Vegetation as an urban climate control in the subtropical city of Gaborone, Botswana

Abstract

The influence of vegetation on the urban climate was studied in the subtropical city Gaborone, the rapidly expanding capital of Botswana with approximately 200 000 inhabitants. Temperature records from an urban and a rural station were analysed for the period 1985-96. In an attempt to explain possible seasonal change in vegetation, NOAA satellite normalized difference vegetation index imagery was analysed. The present urban influence was investigated with temperature loggers at selected urban and rural sites. In addition, mobile measurements revealed spatial patterns in temperature and humidity for different land uses.Seasonal patterns of urban-rural differences in minimum temperatures emerge during the period 1994-96, especially during the winter when the heat island effect is largest. It is shown that differences in urban and rural vegetation over the year partly explain this variation. Mobile measurements reveal a weak nocturnal heat island of 2-3°C during clear and calm nights. There are intra-urban temperature differences that are in the same range as the urban-rural relationship due to the role of vegetation. Evapotranspiration lowers the temperature, which was detected by high humidity in areas of lush vegetation. This becomes apparent at midday, when densely vegetated areas were up to 2°C cooler than rural sites. An oasis effect, therefore, only exists on a highly local basis. In contrast, parts of the city with sparse vegetation are warmer than the countryside.There is an apparent opposed effect of rural and urban vegetation, whereby the former is hindering the temperature from falling and the latter is cooling the environment through evapotranspiration. This can be explained by the overwhelming amounts of imported water in the city promoting evaporative cooling.

Evening evapotranspirative cooling in relation to vegetation and urban geometry in the city of Ouagadougou, Burkina Faso

“…an increase in specific humidity around sunset and intensive cooling has been reported from vegetated urban areas in Gaborone, Botswana by Jonsson (2004). He suggested that the increase in specific humidity was a result of evapotranspiration enhanced by irrigation.…”

“…Many (sub)tropical cities show a clear seasonal difference in NUHI intensities, with 0.5-3 K lower intensity during the wet season compared to the dry season (Okoola, 1990;Jauregui et al, 1992;Jauregui, 1997;Robaa, 2003;Jonsson, 2004;Chow and Roth, 2006;Roth, 2007;Balogun et al, 2009). The lower NUHI intensities during the wet season are explained by the higher amount of cloud cover, but also by thermal admittance due to increased soil-water (Jauregui, 1997;Jonsson, 2004;Jonsson and Lindqvist, 2005;Roth, 2007).…”

“…The lower NUHI intensities during the wet season are explained by the higher amount of cloud cover, but also by thermal admittance due to increased soil-water (Jauregui, 1997;Jonsson, 2004;Jonsson and Lindqvist, 2005;Roth, 2007). The importance of soil wetness to cooling was emphasized by Jonsson and Lindqvist (2005) who compared cloud-free nights during the dry and the wet season, respectively.…”

“…In (sub)tropical as well as in temperate cities, the vegetation fraction of urban areas (Grimmond and Oke, 2002) has been shown to effectively reduce heat storage uptake during daytime through evapotranspirative cooling and shading from trees, hence reducing 3090 B. HOLMER et al air temperature and the heat island and creating large daytime intra-urban temperature differences (Jauregui, 1997;Spronken-Smith and Oke, 1998;Jonsson, 2004). The cooling effect of vegetation also exists during the night.…”

Review of urban climate research in (sub)tropical regions

“…The available work is primarily concerned with the influence of the built form (geometry, height-to-width of street canyons) to promote ventilation, building materials and colour as means to reduce the absorption of solar radiation and storage of heat as well as the role of vegetation for shading and evaporative cooling to moderate the thermal and moisture climate. Studies were conducted in the following climate types and cities: (1) equatorial wet Colombo (Emmanuel, 2003(Emmanuel, , 2005aEmmanuel and Johansson, 2006), Sao Paolo (Ribeiro, 2005) and Singapore (Wong et al, 2003;Wong and Yu, 2005;Chen and Wong, 2006), (2) tropical wet/dry Belo Horizonte (Sad de Assis and Frota, 1999), (3) tropical highland Mexico City and (4) subtropical dry San Juan, Argentina (Papparelli et al, 1996), Gaborone (Jonsson, 2004), Fez, Marocco (Johansson, 2006), various cities in Algeria (Ali-Toudert et al, 2005;Ali-Toudert and Mayer, 2006), Dimona, Israel (Pearlmutter et al, 2007a) and various settlements in the Negev desert of Israel which are reviewed by Pearlmutter et al (2007b) with special attention given to the relationship between urban geometry and thermal stress.…”