Modeling the effect of urban trees on temperature in Khartoum State

Transcription

1 JONARES, Vol. 1, pp 1-5, June Modeling the effect of urban trees on temperature in Khartoum State Tawhida A. Yousif 1 and Hisham M. M. Tahir 2* 1 Freelance Researcher, E -mail: tawhida66@yahoo.com 2 CNRES, University of Bahri, Sudan, E- mail: hisham-7777@hotmail.com Abstract This study was carried out at six sites in Khartoum State namely: Al Kadaru and Al Safia in Bahri town; Wad Nubawi and Al Salha in Omdurman town; and Sunt Reserved Forest and Al Riyadh in Khartoum town. The aim of the study was to determine the effect of urban trees on temperature of Khartoum State. Temperature was recorded throughout a calendar year. Spontaneous temperature readings were made simultaneously in the bare land and under the trees at approximately meters height which represents the environment in which humans live. Seven regression models were built to predict the relationship between temperature under the trees and that on the bare land; six regression models for the six sites and the seventh model, obtained from the pooled data, for Khartoum State. Each model depicts the relationship between the measurements under the trees as the dependent variable and that in the bare land as the independent one. All the models were linear according to the respective scatter plots and the respective most significant coefficients of determination (R 2 ). This study concluded that the quantification of the effect of urban trees on temperature could be obtained with an acceptable level of accuracy using empirical regression models. The reduction in air temperature under trees ranged from 3% to 6% at all sites. I. INTRODUCTION High earth's temperature worldwide is a new phenomenon particularly in big cities and towns. It is caused by the urban heat island, high levels of air pollutants and greenhouse effect. High temperature causes severe disasters such as melting of polar ice caps and rising of sea level. This leads to heat waves, river and coastal floods, landslides, fresh water pollution, drought, storms, hurricanes, spread of diseases and reduction of air quality by the production of poisonous photochemical smog in urban areas. Such disasters lead to climatic changes such as ozone depletion, biodiversity loss, land degradation, dissemination of organic pollutants due to disruption of nitrogen and sulfur cycles, and food poisoning. All these changes will affect the sustainability of the ecological system, food production, human health and human economic activities (McMichael, 2001). High temperature noticed in big cities is created by a combination of factors including paved surfaces, lack of shade, heat retention of buildings and other structures; and high levels of air pollutants (Mayer, 1990). Urban trees reduce ambient temperature and the urban heat island effect in residential areas by shade and evapotranspiration. Thus, they reduce the amount of energy needed for cooling and heating of buildings and reduce the amount of greenhouse gases that is emitted from power plants (Oke, 1989). Trees by reducing air temperature slow down photochemical reactions and increase ozone formation which is temperature dependent (Akbari, 2002; Rosenfeld et al., 1998). Effect of urban trees on temperature vary according to current and recent weather conditions, number of buildings, amount of ground vegetation cover, tree species, tree size, planting arrangement, tree spacing, crown *Corresponding author spread and the vertical distribution of leaf area with height (Oke, 1989; Barlag and Kuttler, 1991). When trees are planted on north and east of a building in the southern hemisphere, or south and west in the northern hemisphere; they shade the buildings and cool the air during summer and warm it in winter. Thereby, they reduce air conditioning cost of energy used for heating and cooling (Nowak, 1995). Cooling of air due to the effect of trees has been well documented in the past through various studies. In Sacramento city, California, Nowak et al. (2002) found that the maximum mid-day air temperature reductions due to trees were in the range of 0.04 C C per one percent tree cover increase. Souch and Souch (1993) in Bloomington, Indiana; Taha et al. (1988) demonstrated that mid-day air temperatures below the trees were 0.7 C C cooler than in the bare land. In Miami, Florida, Parker (1989) found that the average air temperature reduction during summer was 3.6 C under large trees than in bare land. In Sacramento and Phoenix, United States, Akbari and Taha (1992) showed that a 25% increase in the number of trees can reduce the temperature during summer by C. In Tokyo, Japan, Shobhakar and Hanaki (2002) found that the maximum variation in average air temperature caused by green areas was 0.47 C. Ishihara and Katayama (1991) found that maximum temperatures within the green space of individual building sites may be 3 C cooler than outside the green space. In Davis, USA, Scott et al. (1998) found that trees improve air quality by reducing air temperature by C. Because of lack of studies on the effect of urban trees on temperature in the Sudan in general and in Khartoum State in particular, the objective of this research work was to build regression models to explain the effect of urban trees on temperature in Khartoum State. II. STUDY SITES AND METHODOLOGY Air temperature data ( C) were collected from six sites in Khartoum State: namely, Al Kadaru and Al Safia in Bahri town; Wad Nubawi and Al Salha in Omdurman town; and Sunt Reserved Forest and Al Riyadh in Khartoum town. The tree species with the highest relative abundance in each of the six sites were as follows: Conocarpus lancifolius in Al Kadaru, Ficus binjamina in Al Riyad, Azadirachta indica in Wad Nubawi, Peltophorum pterocarpum in Al Safia, Albizia lebbeck in Al Salha and Acacia nilotica in the Sunt Reserved Forest. A digital Oxygen and Temperature-Meter Model was used in data logging. Spontaneous temperature readings were taken on three days every month during a full calendar year from December 2010 to November Temperature readings were taken simultaneously on the bare land and below the trees at an approximate height of 1.5 to 2.0 metres on the assumption that this is the height at which human beings live (Heisler and DeWalle, 1988). The readings were made at intervals of two hours during the period from 8.00 am to 6.00 pm in each of the three days. Six readings were taken daily. The simultaneous readings taken during the period of the study were 162 for the Sunt Forest and 216 for each of the other five sites. Most days of measurements were sunny without clouds except during the autumn (August and September). Measurements were not conducted in Sunt Forest during the months of August, September

