kathmandu Valley in Nepal.” Applied Ecology and
Environmental Sciences, vol. 6, no. 4 (2018): 137-152. doi: 10.12691/aees-6-4-5.
1. Introduction
The urban areas are comparatively warmer than
surrounding rural area that is referred to urban heat island
[1,2]. The urban heat has mainly increased by the
existence of dark color impervious roof’s surface that
absorbs more heat and increased temperature than
surrounding environment condition. The materials such as
asphalts, bricks and concretes in urban surface absorb
more heat than vegetation and release less [3]. The
physical structure of the cities has influence on urban
climate that exaggerates the intensity of urban heat island
[4,5]. Surface materials and canyon urban structure are the
main factor that contributes urban heat [6,7,8,9]. The
impervious and water proof urban surface materials can’t
dispel solar energy and aside urban canyon do trap more
solar radiative energy. Consequently, urban heat has
increased. The relative turn down of urban vegetation
decrease evapotranspirative cooling that serves to develop
urban heat island [10]. There are several potential ways to
reduce Urban Heat Island (UHI) and its negative
consequences in urban environment. Among them, one of
the potential and effective ways is green roofs measures.
Green roofs involve rooftop vegetation growth in
supporting layer of growing media that can be replaced
the vegetated footprints that was destroyed when the
138 Applied Ecology and Environmental Sciences
buildings were constructed [11]. Evapotranspiration process
and low heat absorption of the trees help to control the
temperature. Green roofs have numerous environmental
benefits. It adds as insulation for roof tops [12], maintains
biodiversity [13], carbon sequestration [14] and lower
urban temperature [15]. Green roof reduce temperature by
creating a buffer zone in between roof and sun’s radiation
therefore it shades roof and prevent surface from the heat
exposure [16]. The evapotranspiration from the green roof
maintain cool both in buildings and surrounding areas [10].
The thermal performance of green roof is different than
common roof. It found that daily maximum surface
temperature underneath the green roofs has significantly
lower than the daily maximum temperature in common
roofs surface [16]. Reducing the roof surface temperature
plays a key role in improving thermal condition in
cities where the people were facing urban heat island
[17,18,19].
The roof top vegetation captures the particulate matters
and filters the air. It reduces CO2 levels through
photosynthesis. Living roofs can maintain urban ecosystem
and biodiversity [20]. The lower surface temperature of
roofs can play a key role to improve thermal condition of
the cities [17]. The replacement of traditional roofs
surface with green roofs maintains much lowers summer
temperature. The cooling process of green roof ameliorates
the negative effects of UHI [21]. Beside it, indoor and
outdoor temperatures were significantly cooler in green
roof during the day time [22]. The UHI is influenced by
energy balance modification in urban area that govern by
several factors like urban canyons [1], building materials
and surface thermal properties [23]. Substitution of green
areas with impervious surface limit evapo-transpiration
[24,25] and decrease urban albedo [26]. The variation of
the UHI development factors is depends on the urban
development pattern. The conversion of these available
spaces in to green roof can increase many benefits
in urban area. Green roofs do work by shading and
evapotranspiration [27]. It helps to cool the air
and reduce surface urban heat island. The leaves and
branches of the plants block the amount of solar radiation
that reaches to the ground. Very few portions i.e.
approximately 10-20% of the solar energy reaches the area
below a tree in summer so that the impervious surface
couldn’t get a heat to absorb. The remaining heat absorbed
by leaves for photosynthesis and some being reflected
back in to the atmosphere. Trees and vegetation absorb
water through their roots and emit it through leaves.
Evapotranspiration cool the air by using heat from the air
to evaporate water. Evapotranspiration in combination
with shading are reduced air temperature during peak
summer [27].
Green roof is also named as eco-roof, living roof or
roof garden. It is basically roof with plants. Green roof
enhance the energy efficiency of buildings but it exist
many other benefits. Minimizing urban heat level in cities
is very challenging. Increased population has expedited
surface level temperature. The construction of buildings
and simultaneously growing rate of impervious surface is
high with response to urbanization increased. It has
reported that approximately 40% of worldwide energy
has used in construction and maintenance of buildings.
Globally, buildings are responsible for 33% of green
house emission [28]. The consumption of energy in
buildings is very high and accelerating so that green roof
are often identified as a worth noting strategy for making
building more sustainable. Roofs can represent 20-25%
horizontal surface of built up area [29]. It is important
determinants of the energy flux. The presence of
vegetation layer and soil growing media at roof can reduce
several negative effects of buildings energy consumption.
It dissipates solar radiation and reduces urban heat island
[11]. Living or green roof increases the sound insulation
[30], fire resistance [31] and longetivitiy of roof membrane
[32]. It also reduces the energy required to maintenance of
interior climates [33]. In urban area, surface vegetation
has been replaced by impervious and dark surface paving
materials like asphalt roads and roof that contributes to an
urban heat island [34] where urban area has significantly
warmer than surrounding sub urban and rural areas due to
absorbing, retaining and producing more heat in built
environment than the natural landscape [2]. UHI effects
are mainly observed during summer [35,36]. On a hot
summer day, the exposed urban surface like roofs and
pavements receive more heat from the sun and get dry
that increase the temperature more than the surrounding
air. The high density of vegetation lower the surface
temperature by shading and air temperature through
evapotranspiration in which plants release water to the
surroundings air and dissipate ambient heat. The buildings
roofs, side-walks, roads and parking lots have covered by
dry and impervious surface. Most of the surface has paved
and covered with buildings. Built up area evaporate less
water which contributes to elevated surface and air
temperatures. Mainly, urban heat island depends on urban
geometry and thermal properties like thermal reflectance,
thermal emissivity and heat storing capacity of the urban
surface materials [27].
Modern green roofs were started from Germany.
Several researches have been done to know the multiple
benefits of the green roofs in context to UHI and climate
change. Initially, green roofs were used for aesthetic
purpose [37]. The use of green roof for energy saving has
been widely acknowledged when climate change has
become globally emerged [38,39]. Green roofs can be
classified in to intensive and extensive. It depends on
their purpose and characteristics. Both of the vegetated
roofs have many environmental benefits [40]. Roof cover
with vegetation maintain clean environment and save
energy [41]. Plants can also reduce CO2 concentration in
atmosphere [42]. The concept of green roof has widen and
practiced in several cities of developed countries. In 1997,
a Munich city has initiated and made mandatory for
inclusion of vegetation in all the flat roofs. Green
roofs has also owned by municipalities on their buildings
throughout the Germany. Now, Germany has 10% green
roofs. The reflection has transferred to the Europe and
started green roofs provision as a part of new design in
buildings. Similarly, the government of Switzerland and
Denmark had mandated green roofs in their country
regulation. Switzerland is the world's leader in per capita
uses of green roofs. The countries like Belgium, Singapore,
Japan, USA, Canada, Denmark, Hongkong, China has
enacted regulation to install green roofs in new buildings.
All the mega cities in Europe have already started for
green roofs to address urban heat island. As of 2014, more
Applied Ecology and Environmental Sciences 139
than 54% of the total world population resides in urban
area. It is projected to be a 66% by 2050 [43]. The Asia
will have 63% of global urban population [44]. Asian
urban population has increased continuously. Today, it is
1.5 billion which was 1 billion in early 1990. There will
be added 1.25 billion people in the Asia by 2030. Among
this population, 54% will live in urban area. Nepal is a
least but fastest urbanizing country and Kathmandu is the
one of the fastest growing city in south Asia where the
population density is 10,000/km2 [45].It accounts for one-
third of total urban population of the country [46]. Urban-
rural growth differential in 2011 was 2.4% [47]. In this
context, we aimed to review green roof potential to
mitigate urban heat island. Specifically, urbanization,
urban heat island, modern green roofs practices and
important of green roofs in Kathmandu in response to
urban heat was reviewed.
