Evapotranspiration constitutes an important element that connects the hydrological cycle and the surface energy balance, allowing to maintain some suitable energetic and water levels in the Earth-atmosphere system by means of the mass and energy transfer. Evapotranspiration monitoring has important implications on global and regional climate modelling to assess both the environmental stress affecting ecosystems. Observed warming over several decades has been linked to changes in the large-scale hydrological cycle such as: increasing atmospheric water vapour content; changing precipitation patterns, intensity and extremes; and changes in soil moisture and runoff. In the face of a possible climatic change (Bates et al., 2008) one of the processes that can be more affected is evapotranspiration, which plays a key role in hydrological budget. The increase of evapotranspiration, as a consequence of temperature raising while precipitation remains constant or is reduced may suppose an important decrease in water availability for natural and agricultural systems as well as for human needs (Jackson et al., 2001), especially in arid or semiarid areas, such as the Mediterranean basin. In this way, radiometric measurements provided by remote sensing and GIS climate modelling have proved to be essential to model evapotranspiration because they are the only techniques that allow us to compute it at regional scales in a feasible way. The proposed methodology for modelling actual evapotranspiration is based on the algorithm developed by Anderson et al. (2007), incorporating improvements in the determination of two key variables for this algorithm such as land surface temperature (Cristóbal et al., 2009), air temperature (Cristóbal et al., 2008). It is important to note, that most of the algorithms developed for
the determination of actual evapotranspiration for crops have been developed and its implementation in a Mediterranean vegetation zone represents a major challenge in monitoring this variable and in improving knowledge water and carbon balance in Mediterranean vegetation zone. Finally, to fit and test the model flux towers available in the Doñana Biological Reserve will be used. In addition, we have already designed two experimental plots in order to study the spatial variability (heterogeneity) of actual evapotranspiration that will be operative in May 2010 that will be also used to adjust and validate actual evapotranspiration models.