Experimental methods for estimating green roof evapotranspiration
Résumé
Green roofs (GR) are wellknown for their hydrological performances. Their abilities to retain stormwater are mainly influenced by the initial water content of the substrate prior to a rain event. The evolution in time of the water content is driven by the evapotranspiration (ET) process. The main objectives of this study is to compare three green roof configurations on the ET flux. GRs are instrumented in order to measure thermal, hydrological are climatic variables. All measurements are done over one year. Our work focused on three methods: i) ET is measured by an ET chamber, developed by the Cerema, ii) ET is obtained from the residual calculation of the hydrological balance, iii) a thermal balance is solved to calculate ET. Results show that hourly ET measurements give a good estimation. During summer, ET values are up to 250W/m² while it is less than 10 W/m² in autumn. The values of evapotranspiration are strongly influenced by the LAI, albedo and emissivity of the vegetation. The local climate also affects ET. This study highlights the ability of GRs to participate in urban comfort at the buildingscale and to mitigate urban heat island at urban scale.
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