2. Urbanization
Urbanization has increased rapidly in the world. More
than half of the total world population lives in urban area
in 2014 [43] and estimated 40 mega cities with more than
10 million people in each. At the beginning of 21st century,
urbanization has dramatically increased and continuing in
Asia [48]. The urban population growth in developing
countries was 3 million people per week in last two
decade. Nepal is a slightest but fastest urbanizing country
in South Asia where the total urban population is 38.26%
[49]. The average annual urban population growth rate of
Nepal is about 6% [50]. The large number of people living
in small area and sharing same environmental resources is
common features of urban area. They mostly follow
non-agricultural profession in which Industrialization,
infrastructure development and modernization measure
the level of urbanization. There are different basis to
define urbanization i.e. number of population, size of
cities, level of infrastructure development and available
basic goods and services. In 2014, the most urbanized
region includes Northern America where 82% people are
urban. It is followed by Latin America and Europe
respectively. The Africa and Asia have low urbanization
record where 40% and 48% of their population living in
urban areas respectively but it is faster urbanizing regions.
It has projected to become 56 and 64% urban population
in Asia and Africa respectively [43].
The state of the world's cities in 2008 has reported that
Tokyo is the largest city of 38 million people resides in
urban area. It is followed by Delhi, Shanghai, Mexico,
Mumbai and Sao Paulo respectively. It has projected that
there will have 40 mega cities with more than 10 million
people. China and India is the populous and contributes
more than one third of global urban population in between
2014 and 2050. The urban growth has increased in Europe
and North America in 20th century but in the beginning of
21st century, urbanization has dramatically increased in
Asia [48]. By the mid of the 21st century, the total
population of the world will be doubled that was projected
to 5.3 billion in 2050 compared to 2.3 billion in 2005. In
Asia, global urban population will have 63% [44] which
has been increased. There are mainly three cause of urban
population increased in Asia i.e. industrialization and good
economic opportunities, availability of basic services and
changing legal and administrative provision of urban area.
Globally, more people live in urban area than in rural area.
World Urbanization Prospects, 2014 reported that rural
population has decreased and urban population has
increased dramatically after 1950. The rate of urban
population growth is high than before. In 1950, 30% of
world population was urban. The urban population growth
is more than double within 100 year time interval. The
rural communities are diminishing and urban population
are widening. It increases the consumption rate of the people.
Consequently, urban problems will be increased. The
major problems are excessive emission of greenhouse
gases and urban heat island. Pave surface absorbs more
and increase surface temperature. Similarly, industrialization
and transport sectors consume more fuels. The World
Health Organization (WHO) reported that more than 1
billion Asian people will be exposed to air pollutants. The
level of pollutants exceeds WHO standard. China and
India have high economic growth rate of more than 9%
per year though it has bad experienced of high level of
pollutants than global average.
3. Urban Heat Island
Urban heat island is refers to the warmth of both
atmosphere and surface in cities compared to their
surroundings. It is undesired climatic modification by
changing surface and atmospheric characteristics with
urbanization growth. The UHI form in city comprises the
materials used in construction, the surface characteristics
like building dimensions and spacing, thermal properties
and amount of green space. Beside it, human activities can
also increase the heat island temperature. In general, the
intensity and magnitude of the surface heat islands depend
on urban characteristics and season. It is more in the
summer. The buildings and manmade surface such as
concrete and asphalt retain more heat compared to the
lesser heat retention and cooling properties of vegetation
which is densely abundant in countryside [51]. Simply,
UHI is urban warming phenomena that studied mostly
in terms of the temperature difference between rural
and urban locations [52]. In general, urbanized surface
bring change in energy modification and water balance
processes. It also influences to the dynamics of air
movement [34]. Urban surface contain more heat. It traps
the heat than plants and surrounding rural areas. This
process of increasing heat at the surface of urban area is
known as heat island effect [44].
There are several factors responsible to create urban
heat island. Less vegetation, urban materials, urban
geometry, anthropogenic heat emissions, weather and
geographic location are affecting to the formation of
UHI [27]. Mainly, less vegetation, urban design and
anthropogenic heat are responsible to cause the UHI in the
cities [53]. Now, vegetation are replacing by impervious
and dry surfaces. Urban designs are also affecting on heat
distribution around the surface. Buildings and street
emits infrared radiations that interrupt on surrounding
surfaces and is entrapped in the canyon and absorbed solar
radiation. The bunch of buildings with its high density and
low space create multiple reflections. Consequently, the
140 Applied Ecology and Environmental Sciences
surface heats raised up. Additionally, heat is generated by
human activities by combustion of fuels or stationary
sources. High levels of particulate matters and gaseous
pollutants can change the radiative properties of the
atmosphere in urban areas that increases UHI. It has reported
that urban heat island has formed by the consequences of
several integrated factors of urban environment. The most
important are highlighted by Oke’s in 1991.
The canyon complexity that made complex to
exchange and decreases the long wave radiation
loss
The reduction of evaporating greenery surface and
increase the impervious surface that absorb more
heat i.e. increase sensible heat and less in to latent heat
The thermal properties of the city surface materials
where the storage of sensible heat are increased
The anthropogenic heat released from combustion
of fuels and animal metabolism processes
Urban greenhouse which increase the heat at the
surfaces
Building structure that decrease the effective albedo
Low turbulent transfer of the heat in the city street
The impervious cities area can’t sink the pollutants. The
less vegetation in the cities reduced evapotranspiration
and contributing to warm the surroundings. Globally,
urbanization is the most important entities causing
enormous impacts on the environment. Although, cities
(built up or impervious surface) occupies only 2% of the
global land surfaces [54].The human disturbance due to an
urbanization has significantly altered the natural landscape
all over the world. In the United State, 71-95% of the area
are occupied by industrial area and shopping centers that
has covered by impervious surface. It is estimated that
almost two thirds of all impervious area is in the form of
parking lots, driveways, roads and highways [11] and
remaining one third consists of homes, buildings and other
non vegetated and open soil areas. There are mainly two
types of urban heat island i.e. surface heat island and
atmospheric heat island. The atmospheric heat island
includes the canopy layer and boundary layer heat island.
The canopy layer heat island exist from the ground to
below the tops of trees and roofs but boundary layers
starts from the roof top layers to the area where urban
landscape can influences. This canopy layer typically
extends not more than 1.5 km from the surface [2].The
nature of urban surface and atmosphere exchange specifies
the urban heat island [55]. Based on Oke, 1987, the modified
types and mechanism of the UHI are given in the Figure 1.
The local scale corresponds to an urban neighborhood
with similar surface cover, building density and activities
(Figure 1c). The micro scale (Figure 1d) extends from less
than one meter to hundreds of meters corresponding to the
individual buildings, roads, trees, courtyards, lawn etc.
At the Meso scale, urban impacts occur at the scale
of the whole city typically tens of kilometers in extent
(Figure 1a and Figure 1b). The urban boundary layer
(UBL) can’t extend beyond the planetary boundary layer
which depends on the surface roughness and stability
(Figure 1b). The UBL contains inertial sub layer (ISL),
beneath the ISL lies the roughness surface layer (RSL)
and the lower part of the RSL is refers to as the urban
canopy layer (UCL) which extends from ground up to the
mean height of the main roughness elements like trees,
buildings (Figure 1d). Most of the urban dwellers spend
within this layer where most of an emission takes place
[56].
Figure 1. Schematic of the relative horizontal scales and vertical layers typical of urban areas (a) Meso scale dome (b) Meso scale plume (c) Local scale
(d) Micro-scale
Applied Ecology and Environmental Sciences 141
The urban heat island effect was first observed in
London in the 18th century that was observed when
industrial revolutions begin. London is possibly the
longest studied UHI of any city. Luck Howard was a first
scientist to suggest that the temperature recorded in a city
was likely to be higher than that in the surrounding
countryside. Now, the study of UHI in London is still
continuing. The change in urban canyon, albedo and
vegetation has been changing UHI in the city. Now, it has
gained increasing attention in all developed countries
where summer heats of solar irradiance are experienced. It
has been concern more than a century. One of the earliest
UHI studies in 19th century was conducted in 1964 in
the urban southern Singapore [57]. It was investigated
that temperature was different in between urban and
surrounding where the temperature has more in urban
core. After that, a more process based approach attributes
UHI formation to energy balance change has started
after Oke’s study. Now, many scientists have studied the
spatiotemporal distribution of urban temperature through
different techniques of stationary or mobile observation.
