The effects of green spaces (Palme trees) on the microclimate in arides zones, case study: Ghardaia , Algeria

摘要

The microclimate and air quality in urban spaces are of a great interest for the well-being of citizens. Mitigating urban heat island is a major concern for human health by improving comfort especially in desert regions. While the thermal comfort is relatively easy to obtain in indoor spaces, it is more difficult to provide in outdoor ones. It is often sought by passive means. Vegetation could play an important role in ameliorating urban microclimates. It should be used judiciously in urban planning and designing outdoor spaces. The objective of this work is to explore the influence of green cover during summer on microclimate in the region of Ben-Isguen, a city of a hot-arid climate in the M'zab valley, Ghardaia (Algeria). Many studies have proved that vegetation can produce a cooling effect of 2 to 3 °C on average. Gardens and urban parks in particular, in and around cities contribute to alleviating heat stress by creating the so-called "oasis effect" through public spaces. They improve thermal comfort and reduce cooling energy consumption. The microclimate of different locations, in both green and built-up areas (old town, palm grove, etc.) was characterized with the help of field measurements carried out during summer days. The results confirmed the presence of an important cooling effect of vegetation (PCI). It was shown that it is essentially a nocturnal phenomenon by displaying a maximum value of 10 °C. This result is very important regarding the results of similar studies. The research demonstrated that the oasis effect is best developed around dawn in calm and clear sky conditions. Maximum daytime cooling effect of the green cover was 4.5 °C in reference to old town. Nevertheless this effect showed fluctuation so that it displayed negative values by moments. The cooling effect of the palm grove is due to shade foliage, evapotranspiration, low heat capacity of vegetation and high aerodynamic roughness. In fact the oasis acts as a "heat sink". The old town of Ben-Isguen presented a diurnal heat island and a nocturnal cool island with maximum intensity of 3.5 °C and 4 °C respectively. The investigation of the oasis microclimate revealed that densely vegetated areas display higher cooling effect. The latter varied among different areas due to shade conditions, density of vegetation, sky view factor (SVF), soil characteristics and water surface availability.