2 JONARES, Vol. 1, pp 1-5, June and October because the forest was completely flooded. The collected data were analyzed using the computer packages SPSS 15.0 for Windows and Microsoft Excel version 7. Scatter plots and trend graphs were fitted. Seven regression models were built to predict the effects of urban trees on air temperature in Khartoum State. III. RESULTS In all the temperature line graphs (Figures 1.1, 2.1, 3.1, 4.1, 5.1 and 6.1); the two lines followed the same trend throughout the year at all sites. It is apparent that the temperature in the bare land (Tsun) is higher than temperature under the trees (Tsh). All the results showed linear relationships between temperature under the trees (Tsh) and temperature in bare land (Tsun) with significant coefficients of determination (R 2 ). This is seen in the scatter diagrams (Figures 1.2, 2.2, 3.2, 4.2, 5.2, 6.2, and 7). IV. DISCUSSION The cooling effect of air temperature by trees is associated with shade, transpiration and dispersion of moisture into the atmosphere. The results of this study showed that the reduction of temperature in all sites which could be attributed to trees, ranged from 3% to 6%. These findings are consistent with other results obtained by Souch and Souch (1993), Parker (1989), Akbari et al., (1992), Shobhakar and Hanaki (2002), Ishihara and Katayama (1991), Akbari (2005), Grey and Deneke (1986) and Simpson (1998). All of them concluded that trees ameliorate the climate by reducing air temperature. As a matter of comparison, temperature measurements showed a little variation in the average reduction among different sites in the state during the period of the study. This small variation might be due to differences in the ground vegetation cover, tree species, tree size, planting arrangement, tree spacing, crown spread, vertical distribution of leaf area with height, and stand density at different sites Fig. 1.1: Trend for air temperature ( C) under the tree (Tsh) and in bare land (Tsun) at Al Kadaru Fig. 1.2: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Al Kadaru Fig. 2.1: Trend for air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Al Safia Fig. 2.2: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Al Safia

3 JONARES, Vol. 1, pp 1-5, June Fig. 3.1: Trend for air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Wad Nubawi Fig. 3.2: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Wad Nubawi Fig. 4.1: Trend for air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Al Salha Fig. 4.2: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at ( C) at Al Salha Fig. 5.1: Trend for air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Sunt Forest Fig. 5.2: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Sunt Forest

