4.1. Spatial Variation of the LULC Classes
The increase in urbanization is due to a high population density growth rate and migration toward cities has developed pressure on agricultural land. The agriculture zone is changing into a built-up zone. For instance, the area on the river Ravi bed near Multan Road has been declared an agriculture development zone (ADZ), which is now converted into a built-up zone rapidly. From 1996, spontaneous urbanization stimulated the haphazard LULC changes within the city i.e.; decreasing the cultivated land and expanding high-density built-up land to accommodate the people (Arshad et al., 2021). These LULC changes are not good for the environment of the city. When the temperature was high, the UHI and SUHI intensities reached at their peak.
Table 6
Spatial Growth variation in LULC of Lahore
Year
|
Urban Built-up Zone (Sq. Km)
|
Agriculture Zone (Sq. Km)
|
Water Zone (Sq. Km)
|
Barren Land Zone (Sq. Km)
|
Change in Urban Class Zone
|
Variation
|
1996
|
253
|
937
|
28
|
554
|
14.27%
|
-
|
2008
|
355
|
880
|
23
|
514
|
40.31%
|
26.04%
|
2020
|
700
|
850
|
26
|
196
|
97.18%
|
56.87%
|
Urban built-up zone of Lahore city from 1996, totally was just 253 Sq. Km which is 14.27% of the total area (1772 sq. Km). Due to a high increase in the built-up zone, the area reached to 40.31% in 2008. In 2020, the urban built-up area reached 97.18%. In 24 years, the overall change in the built-up class increased to 56.87%. LULC map (see Fig.
5) depicts 24 years of change in the urban built-up zone and the transformation in the classes. The variation in LULC classes shows in Fig.
3 − 1. Development during 1996–2020 in the city affected the changes in LST. In which temperature raised from 7°C to 14°C at daytime while 5°C to 8°C at nighttime during HWs.
High intensity of SUHI was found in the highly built-up zones, like Shalimar, Ravi, and Data Gunj Bakhsh. During the current study, we analyzed that built-up land in Lahore is increasing towards BRB canal bank sides. Figure 3 − 2 shows the city's directions of built-up area growth.
LULC classes description were broadly identified as shown in the below table 4.
The rapid growth of the built-up areas in the city brought about the development of shanty towns which created urban heat islands. The study area map described the urban built-up zone class which shows the directions of built-up area growth in the city.
The “Urban built-up land” is defined as different surface types, e.g., impervious surface like buildings, and roads. This class zone is important because this covers the central parts (CBD zone) of the city and consists the most of the city area. A simple indicator of a built-up class is the percentage of a space covered by impermeable surfaces like asphalt and concrete.
The built-up area is typically used as a measurement of urban sprawl. It can be measured by identifying the built-up or impervious areas as the primary characteristics of sprawl, which can be identified using topographical maps or remotely collected data. As a result, compared to less developed areas, developed areas have a higher percentage of impermeable surfaces or built-up areas.
Table 7
Description of LULC classes
LULC Class Zone
|
Description
|
Built-up Zone
|
Consisting mostly of highly dense built-up areas with low open space and vegetative cover in between. Areas having the most structures of roads, and buildings were included. E.g., Old walled city, Ichra and Baghbanpura etc.
|
Vegetation Zone
|
Includes agriculture and vegetative land cover. whatever vegetative land cover exists during that time of the month over agricultural fields
|
Barren / Vacant Land
|
Includes the empty land also includes the land cleared up by the construction projects for developing new societies, lastly.