4. Kathmandu Valley: Urbanization and
Environment Problems
Kathmandu valley is situated in the hilly region of
Nepal, it lies between latitude of 27o, 32’ 13” and 27o 49’
10” North and longitude of 85o 11’ 31” and 85o 31’ 38”
East and is located at a mean elevation of about 1300
meters (4265 feet) above sea level. The east west and
north south axis of this valley are about 30-20 km
respectively. The entire valley has a bowled shape
landscape. The total area of the valley is 697 square
kilometer which covers three districts including
Kathmandu, Bhaktapur and Lalitpur (Figure 2). The
Kathmandu valley occupied 85% of the Kathmandu
district, entire part of Bhaktapur district and 50% of Lalitpur
district (KVEO, 2008). Administratively, 1 Metropolitan
city and 10 Municipality of Kathmandu district, 4
Municipality of Bhaktapur district and 1 Metropolitan city,
2 Municipality and 3 Rural Municipalities of Lalitpur district
fall under the Kathmandu valley.
The valley is the highly populous and urban center
of the Nepal which included five major cities: Kathmandu,
Patan, Bhaktapur, Kirtipur and Thimi. Kathmandu
Metropolitan city is the largest city in Nepal and it
is the cosmopolitan heart. It encompasses a compact
zone of temple squares, narrow streets and big urban
canyon dating back 2000 years of old Kathmandu
that corresponds to the current city core. Kathmandu
Metropolitan city is only million plus city in the country
having 2.5 million populations which have 9.72% of
national urban population [45]. Nepal’s demographic
transformation is characterized by fast growing population
density in the Kathmandu valley where urban population
growth is 3.4% per year from 2001-2011. There are 1.04
million household in urban area of the Kathmandu valley
which has projected to increase by 1.13 million in 2025.
More than 80% household of valley are made of with
cement mortar or concrete blocks wall and reinforced
concrete or cement roofs [44]. Mostly, built up area are
dense in middle of the valley where the average annual
temperature is 16.64°C to 18.44°C (Figure 3).
Figure 2. Location map of the Kathmandu valley with major cities and meteorological stations used; the left inset map shows the geographical location
of Nepal
142 Applied Ecology and Environmental Sciences
Figure 3. Land use land cover, 2010 and average annual temperature (1982-2015) based on CRU gridded data set in Kathmandu valley, Nepal
Figure 4. MODIS NDVI distribution in 2000 and 2015 in Kathmandu valley, Nepal
The climate of the Kathmandu valley is sub-tropical warm
temperate with maximum of 35.6°C ambient temperature
in summer. The temperature in winter is range between
2°C-20°C. The average rainfall is 1400 mm where more
than 80% rain fall occured during June to August.
Kathmandu has experienced rapid land use and land cover
change due to rocketed urbanization growth. Unplanned
urbanization has introduced various environmental problems
in Kathmandu and UHI effect is one of the emerging
issues. The early urban growth of Kathmandu was based
on its agriculture surplus. Now, agricultural land in urban
area has been decreasing at an alarming rate. The agricultural
area in the Kathmandu valley is reported to have declined
an annual average loss of 0.5% or 400 hectare [58]. The
valley urban area increased from 3096 to 8378 ha in between
1984-1994. Concurrently, it lost 5282 ha of fertile
agricultural land [59]. The agricultural lands were decreased
from 62 to 42% in between 1984-2000. It has reported that
there will be no agricultural land in Kathmandu valley by
2025 [60]. Another backbone of urban greenery is urban
forest. The urban forest has not been adequately integrated
in to the urban land use and planning process in Nepal. In
Kathmandu, the urban forest coverage is only 3% which
are mainly occupied in surrounding of the valley. In core
city, there are almost no forests in built up area (refer
red color of Figure 3). The MODIS average normalized
difference vegetation index (NDVI) distribution in between
2000 and 2015 (Figure 4) showed that NDVI is very
Applied Ecology and Environmental Sciences 143
less in built up area that ranges from 0.1 to 0.3. The low
NDVI coverage area in city core has increased in 2015
compared to 2000 (refer red and brown color coverage in
Figure 4).
The MODIS NDVI map (Figure 4) was developed
based on MODIS 16-day composite NDVI dataset with
250m spatial resolution, Land Process Distributed Active
Archive Center (LPDAAC), MOD13Q1 Terra product
was downloaded from https://modis.gsfc.nasa.gov /. The
horizontal and vertical tiles used for the study area are
h24v06, h25v06, h24v05 and h25v05 of Terra product
MOD13Q1 0.006 version. The tiles was mosaicked and
re-projected to Albers equal-area projection (STDPR1: 25,
STDPR2: 47, CenMer: 105) and nearest neighbor re-sampling
method with the WGS84 datum using MODIS Re-projection
Tool (MRT) acquired from NASA website. Then, average
annual NDVI map for 2000 and 2015 was developed. The
map showed that NDVI has decreased in core city
especially in built up area. The area less than 0.4 NDVI
was expanded in 2015 compared to the 2000 though
NDVI has increased in overall country report. The open
space categorized as park and gardens are 0.58% in core
area of the Kathmandu [61] which has not significant for
urban dwellers. The availability of green space is 0.25
m2/person which is very lower in compared to WHO
guideline. The required green space based on WHO
guideline is 9m2/person. The topography of valley is bowl
shape and more canyon build up structure in city made the
troposphere with full of the pollution contaminants. It
contributes to increase both surface and air temperature
and decrease the resilience capacities. In Kathmandu
valley, there are poor physical, social, financial, institutional
and infrastructure to improve resilience capacity to tackle
urban heat and its associated problems.
4.1. Temperature Trends in the Kathmandu
Valley
Based on monthly temperature data obtained from 8
meteorological stations (Table 1), the temperature of the
Kathmandu valley has significantly increased. The 4 number
of stations was taken from Kathmandu, 3 from Lalitpur
and 1 from Bhaktapur district (Figure 2). Mann Kendal
test [62,63] and Sen’s slope [64] statistics showed
that average, maximum and minimum temperature has
significantly increased at the rate of 0.04°C yr-1 (p = 0.01),
0.06°C yr-1 (p = 0.001) and 0.03°C yr-1(p < 0.0001)
respectively (Figure 5). The human activities, local and
regional climate has triggered to increase temperature in
Kathmandu valley. The Figure 5 showed that average,
maximum and minimum temperature and trend variation
in Kathmandu valley from 2000-2016. The station wise
average temperatures are given in the Table 1. The average
annual temperature was high in Panipokhari (20.03°C) and
Kathmandu airport (19.50°C) in which both of the stations
was located in urban hub of the city. The maximum average
temperature of these two stations was 26.41°C and 26.24°C
respectively. The minimum temperature was also highin
these two stations compared to the others. The average
annual maximum temperature was ranges between 23.5°C
to 25.5°C, which are more during summer season (June, July
and August). The minimum temperature was also increased
which was approximately 11.5°C in 2000 and became
12.5°C in 2016. Similarly, the high average temperature
in Panipokhari and Kathmandu airport stations showed
that urban core has more temperature than surrounding.
Consequently, it forms more UHI in core of the city.
The warming trend is high in the inner core of
Kathmandu valley ranging from annual temperature trend
of 0.5-0.8°C in between 1976-2006 [65]. Remarkable
change in population growth, high greenhouse gases
emission, greenery lost and increased built up area, urban
canyon and dense impervious surface are responsible for
the warming in Kathmandu valley. These factors also
make the valley more sensitive towards the climate change.
Among above 8 meteorological stations, Nagarkot,
Godavari and Kokhana are located in semi-urban areas.
