Evapotranspiration Measurements and Assessment of Driving Factors: A Comparison of Different Green Roof Systems during Summer in Germany

Abstract

Green roofs have proven to be a space-saving solution to mitigate peak temperatures and control floods in urban areas through evaporative cooling and storm water retention. To encourage a sustainable city design with large-scale green infrastructure networks, a better differentiation between the diverse existing green roof systems is needed. The aim of this study is to demonstrate differences among green roof systems based on comprehensive microclimatic measurements on four small experimental roofs and to assess differences in evapotranspiration with a partial least square regression. The results show that short-wave solar radiation, relative humidity and water availability are the most important drivers of evapotranspiration. The roof system with permanent water storage maintained significantly higher substrate moisture compared to the other roofs and produced peak evapotranspiration rates of 4.88 mm d−1. The highest total evapo-transpiration of 526 mm from April to September was recorded for the roof system with the thickest substrate layer and grass vegetation. In summer, the shallowest roof showed the highest substrate temperature and air temperature at vegetation level. These findings highlight the importance of specifying the characteristics of the various green roofs in order to turn them into useful planning tools for the design of climate-change-resilient cities.