|
Water
|
The class consists of all the water bodies in the study area, from rivers to canals and of identifiable sizes to canals and the river Ravi
|
4.2. UHI Analysis
The magnitude between the UHI and relative humidity were inversely proportional to each other. When relative humidity is lower, then UHI intensity increased, similarly when Relative humidity is high, then UHI intensity decreased. When humidity levels rise in the urban atmosphere, we feel the temperature low concerning to our bodies. This will happen when no change occurs in wind speed. Figure 6 shows the relationship between the UHI intensity and relative humidity at the same height of 2m. Geographical regions have different climate properties. Different countries have different criteria for UHI with respect to regional climate. Different parameters of climate have different effects on UHI intensity. So, therefore UHI intensity can be measured with the regional climate properties. UHI intensity is connected to the physical characteristics of a city in terms of its land use and land cover classification, its magnitude and variability are a function of nonlinear interactions between multiple dynamic factors. Which is the urban canopy, wind-blocking, low albedo materials, deforestation, thermal storage, anthropogenic heat, high population density, and congested areas. The difference in the convective and advective flow of heat energy in the city is also causing urban overheating (Oke, Arnfield, & Kanda). The below diagram shows the formation of UHI in the
Table 8
Difference between SUHI and UHI
Surface urban heat island (SUHI)
|
Atmospheric urban heat island
|
Present at the daytime and nighttime and high intensity rate during the daytime and in the summer
|
Less intensity during the daytime and severe at nighttime
|
More spatial and temporal variation:
Daytime variation 10–15°C
Nighttime variation 5–10°C
|
Less variation:
Daytime variation − 1–3°C
Nighttime variation 7–12°C
|
Indirect measure by remote sensing.
|
Direct measure by fixed weather stations and mobile traverses.
|
Thermal image
|
Isotherm map and Temperature graph
|
4.3. Variations between the UHI2m and HWs
We identified a strong relationship in the variation between UHI2m and HWs. The difference in the net radiations becomes larger during HWs than in normal conditions, suggesting that urban regions get more radiative energy input than rural or suburban regions. The city temperature was magnified during heatwaves, which shows a prominent synergistic effect between UHI and HWs episodes (in Fig. 7). The anthropogenic heat during HWs is one of the main contributors to the synergistic effect during both daytime and nighttime. In high built-up areas, urbanization leads to an increase in sensible heat and a decrease in latent heat flux due to less vegetation cover and soil moisture. The urban-rural difference in humidity becomes larger during HWs. The synergies between UHI and HWs are more intense in densely built-up areas, which is attributed to the vertical buildings hindering natural wind flow, resulting in reduced heat removal. The relationship was a strong synergistic effect, due to the high value of relative humidity in the environment during normal days, on which urban temperature cools down. The intensity of UHI depends on the climate and the different responses to HWs. The main factors, which affect the UHI intensity are morphology, population, climate background, etc.
We analyzed that the magnitude of UHI is highly determined by the relative humidity and wind speed. Wind speed was measured at the 10-meter height level from the surface of the earth (m/s) unit. When relative wind speed is high, relative humidity is high, then UHI intensity becomes lower, similarly inversely proportional with UHI intensity. This is described in Fig. 9.
We found the Variation of the UHI2m with Wind Speed in a dual nature relationship because the speed of wind affects the UHI intensity. When wind speed is high, the UHI intensity becomes lower if the relative humidity is high, otherwise, with lower relative humidity and low wind speed, the UHI intensity becomes high and dangerous to human health and becomes a reason for heat stroke events.
The synergistic effect between UHI2m and HWs during daytime is higher during HW days than during normal days in Lahore in the high built-up zone under current climate conditions. UHI intensity for these zones was remarkably increased during heatwaves episodes, pointing out a synergistic effect between UHI and heatwaves (Zhao et al., 2018).
1.1 SUHI Analysis
For SUHI measurement, data of remote sensing were acquired using Google Earth Engine (GEE). The areas of interest (AOI) were extracted from the Terra MODIS, MYD11A1.006,1000 m, daily product, based on the HWs days. The images of daytime and nighttime were taken separately (Founda & Santamouris, 2017). The average thermal temperature value is equal to one pixel in the image. The spatial resolution of each pixel is 30m. For the processing in ArcGIS, applied image-factor using raster calculator. The pixels of raster data are converted into points form using the tool and these are linked with the geographical zones. Geographical zones are based on built-up and real-time ground earth reality.