Therefore, average annual temperature was lower in
compared to others. Similarly, Nagarkot located in high
altitudinal zone i.e. 2147 m altitude has only 14.69°C
average annual, 19.32°C maximum and 10.06°C minimum
temperature. However, the temperature has increased at
the rate of 0.02°C yr-1 in Nagarkot. The average temperature
has highly increased at the rate of 0.1°C yr-1 at Panipokhari
station which is located at urban core of the valley.
The analysis from landsat images showed that land
surface temperature (LST) was ranges from 15.84°C to
39.17°C in 2000 and 16°C to33.98°C in 2014. In built
up area, the maximum highest mean values were 28.63°C
in 2000 and 28.08°C in 2014 [66] which is relatively
higher than average air surface temperature. The urban
area in the valley has increased sharply by 108%, open
area declined by 26%, bare soil declined by 43% and
water decreased by 8% during 2000-2014.Consequently,
normalized difference built up index (NDBI) values in urban
area of the Kathmandu valley was ranges between 0.12
to 0.22 [66]. The total built up area in Kathmandu valley
in 2016 is 26.06% which was 5.10% in 1989, 11.15% in
1999 and 24.16% in 2009 [67]. Continuously, the built-up
area has increased in the valley. Therefore, heat island
has formed and negative lapse rate occurred in the urban
area of the Kathmandu valley. In this context, green
roofs could be a good solution to combat urban heat and
environmental problems.
Table 1. The detail of the meteorological stations located in Kathmandu valley and the average annual temperature (°C) during 2000-2016
Index. No
St. Name
District
Lon (Degree)
Alt (m)
Ave. tem (°C)
Max tem (°C)
Min tem (°C)
1022
Godavari
Lalitpur
85.37883
1527
17.18
23.14
11.21
1039 Panipokhari Kathmandu 85.32415 27.72864 1329 20.03 26.41 13.64
1071 Budanilkhantha Kathmandu 85.3578 27.7805 1428 18.34 24.24 12.44
1073
Khokana
Lalitpur
85.29672
1309
18.05
25.03
11.07
1014
Kathmandu
Kathmandu
85.33333
1324
18.15
23.84
12.46
1029
Khumaltar
Lalitpur
85.32578
1334
18.58
24.96
12.19
1030
Ktm. Airport
Kathmandu
85.35625
1337
19.50
26.24
12.75
1043 Nagarkot Bhaktapur 85.52086 27.69335 2147 14.69 19.32 10.06
144 Applied Ecology and Environmental Sciences
Figure 5. The average, maximum and minimum temperature trends during 2000-2016 in the Kathmandu valley, Nepal
5. Green Roofs: A Review
Green roofs could serve as a surrogate of urban green
spaces to address urban heat problems and make the
human comfort of being close to nature. Cities, especially
the compact ones, are generally characterized by a severe
shortage of ground-level green spaces due to the scarce
plantable land area. The huge bulk of urban dwellers
trapped in the concrete jungles are largely deprived of the
vital and pleasing natural greenery. Consequently, the
intensity and effects of urban heat island is severe. In this
context, greening the barren roof space provides a promising
pathway to ameliorate urban heat and climate change.
5.1. Modern Green Roofs
The built structure has been a significant impact on
environment and degrades natural environmental quality.
The use of vegetation on a roof top commonly called green
roof is an increasingly practiced to restore environmental
prosperity. Green roof (vegetated roof) is a promising
idea to address UHI. There is several advantages of green
roofs which reduces urban temperature, provide clean
environment and save energy. It develops the resilient
power of the communities. In recent years, several researches
have persuaded in green roofs and UHI mitigation. It is a
flat or sloping rooftop designed to support vegetation.
UHI effects could be mitigated by substantial green roofs
utilization [41]. It is an innovative solution to tackle for
urban environmental problems such as urban noise pollution
control, carbon sequestration, urban heat mitigation, air
pollution control, prevention for urban health, aesthetic
maintenance, controls runoff in to municipal sewers and
drainage system. Modern green roof was originated from
Germany. Reinhard Bornkamm, a first German researcher
published his work on green roofs in 1961.The concept
has widen and practiced in several cities of developed
countries. The countries like Belgium. Singapore, Japan,
USA, Canada has enacted regulation to install green roofs
in new buildings. In 1997, a Munich city has initiated to
add vegetation on all flat roofs. Green roofs has also
owned by municipalities on their buildings throughout the
Germany. Similarly, the government of Switzerland and
Denmark had mandated green roofs in their country
regulation. Switzerland is the world's leader in per capita
uses of green roofs. The use of green roofs to mitigate
UHI is highly acknowledged in developing countries. In
Hongkong, government has encouraged to construct the
green roofs on buildings [68]. Roof top vegetation has
numerous ecological and economic benefits. It also
provides business opportunities for nurseries, landscape
contractor, irrigation specialist and other green industries
members. Globally, there are several types of green roofs
practiced. Mainly, it is intensive, extensive and semi-
intensive types [69]. The intensive green roofs need
frequent monitoring, skill labors, irrigation facilities and
plenty of growing media but extensive green roofs are
easy in comparison where the thin layer of soil can
support vegetation. It can be designed with self sustaining
plants and require minimum maintenance.
Applied Ecology and Environmental Sciences 145
Figure 6. Modern green roofs with the layer from roofing assembly to
growing medium and the vegetation
Semi-intensive types of green roofs are combination of
extensive and intensive green roofs. Green roofs offered to
cater different level of UHI condition. In study of the team
of Hewage in 2011, green roofs design should have
maintain root barriers, drainage layer, filter layer, water
retention, growing medium and vegetation layer as shown
in Figure 6. The root barrier is the first layer which acts as
water proof membrane and prevents the water leakage.
The main purpose of this layer is to protect the roof from
plant's root. If the roots penetrate the building assembly,
the probability of water leakage is high so that it can make
crack and holes where water infiltrates. Green roofs
should also have a drainage layer to prevent the leakage of
water for roof assembly. Good drainage can also maintain
the structural capacity of the roof assembly. Presence of
excessive water in barrier level can encourage the roots
grow so that it increases risk on roof assembly [70]. It also
has a filter layer which is useful to protect infiltration
during draining process. It also maintains the relation
between growing media and vegetation. The main propose
of the filter layers are to prevent drainage layer to block
from water runoff and the particles of the upper layer [71].
Water retention layer control runoff and keep the growing
medium layer moist. The type of green roof, vegetation,
building roofing assembly, weather conditions and soil
saturation determine the retention capacity [72].
The remaining layer is growing medium and vegetation
layer that is very important and needed worth caring
during design. Soil is the natural growing medium. The
clay and organic materials that contain in soil supply
nutrients for plants to their biological function. It also
makes the roots strong to stand during harse environmental
condition. The thickness of the growing medium layer is
depends on the type of vegetation. Small vegetation
requires less depth not more than 2.5 cm for an extensive
green roof system. The organic materials are essential for
plant growth. Larger plants require more nutrients than
smaller one. The organic contents in growing medium for
extensive green roofs is 30% and it is up to 45% for
intensive green roofs [11]. The most popular types of
vegetation on green roofs are sedums and mosses that
meet all the requirements [73] but it depends on local
environment condition. Different level of temperature and
precipitation requires specific vegetation and planting
medium for optimal performance. Intense rainfall can
erode the substrate [74]. The height of sedum and mosses
should not have more than 10 cm but some small trees,
vegetables can have 10 cm to more than 100 cm height
that can be generally used in intensive green roofs.
