Evaporation from (Blue-)Green Roofs: Assessing the Benefits of a Storage and Capillary Irrigation System Based on Measurements and Modeling

Dirk Cirkel,Thijs Van Veen,Bernard Voortman,Ruud Bartholomeus

doi:10.3390/w10091253

Abstract

Worldwide cities are facing increasing temperatures due to climate change and increasing urban density. Green roofs are promoted as a climate adaptation measure to lower air temperatures and improve comfort in urban areas, especially during intensive dry and warm spells. However, there is much debate on the effectiveness of this measure, because of a lack of fundamental knowledge about evaporation from different green roof systems. In this study, we investigate the water and energy balance of different roof types on a rooftop in Amsterdam, the Netherlands. Based on lysimeter measurements and modeling, we compared the water and energy balance of a conventional green roof with blue-green roofs equipped with a novel storage and capillary irrigation system. The roofs were covered either with Sedum or by grasses and herbs. Our measurements and modeling showed that conventional green roof systems (i.e., a Sedum cover and a few centimeters of substrate) have a low evaporation rate and due to a rapid decline in available moisture, a minor cooling effect. Roofs equipped with a storage and capillary irrigation system showed a remarkably large evaporation rate for Sedum species behaving as C3 plants during hot, dry periods. Covered with grasses and herbs, the evaporation rate was even larger. Precipitation storage and capillary irrigation strongly reduced the number of days with dry-out events. Implementing these systems therefore could lead to better cooling efficiencies in cities.

Highlights

Climate change and ongoing urbanization will result in both an increase of the urban heat island (UHI) effect [1,2] and flooding [3]
We investigate the effect of water availability on Ea and the distribution of energy between the latent heat flux (LE) and the sensible heat flux (H) in a rooftop environment in Amsterdam, the Netherlands
The water availability and water demand is quantified for green roofs with

Summary

Introduction

Climate change and ongoing urbanization will result in both an increase of the urban heat island (UHI) effect [1,2] and flooding [3]. The UHI effect is known as the phenomenon where the urban temperature is higher than the surrounding rural environment, due to the modification of land surfaces (i.e., application of energy absorbing surfaces such as asphalt and concrete on roofs and pavements, and changes in reflection and absorption due to the geometry of buildup areas). These surfaces generate an excess of heat due to a lack of evaporation. Aerosol emissions play a role in exacerbating or mitigating the UHI effects and related health impacts, e.g., recent substantial reductions in aerosol concentrations in the southeast of the US resulted in increased surface temperatures [8]

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