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

Dominik Gößner,Justine Jasmin Krespach,Milena Mohri

doi:10.3390/land10121334

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.

Highlights

Hot weather extremes such as heatwaves or droughts as well as the intensity and frequency of heavy rain events have increased during in recent decades and, due to climate change, they are expected to intensify [1]
Even though the Retention Roof started at 50% vegetation cover, together with the Economy Roof, it rapidly increased its cover and reached the 90% mark in the same week as the Nature Roof
The slight decreases in the cover of the Garden Roof in June, as well as of the Retention Roof, the Economy Roof and the Nature Roof in September, can be attributed to roof maintenance and weeding of unwanted species

Summary

Introduction

Hot weather extremes such as heatwaves or droughts as well as the intensity and frequency of heavy rain events have increased during in recent decades and, due to climate change, they are expected to intensify [1]. Urban areas are especially sensitive to these weather extremes because they already exhibit higher temperatures [2,3] and increased precipitation compared to the rural surroundings [4]. Urbanization increases the precipitation over the city by more than 15% [4]. Studies have shown that people living in cities are more likely to be affected by floods [11] and to die a premature death due to heat [12] than the rural population. As the majority of the world population at present lives in urban areas [13], measures for heat mitigation and flood prevention must be implemented in cities

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