On the coupled use of sapflow and eddy covariance measurements: environmental impacts on the evapotranspiration of an heterogeneous - wild olives based - Sardinian ecosystem.

English Abstract:

Sapflow and eddy covariance measurements are attractive methods for evapotranspiration (ET) estimates. We demonstrated that in Mediterranean ecosystems, characterized by an heterogeneous spatial distribution of different plant functional types (PFT), the combined use of these techniques becomes essential for the actual ET estimates. During the dry summers these water-limited heterogeneous ecosystems are characterized by a dual PFT structure with draught-resistant woody vegetation and bare soil, since grass died. The study has been performed in the Orroli site in Sardinia (Italy), where landscape is a mixture of Mediterranean patchy vegetation types: wild olives and herbaceous species. An eddy covariance – micrometeorological tower has been installed from 2003. Where patchy land cover leads and the surface fluxes from different cover are largely different, eddy covariance method hypothesis are not anymore preserved and ET evaluation may be not robust enough. In these conditions the sapflow measurements provide robust estimates of the transpiration from woody vegetation. Based on the Granier technique, 33 thermo dissipative probes have been built and 6 power regulators have been assembled to provide a constant current of 3V to the sensors. The sensors have been installed into 13 wild olives clumps, which characteristics have been analyzed to estimate innovative allometric relationships between sapwood area, diameter and canopy area and the wind and solar exposition effects on the tree transpiration. These relationships allowed the correct upscale of the local tree measurements to the site plot larger scale using an innovative scaling procedure. Through the coupled use of the sapflow sensor observations, a 2D footprint model of the eddy covariance tower and high resolution satellite images, for the estimate of the footprint land cover map, the sapflow ET at field scale has been compared to the eddy covariance ET, showing the impact of climate dynamics on ET.