Measurement of evapotranspiration on small urban surfaces. Application to green roofs.

The greening of cities has become a major component of current urban development policies. This greening is taking new and variable forms, and is increasingly associated with rainwater management techniques. This increase in vegetated surfaces can potentially encourage evapotranspiration. Increasing this process has a twofold advantage. On the one hand, It provides stormwater runoff reduction benefits and, on the other, it promotes cooling in the urban environment. However, in order to better quantify and optimise evapotranspiration, it is necessary to be able to measure it for different kind of urban surfaces. In an urban environment, where surfaces are very heterogeneous, it is therefore necessary to have continuous measurements, over the long term (several seasons) and, if possible, on different types of surfaces with relatively small areas: just a few dozen m². In order to verify whether Eddy Covariance measurements can be adapted to these singular surfaces, measurements were carried out on a green roof (situated in Broué (28), France) covering an area of approximately 1900 m² (16.5 m x 115 m) during the summers of 2022 and 2023. Two measurement systems were used (an IRGASON from Campbell Scientific and a Gill WindMaster associated with a LI-COR LI-7500). In an attempt to limit the footprint to the roof surface, the instruments were placed at a height of 50cm above the roof surface. In addition to these measurements, the energy balance was monitored and complementary measurements were made using a transpiration chamber. The latent heat flux measurements obtained by EC indicate fluxes generally below 150 W.m2, which represents between 10 and 45% of the net radiation depending on the soil water content. Comparison with the transpiration chamber shows that the latter underestimates the fluxes compared with the EC measurement. However, the final objective is to determine whether measurements at such a low height are realistic and can limit the footprint at the roof surface, spectral analyses of these measurements and footprints are currently being studied.