Attributing urban evapotranspiration from eddy-covariance to surface cover: bottom-up versus top-down

Evapotranspiration (ET) is a key process in the hydrological cycle that can help mitigate urban heat. ET depends on the surface cover, as the surface affects the partitioning of precipitation between runoff and evapotranspiration. In urban neighborhoods, this surface cover is highly heterogeneous. The resulting neighborhood-scale ET is observed with eddy-covariance systems. However, these observations represent the signal from wind- and stability-dependent footprints resulting in a continuously changing surface cover composition. This continuous change prevents quantitative analysis of the separate types. Here, we disentangle this neighborhood-scale ET at two urban sites in Berlin attributing the ET dynamics to the four major surface cover types in the footprint: impervious surfaces, low vegetation, high vegetation, and open water. Starting from the surface, we reconstruct ET based on patch-scale observations and conceptual models. Alternatively, we start with the eddy-covariance observations and attribute ET to the surface cover types solving a system of equations for four eddy-covariance systems with different footprints. Although starting at the surface yields more robust results, both approaches indicate that vegetation is responsible for more ET than proportional to its surface fraction, and evaporation from impervious surfaces although less cannot be neglected. The behavior of each surface cover type is separated allowing to study the response to rainfall for each type separately. Impervious surfaces exhibit a strong ET peak directly after the rainfall event, while open water is insensitive to the rainfall. High vegetation starts with high ET after rainfall and limits ET within the first days, but low vegetation does not start limiting ET until the end of the warm season likely reaching the soil moisture limit. Additionally, we confirm the intuitive relation between ET and the surface cover fractions based on a wide range of surface compositions.