Abstract. For an assessment of the roles of soil and vegetation in the
climate system, a further understanding of the flux components of
H2O and CO2 (e.g., transpiration, soil respiration) and
their interaction with physical conditions and physiological functioning of
plants and ecosystems is necessary. To obtain magnitudes of these flux
components, we applied source partitioning approaches after Scanlon and
Kustas (2010; SK10) and after Thomas et al. (2008; TH08) to high-frequency
eddy covariance measurements of 12 study sites covering different
ecosystems (croplands, grasslands, and forests) in different climatic
regions. Both partitioning methods are based on higher-order statistics of
the H2O and CO2 fluctuations, but proceed differently to
estimate transpiration, evaporation, net primary production, and soil
respiration. We compared and evaluated the partitioning results obtained with
SK10 and TH08, including slight modifications of both approaches. Further, we
analyzed the interrelations among the performance of the partitioning
methods, turbulence characteristics, and site characteristics (such as plant
cover type, canopy height, canopy density, and measurement height). We were
able to identify characteristics of a data set that are prerequisites for
adequate performance of the partitioning methods. SK10 had the tendency to overestimate and TH08 to underestimate soil flux
components. For both methods, the partitioning of CO2 fluxes was
less robust than for H2O fluxes. Results derived with SK10 showed
relatively large dependencies on estimated water use efficiency (WUE) at the
leaf level, which is a required input. Measurements of outgoing longwave
radiation used for the estimation of foliage temperature (used in WUE) could
slightly increase the quality of the partitioning results. A modification of
the TH08 approach, by applying a cluster analysis for the conditional
sampling of respiration–evaporation events, performed satisfactorily, but did
not result in significant advantages compared to the original method versions
developed by Thomas et al. (2008). The performance of each partitioning
approach was dependent on meteorological conditions, plant development,
canopy height, canopy density, and measurement height. Foremost, the
performance of SK10 correlated negatively with the ratio between measurement
height and canopy height. The performance of TH08 was more dependent on
canopy height and leaf area index. In general, all site characteristics that
increase dissimilarities between scalars appeared to enhance partitioning
performance for SK10 and TH08.
Observed Response of the Raindrop Size Distribution to Changes in the Melting Layer