2 Suhi Intensity Under Contribution Index
With the increasing urbanization, the built-up area of the city reached 56.17% from 1996 to 2020. The eight multifactor-oriented zones (MFOZs) appeared significant changes in terms of zone-wise area and landscape. The MFOZs with the distribution of area weightage are shown in Table 6.
The SUHI intensity was measured by using the CI multifactor-oriented zones (MFOZs) method. SUHII was calculated based on the HWs episodes which occurred during 2011–2021.
Table 9
Contribution Index of HWs (Daytime and Nighttime) under the MFOZs
MFOZs
|
ZONE ID
|
Area (Km2)
|
Average Temperature Daytime
|
Average Temperature Nighttime
|
Contribution Index Day
|
Contribution Index Night
|
Agriculture Development Zone
|
ADZ
|
849
|
40.81
|
27.29
|
-0.1849
|
-0.4345
|
Water Bodies Zone
|
WBZ
|
23.8
|
39.05
|
27.61
|
-0.0289
|
-0.0078
|
Parks and Recreational Zone
|
PRZ
|
14.5
|
41.72
|
30.94
|
0.0225
|
0.0043
|
Industrial Development Zone
|
IDZ
|
69.94
|
41.26
|
28.84
|
0.0256
|
0.0205
|
Mix Built-up Zone
|
MBZ
|
95.5
|
41.55
|
29.94
|
0.0938
|
0.0389
|
Central Business District
|
CBD
|
71.1
|
41.7
|
30.61
|
0.0967
|
0.0540
|
High Built-up Zone
|
HBZ
|
315.14
|
42.02
|
29.05
|
0.1509
|
0.1470
|
Planned Built-up Zone
|
PBZ
|
333.02
|
41.4682
|
29.2128
|
0.1903
|
0.0504
|
3 The Evaluation Of Suhii Under The Mfozs
Figure 15 represents the difference in SUHI intensity during daytime and nighttime under the contribution of the eight MFOZs (see Table 6).
The average SUHII differences between each zone were evaluated (Table 6), and the HWs during daytime and nighttime during 2011–2021 were chosen to calculate the contributions of the eight MFOZs. Overall, during the day, the average temperatures in the IDZ, HBZ, MBZ, CBD, and PBZ were higher than those in the ADZ, WBZ, and PRZ during 2011–2021 (in Fig. 16). The mean temperature was high in the high built-up zone (HBZ) and industrial development zone (IDZ). This happened due to industries running day and night and releasing vehicles' heat into the environment. the heat energy is stored in the environment during the daytime, due to dense built-up infrastructure and high population rate, and the heat energy is less released in the environment at nighttime.
Distribution Of Suhi Intensity Under The Administrative Zones
The Lahore administrative zones are shown in Table 7 with population density year 2020. We analyzed the SUHI intensity under the administrative zones using the severity of the surface urban heat island formula, in which indexing zones with region weightage. The following Table 7 shows the zone-wise evaluation index and SUHI intensity in °C.
Table 10
The Administrative zone wise Population, evaluation index and SUHII in °C
ADMINISTRATIVE ZONE
|
EVALUATION INDEX
|
SUHII
|
POPULATION DENSITY 2020
|
DATA GUNJ BUKHSH
|
0.068
|
1.24
|
40257
|
SHALIMAR
|
0.012
|
0.69
|
28291
|
RAVI
|
0.034
|
0.54
|
34560
|
AZIZ BHATTI
|
0.017
|
0.45
|
11597
|
GULBERG
|
0.010
|
0.44
|
25218
|
SAMANABAD
|
0.007
|
0.31
|
36212
|
LAHORE CANTONMENT
|
0.042
|
0.17
|
11200
|
WAGAH
|
0.032
|
0.12
|
1965
|
NISHTER
|
0.029
|
0.12
|
3578
|
IQBAL
|
0.002
|
0.08
|
2450
|
We can see Fig. 18 which shows the highest SUHI intensity zones are Data Gunj Bukhsh town, Shalimar town, and Ravi town. And these zones have the highest population density and built-up ratio.