Environmental Science and Engineering

Quantifying Evaporation and Transpirational Water Losses from Greenroofs and Greenroof Media Capacity for Neutralizing Acid Rain

Publication Date: August 2007
Cooperating Institution: The Pennsylvania State University
Sub-Grantee: Robert Berghage
Project Budget: $75,000
Project Identifier: 04-DEC-10SG


Green roofs are becoming increasingly common in North America, where they are being promoted as a stormwater management BMP. Although green roofs have been used in Europe, particularly Germany for over 30 years, the North American Industry is still relatively new and installation, management and performance standards are relatively poorly developed. Research on the performance of North American green roofs has really only been done for the last decade, and although we have learned much, there are still many unanswered questions. There is ample evidence that green roofs can reduce stormwater runoff in Eastern North America by 40 – 60%, but the relative contribution of the media and plants to this stormwater retention has not been characterized or quantified. Further for this retention and evapotranspiration to be of use to stormwater engineers, and developers, tools to predict the retention and detention of stormwater on a green roof are needed. This project describes studies of the evaporation, and evapostranspiration of water from green roof modules planted with three common green roof plant species. Green roof plants like sedum and delosperma used water quickly when it was available and reduced their water use rate when they were drought stressed. This makes sedums and delosperma ideal plants for green roof use. Plants contribute as much as 40% of the roof capacity to retain stormwater depending on the frequency and intensity of the storm events. This data and other runoff data from larger field study roofs provided the basis for models that describe and predict the function of a green roof described in this report. In addition to influencing the quantity of runoff, greenroofs can also influence the quality of runoff. One of the most consistently reported benefits of a green roof for runoff quality is the neutralization of acid precipitation. It is clear however, that this is a finite property of the medium, controlled by the potential buffering capacity of the medium. To maintain this capacity and hence the water quality benefit greenroof maintenance should include periodic liming to replace the neutralized media buffer. This project describes the buffer potential of two commercial green roof media, and details a testing procedure. The testing procedure allows a green roof manager to estimate when lime will be needed, and what the potential buffering capacity of a green roof media will be. With the two media evaluated there were slight differences in total buffer potential, however the differences were not great and the response to acid addition was similar for both media, with both having sufficient buffering capacity to neutralize acid precipitation in Central Pennsylvania for at least 10 years before liming would be required.

Associated Documents:

Final Report