Can eddy covariance flux magnitudes and uncertainties be explained by surface heterogeneity? - A combined multi-tower – hyperspectral remote sensing approach in a Mediterranean Savanna ecosystem

Abstract:

Several studies in recent literature focus on uncertainty estimates of turbulent fluxes as measured by the eddy covariance method. The knowledge of the uncertainties is important for the interpretation of results, but also for model-data-integration where uncertainties of input parameters play an important role for parameter estimations. The estimates of uncertainties rely either on time series analysis or on comparisons between collocated towers or comparisons in time under similar bio-meteorological conditions. We will present a unique data set from 3 collocated EC towers (distance of 500 - 600 m) at the long-term EC site ‘Majadas del Tietar’, Spain. The site is a Savanna type ecosystem with oak trees and grass in the understory. Cattle are grazing the area from autumn to early summer. The combination of EC data and hyperspectral remote sensing data from aircraft flight with the CASI system allows to associate flux differences to differences in the spectral properties of the surface within the footprints. The drawback is that maps of VI can only be used as long as the vegetation (structure or phenology) is not changing. But not only vegetation indices (VI), also differences of the full spectra will be used to determine differences of footprint areas. First results show correlations between flux differences and differences of VI within the footprint areas. This indicates that multi-tower approaches and paired observations from one tower (if the footprint area is not identical) may overestimate the uncertainty of eddy covariance measurements.