Sedum species are highly preferred and it can perform
even in low media in absence of irrigation. These plants
open their stomata during night and absorb carbon-dioxide
and store it as malic acid. During the day time, the plants
use the stored carbon dioxide for photosynthesis [75]. In
general, extensive green roofs plants are perennials. It has
shallow roots that spread rapidly and required minimum
nutrients. It tolerates sun, wind and extreme fluctuation of
temperature. Intensive green roofs have deeper growing
media which allows them to incorporate larger plants
including shrubs, bushes and trees in their design. Greens
roofs plants should tolerate radiation, high wind speed,
high temperature and scanty rain to the limitation of the
water [76]. Preferred green plants are perennials with
shallow-rooted. It is limited in growth and prevent toppling
and desiccation [77]. Some of the plant species like
Spanish moss (Tillandsia Usneoides) can be used which
has multiple functions and bio-monitoring for heavy
metals and dust. Similarly, green curtain, cotton candy,
grasses; flowering plants etc. can be selected depending
on substrate depth. Several variety of plants available for
green roof design such as low growing succulents,
herbaceous perennials, annual and biennial plants, turfs,
small shrubs and trees can be used but it depends on local
climate and propose of green roofs. Some of the air plants
can also be used which does not required substrate as well.
Extensive and intensive green roofs have some special
distinct characteristics as shown in Figure 7 [78].
It has different module of design and used based on
purpose and available supporting environment. Extensive
green roofs have low weight, low capital cost, low plant
diversity and requires minimal maintenance but intensive
green roofs require deeper soil and greater weight, higher
capital cost, increase plant diversity and require more
maintenance (Table 2 & Figure 7). The concept of green
roofs became popular within very short period of time
due to it’s numerous environmental benefits. Green
roofs can be installed not only on conventional flat roofs
but also on slope roof. Roofs with a slope of more than
45o are normally not suitable for a green roof system built
up [79]. The slope below 10o should have a special
precaution for the mitigation of shear force and erosion.
However, both of the extensive and intensive green roofs type
have advantage and disadvantage [80] as shown in Table 2.
In addition to the extensive and intensive green
roofs, some modular types of green roofs can also be
practiced. It is becoming more popular in some countries
[81]. Modular system retains all the benefits of the green
roofs with addressing in built up limitation. Modular
designs are self contained including its different types
such as mat system, tray system and sack system. Now a
day, modular tray systems are commonly in practice. In
modular tray system, plastic interlocking containers are
filled with a drainage system, growing medium and
vegetation prior to installation [30]. It can easily be
removed or replaces without affecting the original
structure or other plants. Modular tray green roofs are
simple to install and less expensive.
146 Applied Ecology and Environmental Sciences
Figure 7. Extensive vs. intensive green roof type
Table 2. Comparison of Extensive and Intensive Green Roofs System
Extensive Green Roof Intensive Green Roof
Thin growing medium; little or no irrigation; stressful condition for plants;
low plant diversity
Deep soil; irrigation system; more favourable condition of plants;
high plant diversity
Advantage Advantage
Light weight: roof generally does not require reinforcement
Suitable for large area
Suitable for roofs with 0-3°C slope
Low maintenance and long life
Often no need for irrigation and specialised drainage system
Less technical expertise needed
Often suitable for retrofit projects
Can leave vegetation to grow spontaneously
Relatively inexpensive
Looks more natural
Easier for planning authority to demand as a condition of planning approval.
Greater diversity of plants and habitats
Good insulation properties
Can simulate a wildlife garden on the ground
Can be made very attractive visually
Often accessible with more diverse utilization of roofs i.e. for
recreation, growing food as open space
More energy efficiency and storm water retention capacity
Longer membrane life
Disadvantage Disadvantage
Less energy efficiency and storm water retention benefits
More limited choice of plants
Usually no access for recreation or other uses
Unattractive to some, especially in winter
Greater weight loading on roof
Need for irrigation and drainage systems, requiring energy, water,
materials
Higher capital and maintenance costs
More complex systems and expertise
5.2. Green Roofs to Mitigate UHI
Many researchers have been investigated to identify
the potentiality of green roofs to reduce urban heat island.
It found that green roofs can reduce urban temperature.
The temperature measurement at the surface of green
roofs is lower than the common roof surface. The
relationship between green roofs and temperature is
negative whether the green roofs area increases the
temperature decreased. The relation between high green
roofs area and low temperature has been established
[11,15,16,17,21,24,34,40,41,74,82,83,84]. Beside it, many
more researches have focused on green roofs mitigation to
the urban heat. Undoubtedly, urban vegetation is a
possible mitigation strategy for the UHI. The highly
populous urban areas have few residential and open
Applied Ecology and Environmental Sciences 147
spaces. In this context, barren roofs could be a one
solution to convert in to green areas. Green roofs can
perform well in summer which decreases in uses of
buildings energy [85]. The field and modeling studies in
various cities have reported that green roofs reduced 15-
45°C in peak surface temperature and 2-5°C in peak air
temperature on hot days. Energy saving in air conditioning
electricity in summer could amount to 10-80% for
individual buildings [11]. It reduces the UHI by
evapotranspiration and altering the urface albedo that
directly reduces the building cooling demands [86]. There
are mainly three processes performed by vegetation i.e.
shading, evapotranspiration and plants photosynthesis that
helps to mitigate urban heat island [87,88,89]. The
insulating properties of the vegetated structure,
evapotranspiration and substrate as a thermal mass save
the energy than conventional roofs. The combined effect of
shading, insulation and evapotranspiration protect the
buildings and cooled it passively [90].
The green roofs design in buildings focus on heat gain
control and heat dissipation that improve the indoor
thermal comfort with low energy consumption. Likewise,
green roofs perform passive cooling process in the
buildings. The substrate and vegetation layer uses the
heat for water release via evapotranspiration rather than
the common roofs absorb more heat. The individual
building those who were adopted green roofs decrease
the energy need for air conditioning and heating. The
energy needed for buildings cooling will reduce by the
presence of vegetation in roof because the buildings
are protected from the sun radiation of environment
[91,92,93]. Wide spreading of green roofs design could
mitigate the harsh aspects of the UHI effect [90]. Green
roofs tend to store less heat but traditional roofs store
more heat throughout the day and dissipate at night.
The materials of traditional roofs emit and transfer
the heat by convection to the surrounding air consequently
exacerbates the UHI effects [6]. Green roofs have versatile
values in thermal urban environment. Green roofs
vegetation absorbs solar energy to fuel photosynthesis. It
also protects the roof membrane against solar radiation. It
increases the longevity of roofs by protecting it from
ultraviolet light. The green roofs expectancy is estimated
at 40 years compared to the 7-13 years for a typical
roof [94]. It lowers roof temperature and minimizes
high fluctuation. During summer, green roofs reduced
temperature in the roof membrane by 5-7°C. However, it
depends on the types of green roofs, plant materials and
local weather condition [95]. White roofs cool the interior
of the buildings significantly less than green roofs [90].
The heat change in urban areas is alarming due to
greenery lost and built-up increased. Several attempted
have been introduced in city system like increased
vegetation, adopting cool roofs and cool paving materials
but roofing system has central issues of research and
practices. Current research in roofing system only assess
the capacity of green roofs to reduce the heat fluxes [82],
decreasing air temperature and energy use for cooling
[96,97] though it has several bio-physical and social
environmental significance.
Table 3. Some of the studies related to the effect of green roofs in urban heat island
Research Methodology Findings
Harazona, Terako, Nakase
and Ikeda, 1990 Field measurement method The green roof vegetation system can successively moderate hotter and drier
climate during the summer in urban area
Bass et al, 2003 Meso-scale community compressible
(MC2) model
A regional climatic simulation model projected that if 50% green roof
coverage are distributed throughout the Toronto, it reduces temperature by
2°C-3°C
Rosenzweig et al, 2006
Regional climate model (MM5) in
combination with observed
meteorological, satellite and GIS data.
