Greenroofs as a part of urban water management

Abstract

Urban water management has long been determined by a quick removal of rainfall to prevent floodings. Nowadays, policy makers are more interested in integrated water management. One of the current management options consists of keeping rainfall as long as possible at its location for example by means of greenroofs. On greenroofs, a vegetation (mostly Sedum species) is cultivated on a substrate on top of a roof. New techniques have made it possible to build greenroofs on slopes of up to 45'. During the last two decades non-sloping greenroofs have been intensively studied in Western Europe. In these studies peak andlor yearly runoff were determined at specific locations. Since the evapotranspiration rate, rainfall distribution and intensity vary considerable from one area to another, the results are however not transferable to other regions. Therefore a full water balance model is needed. The paper presents (i) a first draft of a water balance model for a greenroof, (ii) the experimental set-up to measure water retention and to validate the simulations and (iii) initial results. The water balance model considers not only climatic characteristics, but also the orientation and slope of the roof as well as characteristics of various types of greenroofs. The experimental set-up consists of 78 lysimeters. Three different slopes (1.5%, 20 and 40) and four orientations (north, east, south and west) are considered. Drainage water from the lysimeters is measured by rain intensity meters. The climatic conditions are recorded at the site. Initial results indicate that the evaporation rate is strongly determined by the orientation and slope of the greenroof. This demonstrates the need to consider those variables as essential variables in the water balance model. Transactions on Ecology and the Environment vol 61, © 2003 WIT Press, www.witpress.com, ISSN 1743-3541