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.

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 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 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/km^{2 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 21^st 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 20^th century but in the beginning of 21^st century, urbanization has dramatically increased in Asia ⁴⁸. By the mid of the 21^st 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 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.

The urban heat island effect was first observed in London in the 18^th 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 19^th 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 27^o, 32’ 13” and 27^o 49’ 10” North and longitude of 85^o 11’ 31” and 85^o 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).

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 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.

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.

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.

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.

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 45^o are normally not suitable for a green roof system built up ⁷⁹. The slope below 10^o 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.

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 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.

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.

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).

Where,

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.

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.

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 4^th 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 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.

References