26 Jun 2018

To what extent can rooftop greening systems improve cities’ microclimate?

A few weeks ago, we explained the importance of roof greening to improve the microclimate in urban areas. There are no doubts that these urban greening elements are able to reduce the air temperature by plant/water-atmosphere-interaction through evaporation. It is, however, hard to evaluate the efficiency of these systems regarding their effects on the microclimate, e.g. decreasing the air temperature or binding pollutants. Furthermore, to provide these beneficial effects, it has to be ensured that the plants can survive under the local meteorological conditions.

In general, there are two types of roof greening systems: extensive and intensive roof greening. The extensive roof greening is characterized by a small soil layer of succulent plants with a shallow root system. These plants are able to store great amounts of water in the plant’s body and can compensate water deficits during droughts. However, on long lasting dry spells the soil’s physical characteristics become nearly similar to sealed surfaces in cities, which displays the great disadvantage of the extensive roof greening. The intensive roof greening system on the other hand features a thicker substrate layer and can therefore only be implemented on roofs with a solid building construction. The plants used for intensive roof greening are typically non-succulent plants like bushes and small trees with deep root systems as found in parks and thus need additional irrigation.

In order to evaluate the efficiency of these roof greening systems, complex measurements, e.g. eddy-covariance for latent heat fluxes, are needed. These real measurements have to be performed over weeks or longer and depend on prevailing weather conditions.

A much easier way to evaluate the beneficial effects is by modelling these systems. In order to find the best combinations of roof greening systems for the specific location and weather period (i.e. wet or dry period), models can be run using different scenarios to find optimized solutions. Such models can not only calculate the effects on the surface and air temperature, but also the soil water content and the dynamic water supply and thus allow a quite accurate estimation of the effects on the microclimate in urban areas.