The cooling efficiency of urban landscape strategies in a hot dry climate

This paper describes a climatic analysis of landscape strategies for outdoor cooling in a hot-arid region, considering the efficiency of water use. Six landscape strategies were studied, using different combinations of trees, lawn, and an overhead shade mesh. The effects of these treatments were tested during the summer season in two semi-enclosed courtyards located at an urban settlement in the arid Negev Highlands of southern Israel. Compared to a non-vegetated exposed courtyard, which on average reached a maximum air temperature of 34 °C in mid-afternoon, a similar courtyard treated with shade trees and grass yielded a daytime temperature depression of up to 2.5 K, while shading the courtyard with a fabric shading mesh, counter-intuitively, caused a relative increase of nearly 1 K. Unshaded grass was found to cause only a small air temperature depression and had the highest water requirement. However when the grass was shaded, either by the trees or by the shade mesh, a synergic effect produced greater cooling as well as a reduction of more than 50% in total water use. The “cooling efficiency” of these strategies was calculated as the ratio between the sensible heat removed from the space and the latent heat of evaporation, with the latter representing the amount of water required for landscape irrigation. This measure is proposed as a criterion for evaluating landscape strategies in arid regions, where water resources are scarce.