Green roofs vegetation can decrease the near surface air temperature between
0.4°C-1.3°C
Arabi et al. 2015 Reviewed and analysed the previous
studies
Using green roofs as a main strategy can decrease the UHI level and its
harmful impacts
Wong et al. 2007 Field measurement, Satellite imagery
analysis and Simulation approach
The potential reduction of 14.64-25.82% of temperature level by covering
40% of buildings roofs in NUS in Singapore
Alexandri & Jones, 2008 Two dimensional micro-scale model Buildings roofs vegetation have an important potential of lowering urban
temperature
Hui, 2009 Two dimensional micro-scale model Approximately 42% of outdoor temperature can reduce by the green roofs
Pompeii, 2010 Simulation through hard scale model Green roofs do have beneficial effects on UHI by lowering the temperature
within the city of Pennsylvania by 2°C
Smith & Roebber, 2011 Weather research and forcasting and
urban canopy model
The adoption of green roofs vegetation increase albedo and
evapotranspiration consequently it reduces the temperature in the urban
environment by 3°C
Susca et al, 2011 Multi-scale approach: an urban and
building scale
There is an average of 2°C temperature difference in between most and least
vegetated areas
Scherba, Sailor, Rosenstiel
& Wamser, 2011 Field experiment models Green roofs decrease the total sensible heat flux by 50%
Santamouris, 2012 Review the previous studies Green roofs has potential benefits to mitigate the UHI
Sun et al, 2012 Field measurement The green roofs can have potential to cool the ambient air temperature in
Tawain
Kolokotsa et al, 2013 Parametric study Green roofs can decrease the energy demand and simultaneously improve the
urban thermal environment
148 Applied Ecology and Environmental Sciences
Green roofs have several environmental values for reducing
urban heat island. It can be practiced as a multi-purpose
strategy to mitigate urban temperature. At the same time,
it compensates urban greenery lost [21]. The research
suggested that, hottest spot of the city can be converted to
the coolest one by using green roofs in large area. It also
helps to purify the air. A green roof acts as a wind buffer
and filters the air. Urban vegetation has air purification
capability which removes the air pollution by dry deposition
mechanism [69]. It helps to reduce the concentration of
pollutants at the urban climate so that re-radiated heat
from the urban surface can escape out to the atmosphere.
Otherwise, it could traps the heat and make the surface
warm. Similarly, green roofs vegetation can sequestrate
carbon directly from the air and store it [69]. The average
CO2 concentration at green roofs is lower than the other
places [42]. In the day time, photosynthesis process is
very active due to the influence of solar intensity resulting
in lowering the CO2 concentration in the surrounding. The
CO2 concentrations and amount of CO2 absorbed by the
trees are difference on the road side and inside the park in
Rome [98]. It showed that trees had an important effect on
the difference of CO2 concentration between two sites.
The park side CO2 concentration was lower than the road
side during day time observation. Ultimately, it affects on
temperature maintenance in urban environment. Moreover,
some of the studies have been done in green roofs to
mitigate urban heat island are listed as followings.
5.3. GIS Extension Model and UHI Study
Urban temperature has been escalating less greenery
and high built up areas. It is enduring in major cities
where population is more than 10 million. Some of the
research has projected possible impacts on urban climate
that is triggering to develop some urban resilient capacity.
Several researches have been done to estimate the impact
of different hypothetical greenery scenario on temperature
in urban areas. It has increased great attention of the
greenery projection [4,23,83,99]. Oke’s model with using
height and width (H/W) ratio of urban geometry was
considered in a beginning step in simulation research.
Some of them are used mesoscale climate model with land
use and land cover input but some are used urban
geometry and canyon structure. Some of the models were
developed for urban study based on grid network of cells
[100,101]. Whatever the used, simulation could be
estimated the impacts of greenery to thermal heat change.
Now, GIS extension model to calculate urban heat island
intensity has highly used where different input variables
like urban geometry, land cover change (greenery and built
up) and other urban parameters used for interpretation of
their influence on urban temperature [99]. There were
different physical parameters including greenery and built
up ratio that influences to the UHI formation. Similarly,
the Oke’s model was fitted in different places with
different physical parameters [23]. Now, urban greenery
coverage has becoming a central issue as a main limiting
factor for urban heat Island (UHI) change in the cities
which can be restored by the utilization of waste land i.e.
available roof top in the cities. Projection in the presence
of different green roofs condition in the cities and its
influence to the urban temperature has highly prioritized.
Arc GIS 10 [99] used the GIS extension model to
estimate UHI intensity based on Oke’s model. Recently,
there are two important factors i.e. urban geometry and green
space that contributes UHI in the cities. Urban geometry is
depends on the design factors like building height, aspect
ratio (H/W ratio). These factors are made complex to pass
long wave radiation. Consequently surface heat has
increased [102]. Similarly, urban green space can be studied
by using green plot ratio [103]. Ong use the green and
building plot ratio for sustainable urban planning. The design
factors and impacts can be obtained from surveying,
photographic methods and GIS system [102]. Basically,
Oke,s used the urban geometry of relationship between
the height of the buildings and the width of the street
canyon i.e. H/W ratio as an indicator of UHI. The Oke’s
empirical model adjusted for the H/W ratio is as follow (with
R2=0.89).
()
Tu r max a bIn(H / W)∆− =+
Where,
Tu r(max)=Maximum urban heat island (function of
urban and rural difference).
H/W = Relationship between height and width.
R2 = value of coefficient of determination (significant
relationship between the elements).
a & b = thermal coefficient value, function of the thermal
admittance of the urban and rural environment.
The calculation of H is the mean height of all buildings
on both sides and the calculation of W is performed based
on the sum of the mean values away from the building of
the axis of the right and left side.
h1 h2 h3 . hx
Hx
+ + + …… +
=
Dr1 Dr2 Dr3 . Dry
Wy
Dl1 DL2 DL3 . DLz
Z
+ + + …… +
=
+ + + …… +
+
Where,
H = average height.
h = height of each built.
W = average width.
Dr = distance of each building to axis, from block on the
right side.
Dl = distance of each building to axis, from block on the
left side.
Finally, the outputs provided by GIS are average height,
average weight, H/W ratio and Maximum UHI. Likewise,
the others physical parameters that influences UHI can be
used for simulation [104] and can adjust to Oke’s model
[23]. The Oke’s study has limited only urban geometry
but UHI can also depend on others physical factors like
land cover change, greenery and built up land. In case of
green roofs mitigation to UHI study, the ratio of greenery
and build up area can be fitted on Oke’s model to estimate
the impact of hypothetical 10%, 20% and 50% of rooftop
greenery to temperature change in urban environment. In
this case, The GIS will provide average greenery area,
average build up area, greenery/built-up ratio and
maximum UHI. This simulation model can be effective to
study in the Kathmandu valley.
Applied Ecology and Environmental Sciences 149
6. Green Roofs to Combat UHI
in the Kathmandu Valley
The concept of green roofs is newly emerging issues in
Nepal. Although, it has practiced in major cities around
the world. Extensive green roofs are not started in Nepal
but some of the intensive green roofs have been practiced.
Both of the communities and policy makers are not
scientifically aware on green roofs and its environment
significance. The surface and air temperature has been
increasing in the cities. The NDVI has low but NDBI is
high. The urban growth has been dramatically increased.
The dust and air pollutant has heavily affected to the
environment and people. Gradually, the government,
private and public sectors are interested to make the city
smart and eco-friendly. The type of roof assembly,
selection of plant species, use of polymers, drainage layer
and materials, growing media and its sensitivity in term of
nutrient composition and maintenance is the part of design
that has not well familiar and practiced in Nepal.
Government of Nepal has some specific environmental
law and policy. Environment Protection Act (EPA, 1996)
and Environment Protection Rule (EPR, 1997) isactive.
The act has made mandatory to conduct Environment
Impact Assessment (EIA) and Initial Environment
Examination (IEE) before starting any kind of
developmental projects. Furthermore, Government of
Nepal has preparing Strategic Environment Assessment
(SEA) guideline for environment screening. The
Government of Nepal, Ministry of Urban Development
has prepared guiding principles of urban development
where urban greenery issues such as green open space,
urban agriculture and forestry were highly prioritized [47].
Most of the developed countries and large cities have
enacted regulation to install green roofs in newly design
buildings and retrofitting. The city like Munich, Basal,
Copenhagen, Tokyo, Chicago, Toronto, Hongkong,
Belgium, Singapore, China and New York have regulated
green roofs policy in design, plants selection, media
adjustment, incentives criteria and subsidy with
prioritizing their available native resources.
