Abstract: Urban Heat Island (UHI) is the name given to the characteristic warmth of both the atmosphere and the Lithosphere in Cities (Urban) compared to their rural (non-Urbanized) Surroundings. This study investigates the applicability of Land SAT/ETM in the study of UHI. The study employed the normalized difference method to estimate Land Surface Temperature (LST). Results demonstrate that LST positively correlate with concentration of Urban Structures, population density and human activities. Temperature gradients are formed at the Central Business District (CBD) in the downtown areas and progressively lowered to the suburbs. The study suggested the use of high density green cover, reflective roofing materials, building massing (arcades) and Lightening of pavements as adaptive/mitigation measures against UHI in Enugu Urban.

INTRODUCTION

Urban Heat Island (UHI) is the name given to the characteristic warmth of both the atmosphere and the Lithosphere in Cities (Urban areas) compared to their rural (non-Urbanized) surroundings (Voogt, 2004). UHI may be up to 10-15°C under optimum conditions (Oke, 1982).

For almost 200 years, climatic differences between Urban and rural environments have been recognized (Taha, 1997), of which temperature is the most obvious (Unger et al., 2001).

This phenomenon has been well documented and found to be universally typical by Shashua-Bar and Hoffman (2002). UHIs develop when a large fraction of the natural land-cover in an area are replaced by built surfaces that trap incoming Solar radiation during the day and then re-radiate it at night (Oke, 1982; Quattrochi et al., 2000).

The microclimate caused by Urban Heat Island (UHI) has the effect of increasing the demand for cooling energy in commercial and residential buildings. Increased demand for energy can cost consumers and cities thousands of additional naira in air conditioning bills in order to maintain comfort levels. In addition, increased electricity generation by power plants leads to higher emissions of Sulfur dioxide, Carbon monoxide, nitrous oxide and suspended particulates, as well as CO_2, a Green House Gas (GHG) known to contribute to global warming and climate change.

In particular, the energy implications of the climatic changes induced by UHI have received very little attention in urban design and planning. This is especially the case in Enugu, where Urbanization is at its peak. Enugu is a city with an area of about 100 km². Here, the traditional ground observation method of UHI may not be able to reflect in details the Urban heat Island distribution characteristics, which satellite remote sensing technique can offer and that is the reason for the choice of this technique to study UHI in Enugu Urban.

Land SAT/ETM is a passive imaging instrument able to collect reflected and emitted electromagnetic radiation in 7 different spectral regions, spanning from the visible to the thermal infrared. Land SAT/ETM data of October, 2007 was used in this study.

The goal of this study, therefore, is to determine the spatial extent and behaviour of urban heat Island in Enugu Urban.

The study area: Enugu Urban is the study area. It is located on the Eastern fringe of Udi escarpment at an attitude of about 223 m and lies on 6°.20'N, 7°.25'E and 6°.30'N, 7°.30'E (Government of Anambra State, 1978). Its origin dates back to 1909 when Mr. Kitson, a British Minning Engineer, discovered coal at the foot of Udi escarpment together with British Geological Exploration Team (Nnamani, 2002).

Coal Mining activities and extension of railway line from Port Harcourt to enable the coal to be transported to the coast for export; attracted waves of migrants from the surrounding villages to the extent that in 1917, it attained a township status under Lord Lugard’s Ordinance.The growth of the city is evident from the various population Census figures from 1952-2006. It recorded a population of 62,764 in 1952; the 1991 census puts the population figure of Enugu to be 464, 514, accommodated in 28 residential Layouts. The 2006 Census records the population of Enugu to be 722, 664 (NPC, 2006).

Enugu, since 1929 has remained an administrative headquarters. It served as the capital of Southern Provinces (1929), Capital of Eastern Provinces (1939), Capital of Eastern Region (1951), Capital of defunct Republic of Biafra (1956), Capital of East Central States (1970), Capital of old Anambra State (1976) and Capital of Enugu State from (1991) till data.

Location of Enugu puts it firmly within the tropics; as such solar radiation is high all year round. Rainfall over Enugu is associated with the presence of moist MT and the location of ITD. Rainfall in Enugu is of 2 major types conventional and rainfall associated with disturbances. The mean annual rainfall is heavy, varying from 1500-1750 mm. The study area lies within the rainforest-savanna Ecotone.

MATERIALS AND METHODS

The present study explores, the conceptual and methodological issues of normalized surface temperature as shown in Fig. 1.

Figure 1 shows the date processing flow in the study. The first step in the analysis was to make the Land-cover Classification using the Erdas Imagine. In the study, supervised classification was employed to categorize the Imagery into Built- up1 and 2, Forest 1 and 2 and Farmlands.

