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

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.

Nature Based Solutions for Urban Resilience: A Distinction Between No-Tech, Low-Tech and High-Tech Solutions

“…It seems that local actors assume that by implementing green roofs all potential ecosystem services and benefits are "delivered, " independent of the type of vegetated roof system used. Research in Dutch cities has already demonstrated that the level of ecosystem performance, provided by different vegetated roof systems, varies greatly depending on used materials, soils, water management systems and vegetation types (Solcerova et al, 2017;Cirkel et al, 2018). "To vegetate or not to vegetate" is no longer just the question; the actual performance of NBS should be discussed, characterized and rewarded (Dorst et al, 2019).…”

“…These man-made vegetated systems can regulate (rain)water effectively, because the growing site is designed for such performance. By improving and upgrading soil conditions, and adding additional engineering solutions like retention, capillary irrigation and water level sensing and controls, vegetated roofs boost improved water retention and detention during peak rain events and will continue to cool the urban environment for a longer period of time with retained rainwater during prolonged dry spells (Cirkel et al, 2018).…”

Dynamic Distributed Storage of Stormwater in Sponge-Like Porous Bodies: Modelling Water Uptake

“…Hence, the average outflow rate would be about 50 times smaller than the inflow rate of design events. Theoretically, water may be released, e.g., by changes in capillary action, by mechanisms applying pressure to stored water, and by evaporation of water into the atmosphere [47][48][49][50][51]. All three measures have different time-scales, and their design would need to meet the time limit on the full drawdown of the storage facility, typically specified as 48 h (such limits depend on the type of storage facility and the local rainfall regime).…”

Wetland Roofs as an Attractive Option for Decentralized Water Management and Air Conditioning Enhancement in Growing Cities—A Review

“…Supply to the terrestrial vegetation is based on the capillary effect. Blue-green roofs of this type can achieve a significantly higher cooling effect as a result of the better availability of water relative to simple extensive green roofs [32], and they can also contribute significantly to rainwater retention [33]. In the case of wetland roofs, rainwater can be stored partly within a special plant carrier and water storage mat (see Figure 3), since these plants can actively supply oxygen to their root zones [34] via their aerenchyma tissue.…”