The warming has occurred with an annual rate of
0.06°C in Nepal [105]. It increases the vulnerability in
urban environment. The major vulnerable sectors of
climate change impacts in Nepal are agriculture, food
security, forest and biodiversity, water resource, energy,
public health, urban settlements and infrastructures [106].
One of the major vulnerable sectors is urban settlements
and infrastructures. Climate change risk atlas 2010 ranked
as Nepal is 4th most vulnerable country worldwide
indicating the extreme vulnerability that the country faces
[107]. The vulnerability projection showed that Nepal is
under significant vulnerability category for static
adaptation capacity [108] in which Kathmandu has the
highest vulnerability ranked of 0.78-1 [106]. Human
assets, natural assets, social assets, financial assets and
physical assets are major determinants of adaptive
capacity [109]. All of these determinants are very week in
Kathmandu valley to resists urban heat in the city. UHI
assessment has been done in several cities in Europe and
Asia. It has more than 500 research recorded in Scopus
data base in 2015. Most of the research has used remote
sensing images and GIS for projection. The research
showed that there exists high urban heat island in cities
due to land use change by impervious surface and human
activities. Now, the critical issues are mitigation of urban
heat. The temperature in Kathmandu is high than
surroundings but the mitigation practices are poor. On the
one hand, greenery and urban forestry has been decreasing
and on the other hand temperature has been increasing.
High amount of energy are consumed in cities during
summer peak. Very few spaces are ponds, lakes, forestry,
open space and greenery to resist urban heat in
Kathmandu. In this context, green roofs (roof top vegetation)
are better option to deal. Therefore, the research on green
roofs to mitigate urban heat island is imperative. It
remains to be assured and addressed through the intensive
researches in Kathmandu valley.
Kathmandu valley is densely populous area. The people
are facing several extreme environmental problems like
high pollution, increased land surface temperature, health
impacts and energy shortage which can be observed
mostly during summer season. People have not getting
good alternative solution because they have limited
land owned area, open spaced and less assets to adapt
the problems. They have wasting freely available roof
surfaces. The green roofs practices can also create the
market opportunities from nursery entrepreneurs to large
scale contractors and experts. Green roof are not only an
option to reduce urban heat island, there can be practiced
cool roof and cool pavements which can reduce urban
temperature but green roofs have multiple benefits. Cool
roof can’t sequestrate CO2 like vegetation in green roofs
use CO2 for photosynthesis. Cool roof does not have
shading effect, energy demand could reduce but in green
roofs shading is important functions that block the solar
radiation to reach the ground surface. Thermal absorption
and emission of cool pavements can be intervened by
human activities. Green roofs have neither implemented
not regulated however building’s roof are flat and
favorable for retrofitting and new design. Urban geometry,
canyon, unplanned buildings, land use and human
activities are the main detrimental factors in Kathmandu
valley so that green roof is demanded in city. In summary,
urban heat island and green roofs mitigation potential
should be investigated from different prospective in
Kathmandu valley. The simulation model can be used to
identify required vegetation in Kathmandu. The initiation
of green roofs as a surrogate of urban forestry from both
practices and planning can be flourished after the detail
investigation of bio-physical and social environment in the
valley.
7. Conclusion
Kathmandu valley is the highly populous and urban
center of the Nepal where the air surface and land surface
temperature is high where the surface temperature has
significantly increased at the rate of 0.04°C yr-1 in between
2000 to 2016. During same period, maximum and minimum
temperature has also increased significantly by 0.06°C yr-1
and 0.03°C yr-1 respectively. The normalized difference
vegetation index is less and normalized difference built up
index is high in urban area of the valley where the urban
150 Applied Ecology and Environmental Sciences
area has increased dramatically. It creates possible risk of
urban heat island. In this study, urban heat island and
globally practiced green roofs technology were reviewed
to relate the emerging urban heat mitigation in Kathmandu
valley. Urban temperature has been escalating less
greenery and high built up areas. In this context, Green
roofs have cost effective technology which can be restored
urban forest and healthy environment. The review findings
of the green roofs help to replace the traditional roof
surfaces by green roofs that offer much lower temperatures
in summer to enhance their thermal performance. The
impact of projected greenery scenario on urban temperature
could be identified from remote sensing and GIS
extension model. Urban activities are highly responsible to
use more energy and green house gases emission. At the
same times, resilience capacities were loosed due to
excessive use of natural resources for urban development
activities. Therefore, green roofs techniques can be
practiced to mitigate urban heat island and environmental
problems in Kathmandu to make smart, eco-friendly,
highly resilient and sustainable city. However, the detail
study about green roofs, building designs and policy
intervention is required to implement green roofs in the
future.
Acknowledgements
We would like to express our sincere gratitude to
the Institute of Science and Technology, Tribhuvan
University, Nepal and Faculty of Environmental Management,
Princes of Songkla University, Hatyai, Thailand. The
authors would also like to thanks Department of
Hydrology and Meteorology, Nepal for temperature data.
We acknowledge MODIS team for NDVI data, Climate
Research Unit, University of East Anglia for time series
temperature data and Ministry of Urban Development,
Nepal. Finally, we are delighted to the reviewers for
providing valuable comments on the review manuscript.
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The growth of tourism often expedites the process of urbanization in terms of expanding built-up areasBuilt-up area and other impervious surfaces at the cost of the original natural serenity. The coastal resort town of Digha, situated at the Medinipur coastal plain of West Bengal, India, had been developed after substantial land use conversionsLand use conversion and loss of perennial interdunal wetlandsInterdunal wetland. The effects of wetland transformations are more evident in terms of continuous ecosystem degeneration and reduction of subsistence-based livelihood provisions for the local populace. However, this trend of urban expansion was severely contested by the ‘smart citySmart city’ concept which the State Government had envisaged for the sustainable development of Digha Township in recent years. This new concept advocated for a balanced land use planning giving adequate attention to its green infrastructure and urban ecological standards towards developing the cities of the new century. An appropriate assessment of the present patterns of urbanization and consequent environmental transformations thus becomes the prerequisite of any such development endeavour. In this context, the present study aimed to quantify the cumulative stress of expanding buildups on the interdunal wetlandInterdunal wetland ecosystems around the renowned coastal resort town of Digha of this region through a coupling of geospatial technologies with statistical analyses. For this purpose, Normalized Difference Built-up Index and Soil Moisture IndexSoil Moisture Index (SMI) were derived from the multispectral Landsat images of the year 2000 and 2018, respectively, as indicative physical and bio-physical parameters. The linear maximum likelihood regression model was then applied on these derivatives to infer the spatio-temporal relationships between the expansion of buildups and changes in wetland characteristics of this area. Results indicated that the magnitude of wetland encroachment was more severe within the newly developed high-density built-up areasBuilt-up area. Moreover, the interdunal wetlandsInterdunal wetland were found to be shrinking more rapidly in 2018 compared to that of the 2000 scenario in direct correspondence with the enhanced growth of built-up zones. Remarkably, a few sites in the rural fringes were also experiencing aggravated loss of soil moisture contents chiefly due to the establishment of isolated resort compounds and gated housing complexes in spite of being quite far from the core urban zones. Incessantly changing tourist preferences towards secluded lifestyles and demand for serene landscapes as well as lacklustre implementation status of land development regulations were primarily attributed to this sporadic nature of land use conversionsLand use conversion in this region. Based on the findings, a few realistically attainable management guidelines have been recommended towards developing a true ‘smart citySmart city’ in terms of both ecological composure and sustenance of tourism initiatives.
... Atmospheric processes in an urban area is studied within the defined climatic scales and vertical layers (Figure 1). On the horizontal scale, we can distinguish microscale or street canyon scale, local scale or neighborhood scale, mesoscale or city-scale [31,38]. Due to the air turbulence, an increase in spatial scale decreases the temperature difference between UCL and UBL layers [34]. ...
