Attributing urban evapotranspiration from eddy-covariance to surface
cover: bottom-up versus top-down
Abstract
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.
We confirm the intuitive relation between ET and the surface cover
fractions based on a wide range of surface compositions.