4 JONARES, Vol. 1, pp 1-5, June Fig. 6.1: Trend for air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Al Riyad Fig. 6.2: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) at Al Riyad Akbari, H., 2002.Shade Trees Reduce Building Energy Use and carbon dioxide emissions from Power Plants, Environmental Pollution,116: Fig. 7: Relationship between air temperature ( C) under the trees (Tsh) and in bare land (Tsun) in Khartoum state V. CONCLUSION The main finding of this study is that quantification of the effect of urban trees on temperature could be obtained with an acceptable level of accuracy using empirical regression models. VI. RECOMMENDATIONS Further research work on the effect of urban trees on temperature is needed, giving consideration to other factors in the study area. These factors include: - Tree species, spacing, density, crown spread and leaf area index - Microclimate, buildings and other constructions VII. REFERENCES Akbari H., Davis S., Dorsano S., Huang J. and Winnett, S., 1992.Cooling our Communities: A guidebook on tree planting and light colored-surfacing. U.S.E.P.A., Office of Policy Analysis, Climate Change Division, 220 P. Akbari, H Energy Saving Potentials and Air Quality Benefits of Urban Heat Island Mitigation. Lawrence Berkeley National Laboratory. 510: Akbari, H., and Taha, H., The Impact of Trees and White Surfaces on Residential Heating and Cooling Energy Use in Four Canadian Cities. Energy, 17 (2): Barlag, A. and Kuttler, W., The Significance of Country Breezes for Urban Planning. Energy and Buildings, 16: Grey, G.W. and Deneke, F.J., Urban Forestry, Second edition, John Wiley and Sons. NY, USA, 299 P. Heisler, G.M. and DeWalle, D. R Effects of windbreak structure on wind flow. Agriculture, Ecosystems and Environment, 22-23: Ishihara, O. and Katayama, T., Study of the effect of green areas on the thermal environment in an urban area. Energy and Buildings, 15-16: Mayer, H., Human bio meteorological evaluation of urban environment. In: Association of German engineers, VDI Commission of clean air (ed) environmental meteorology. VDIseries 15, pp , English. McMichael, A. J., Human Frontiers, Environment and disease: Past patterns, Uncertain Futures. Cambridge University Press, Cambridge, UK. 413 P. Nowak, D. J., Benefits of Community Trees. Brooklyn Trees, USDA Forest Service General Technical Report. Nowak, D. J., Crane, D.E., Stevens, J. C., and Ibarra, M., Brooklyn s Urban Forest. USDA Forest Service Gen Tech. Rep. NE P. Oke, T. R., The Micrometeorology of the Urban Forest. Phil. Trans. R. Soc. Lond., 324: Parker, J. R., The Impact of Vegetation on Air Conditioning Consumption. Controlling Summer Heat Island. In: Akbari, H., Garbesi, K., and Martien, P. (eds.): Proceedings of the Workshop on: Saving Energy and Reducing Atmospheric Pollution by Controlling Summer Heat Island, University of California, Barkley, California, pp Rosenfeld, A. H., Akbari, H., Romm, J. J., and Pomerantz, M., Cool Communities: Strategies for Heat Island Mitigation and Smog Reduction, Energy and Buildings, pp

5 JONARES, Vol. 1, pp 1-5, June Scott, K.I.; McPherson, E.G.; Simpson, J.R., Air Pollutant Uptake by Sacramento s Urban Forest. Journal of Arboriculture 24 (4): Shobhakar, D., Hanaki, K., Improvement of Urban Thermal Environment by Managing heat Discharge Sources and Surface Modification in Tokyo energy and Building 34: Simpson, J. R., Urban forest Impacts on Regional Cooling and Heating Energy Use: Sacramento County Case Study. J. Arboric. 24(4): Souch, C. A., Souch, C., The Effect of Trees on Summer time below Urban Climate: a Case Study Bloomington, Indiana. Journal of Arboriculture 19(5): Taha, H. G., Akbari, H. and Rosenfeld, A. H., Vegetation Micro- climate: A field Project in Davis, California. Lawrence Berkley in Davis, Laboratory Report , Berkley, CA.