The potential for evaporative cooling from Vienna's green roofs

Increased vegetation and the removal of sealed surfaces have been major efforts to reduce urban heat. Nevertheless, a lot of green facades and roofs are not irrigated, which causes drought stress in the plants during future, longer, or more intense summer heat waves without precipitation, leading to lower actual evapotranspiration. As a result, the anticipated cooling effect may not be achieved by many non-irrigated green roofs.

In our study, four green roof sites in different local climate zones and elevations regarding average roof level (compact-midrise above rooftop, compact-midrise below rooftop, open mid-rise, and large low-rise) within Vienna, Austria were investigated, over a two-year period. The impact of microclimatic site conditions on typically applied extensive and intensive green roof set ups, respectively, on actual evapotranspiration were measured under rainfed conditions by mini-lysimeters combined with meteorological in-situ measurements. The extensive green-roof set-up represented a soil substrate with a depth of 10 cm covered with drought stress tolerant plant species, whereas the intensive set-up represented a soil substrate with a depth of 25 cm covered with plant species of higher biomass growth potential, but more sensitive to drought stress. The measured actual evapotranspiration rates on a daily base were further used to calibrate the FAO-approach (Allen et al., 1998) for calculating actual evapotranspiration for the applied green roof set-ups and applied for different scenarios (irrigation and climate scenarios).

The results revealed that the green roof site conditions (especially wind speed) as well as the type of substrate and vegetation influence the temporal dynamics of actual evapotranspiration significantly. Also, it was found that actual evapotranspiration rates (and related cooling potential) were strongly limited under rainfed conditions during hot and dry summer conditions. Support irrigation to enhance actual evapotranspiration and avoid plant damages during drought periods and heat waves may need considerable amounts of water over the city of Vienna, showing a need for effective irrigation systems and irrigation water management.

 

^{Reference: Allen R, Pereira L, Raes D, Smith M (1998) Crop Evapotranspiration – Guidelines for computing crop water requirements. Irrigation and Drainage Paper Nr. 56. Rome, Italy.}