The classification result was used to provide the emissivity of the land-cover categories. Emissivity was then used to estimate the Land surface temperature from brightness temperature value in the thermal band image using the following steps:

Step 1: Conversion of Digital Number (DN) to Spectral Radiance (L_x)

(1)

Fig. 1:	Processing flow

where:

L_{mins and} L_min are the Spectral radiance for band 6 at DN 1 and 255, respectively.

Step 2: Conversion of the Spectral Radiance to Temperature using the following formula.

(2)

where:

Step 3: Computation of the emissivity corrected surface temperature using the formula.

(3)

where:

RESULTS AND DISCUSSION

Land SAT/Enhanced Thematic mapper of October, 2007 was used for this research. The data acquisition date has a highly clear atmospheric condition and the Image was acquired through the USGS Earth Resource Observation systems Data Center, which has corrected the radiometric and geometrical distortions of the images to a quality level of I G before delivery.

Fig. 2:	Transect points

The Land SAT image was further rectified to a common Universal Transverse Mercator coordinate system based on 1:24,000 scale topographic maps and was reassembled using the nearest neighbor algorithm with a pixel size of 30 by 30 m for all bands.

To examine the spatial patterns of temperature within Enugu Urban, 30 transects (profile) were drawn to across the image through the center of the city as shown in Fig. 2.

Since, these transects pass over various landscapes with different environmental settings, an inquiry into the UHI characteristics of the profile will help to understand the factors shaping the city’s thermal landscape. These factors include the spatial pattern of different land-cover classes, the occurrence of water bodies, parks, buildings and population densities and the division of the city’s functional districts (Weng, 2003).

Figure 3 shows the distribution of surface temperature values. The radiant temperature ranged from 26-39°C. An Isotherm Map (Fig. 4) and a choropleth map (Fig. 5) were produced based on the classification scheme of standard deviation, in which class 1 data values are less than one standard deviation below the mean, class 2 values fall between the mean and so on (Smith, 1986).

Also, a Digital Elevation Model (DEM) was constructed to further buttress the behaviour of temperature in Enugu Urban (Fig. 6).

Each Component surface in Urban Landscapes exhibits a unique radiative, thermal moisture and aerodynamic properties and relates to their surrounding site environment (Oke, 1982).

This result correlates very well with the result of processed imagery shown in Fig. 7 and 8, respectively.

It is evident from the maps that there is a thermal gradient as progressed from the Central Business District (CBD) out into the country side. Some hot spots or UHI can be easily identified. The most extensive UHI was distributed in the central part of the CBD, comprising Ogbette market, Okpara Avenue, Zik’s Avenue and presidential road.

There were also, many smaller UHIs along the highways that crossed the town as well as on the residential Layouts of New Layout, New Heaven, Abakpa, Emene, Garriki, Uwani and Achara Layouts. However, there did not exist an extensive UHI in the North-West and South-East, except for a few small ones. Apparently, forest and agricultural uses in these areas contain the development of UHI.

The result of the processed imagery shows clearly that commercial and industrial areas exhibited the highest temperature, followed by residential areas. The lowest temperature was observed in forest zones followed by farmlands.

Fig. 3:	Surface temperature values

Fig. 4:	Isotherm map

This implies that urban development raised the temperature by replacing natural environment (forest, water and pasture) with non-evaporating, non-transpiring surfaces.

Adaptation measures: The study, based on the findings, proposed some design strategies for the mitigation of Enugu Urban heat Island. Most of these strategies are applicable to the downtown location. Employing these stategies will result in substantial green cover increase in the downtown locations while street level thermal comfort is enhanced by avoiding compact designs.

Fig. 5:	Choropleth map

Fig. 6:	Digital elevation map

The building design should be such that taller buildings are on the north-east side of city blocks while green areas are at center and to the north-side.

Two other design strategies aimed at enhancing the thermal properties of the high-density residential areas and multi-family residential neighborhoods includes: The design strategy for the former proposes a 50% increase in tree cover. Suggestions for the multi-family residential areas include a courtyard built-form with grass-covered inner lawns to allow for building shade.

Fig. 7:	Classified map of enugu

Fig. 8:	Classified temperature range of enugu

Finally, using light-colored materials for roofing of downtown locations as well as improving the reflectivity of pavement within the urban centers will reduce UHI.

CONCLUSION

The Land SAT/ETM imagery instrument employed in this research was able to determine the spatial extent and behavioral patterns of UHI in Enugu Urban Center. UHI correlated positively with population density and concentration of human activities. The instrument was able to determine the temperature ranges of different land-use classes in Enugu Urban.