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Economic and social development of urban and rural areas continues in parallel with the increase of the human population, especially in developing countries, which leads to sustained expansion of impervious surface areas, particularly paved surfaces. The conversion of pervious surfaces to impervious surfaces significantly modifies local energy balance in urban areas and contributes to urban heat island (UHI) formation, mainly in densely developed cities. This paper represents a literature review on the causes and consequences of the UHI and potential measures that could be adopted to improve the urban microclimate. The primary focus is to discuss and summarise significant findings on the UHI phenomenon and its consequences, such as the impact on human thermal comfort and health, energy consumption, air pollution, and surface water quality deterioration. Regarding the measures to mitigate UHI, particular emphasis is given to the reflective and permeable pavements.
... Kathmandu city began to witness a population influx during the 1950s, and the rate increased exponentially after the 1980s (NWCF/NTNC, 2009). Normalized difference vegetation index has also shown risen urbanization in the Kathmandu Valley (Baniya et al., 2018). With the expansion of settlements, the volume of waste entering the river has increased. ...
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Identification of pollution in the river helps to know the state of the river ecosystem. The study aimed to assess the water quality of the Bagmati River by analyzing the physical and chemical condition and comparing it with national and international standards. The water samples were taken from 10 different sampling sites along the length of the Bagmati River inside Kathmandu Valley, i.e., from Sundarijal to Saibubhanjyang. A total of 30 physical and chemical parameters were examined. The results showed that the pH ranges from 6.0 to 7.5 in different sampling locations. The highest dissolved oxygen (DO) (8.5 mg/L) was found at the upstream while the lowest, i.e., 3.4 mg/L and 3.5 mg/L, was found at the urban core of the valley, i.e., Teku and Thapathali, respectively. The BOD, COD, oil, and grease considerably exceeded the WHO and national generic effluent standard. Most of the heavy metals in the river water were below the range of standard. The concentrations of all pesticides were found below 10 µg/L except heptachlor exoepoxide. The highest concentration of heptachlor exoepoxide (75 µg/L) was found at Balkhu, followed by Thapathali (69 µg/L) and Teku (62 µg/L). The result showed that the middle-urbanized segment, i.e., from Gokarna to Teku, is heavily polluted than the upstream and downstream segments of the river. The results are of great significance for policy formulation and implementation of the ecosystem restoration project of Bagmati River in the Kathmandu valley, Nepal.
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This study emphasizes the use of remote sensingRemote sensingparameters NDVI and LSTLand Surface Temperature (LST) to identify decadal changeDecadal change of three different years 1999, 2009, and 2019 in green cover in Dehradun cityDehradun city. Data analysis was done by using satellite imagery Landsat 8 OLI and Landsat 5 TM. The relationship between NDVINormalized Difference Vegetation Index (NDVI) and LST are inversely proportional. The result shows the continuous decrease in green cover with an increase in temperature over the time of Dehradun cityDehradun city. From this study, we came to know that the area under NDVI (Very High) value is decreasing from 10% in the year 1999 to 3.7% in the year 2009 and further 0.6% in the year 2019 and in contrast, the area under LSTLand Surface Temperature (LST) range(>35 °C) is increasing from 1.68% in the year 1999 to 16.55% in the year 2009 and finally reaches to 39.75% in the year 2019. This results in environmental damage of Dehradun cityDehradun city over the time period. This study helps the decision makers of different organizations who want to do urban planning, climate change analysis, and forest fire risk management study in reducing sustainable damage.
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In this research work the number of changes developed in the land use/land cover is investigated that got developed in the mountainous region of Nepal due to the Landslide disasters that occurred in the month of June 2013. The multispectral satellite data is obtained from the United States Geological Survey (USGS) through the Landsat 8 satellite. The data represents the multispectral images before and after the landslide disasters. Here we have used a post-classification change detection texture-based technique to quantify the number of changes that got developed in the value of the feature. The pre landslide and post landslide images are converted into gray-level images. The features value is computed and quantified through a grey level co-occurrence matrix-based technique. The pre landslide and post landslide images are compared for the texture features varying from 0° to 135° having a step size of 45°. This research work showcases the possibility of using a texture-based change detection technique for quantifying natural disasters like landslides. The process can be further extended to identify and quantify events other than landslides like flood, drought, soil moisture retrieval.
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The Kathmandu Valley of Nepal epitomizes the growing urbanization trend spreading across the Himalayan foothills. This metropolitan valley has experienced a significant transformation of its landscapes in the last four decades resulting in substantial land use and land cover (LULC) change; however, no major systematic analysis of the urbanization trend and LULC has been conducted on this valley since 2000. When considering the importance of using LULC change as a window to study the broader changes in socio-ecological systems of this valley, our study first detected LULC change trajectories of this valley using four Landsat images of the year 1989, 1999, 2009, and 2016, and then analyzed the detected change in the light of a set of proximate causes and factors driving those changes. A pixel-based hybrid classification (unsupervised followed by supervised) approach was employed to classify these images into five LULC categories and analyze the LULC trajectories detected from them. Our results show that urban area expanded up to 412% in last three decades and the most of this expansion occurred with the conversions of 31% agricultural land. The majority of the urban expansion happened during 1989-2009, and it is still growing along the major roads in a concentric pattern, significantly altering the cityscape of the valley. The centrality feature of Kathmandu valley and the massive surge in rural-to-urban migration are identified as the primary proximate causes of the fast expansion of built-up areas and rapid conversions of agricultural areas.
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Under the same urbanization pressure, the local climate in large metropolitan areas is also altered. This is especially apparent when certain climatic characteristics are considered, e.g. temperature, humidity and wind. In fact, all the main meteorological parameters are severely affected, resulting in the development of a local climatic regime, which is characterized by increases in temperature (the heat-island effect) and reduction of humidity and wind. Furthermore, in central areas particularly, the continuous replacement of vegetation with buildings and roads severely affects the radiation balance and this further influences the temperature regime of the environment. Under these circumstances the comfort index for those living in big cities is quite different from that for those living in suburban and rural areas.
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This paper presents the development of a simulation model, which was incorporated into a Geographic Information System (GIS) in order to calculate the maximum intensity of urban heat islands (UHImax) based on urban geometry data (using a H/W parameter). This tool is called THIS – Tool for Heat Island Simulation. The urban heat island phenomenon is defined by the temperature rise in dense city centers compared with the surrounding countryside. The methodology of this study is based on a theoretical-numerical basis (Oke model), followed by the development of a calculation algorithm incorporated into the GIS platform, which is then adjusted and applied as exemplification. This adjustment was made by calibrating the Oke model for a case study based on two Brazilian cities and different various trends for different roughness length ranges were found. As a consequence, this work has resulted in the automation of an algorithm to obtain maximum intensity values of heat islands based on a simplified model. After finishing the subroutine, the application of the THIS in a simulation of different urban scenarios showed different trends in the UHImax value for the H/W ratio and the roughness length. The UHImax increases when the H/W ratio increases, but the urban canyons with greater roughness (larger areas of facades and more heterogeneous heights, Z0 ≥ 2.0) result in UHImax values of approximately two times smaller than canyons with less roughness (homogeneous with highest average areas occupied by buildings, Z0 < 2.0) for the same value as the H/W ratio. Overall, the developed tool has one aim: to simulate the effect of the isolated variable of urban geometry on the maximum intensity of nocturnal heat islands, considering different urban scenarios.
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Heat islands are urban and suburban areas that are significantly warmer than their surroundings. Traditional, highly absorptive construction materials and a lack of effective landscaping are their main causes. Heat island problems, in terms of increased energy consumption, reduced air quality and effects on human health and mortality, are becoming more pressing as cities continue to grow and sprawl. This comprehensive book brings together the latest information about heat islands and their mitigation. The book describes how heat islands are formed, what problems they cause, which technologies mitigate heat island effects and what policies and actions can be taken to cool communities. Internationally renowned expert Lisa Gartland offers a comprehensive source of information for turning heat islands into cool communities. The author includes sections on cool roofing and cool paving, explains their benefits in detail and provides practical guidelines for their selection and installation. The book also reviews how and why to incorporate trees and vegetation around buildings, in parking lots and on green roofs.