The moderating effect of water-efficient ground cover vegetation on pedestrian thermal stress

The level of thermal stress experienced by pedestrians in an urban environment is affected by their exchange of energy with their surroundings, and the properties of ground-cover vegetation can have an important influence on this energy exchange - particularly in terms of radiation. Although vegetated surfaces have a low albedo and thus absorb a large part of the solar radiation incident upon them, they can maintain lower temperatures than typical paved surfaces because they are cooled by evapotranspiration. The purpose of this research is to examine the cooling effect of "succulent" surface cover plants that are adapted to arid climates, in comparison to plants such as grass that require substantially more irrigation. The research site, in the Negev desert of southern Israel, consists of several small test plots with different species of succulents and other plants, along with dry ground surfaces. Measurements of surface temperature and albedo provide input for comprehensive pedestrian thermal comfort modeling using the Index of Thermal Stress, assuming an open space scenario with various surface cover treatments. Water requirements of the different plant are examined through an irrigation experiment. Results indicate that although there are thermal differences between the different plant types, these differences are small when compared with non-vegetated surfaces. While the non-succulents are slightly cooler and less stress-inducing than the desert-adapted plants, their water requirements are higher, making them less efficient in terms of the ratio between cooling "benefits" and water "costs"-both of which are expressed in terms of their equivalent energy.