干旱区生态环境知识资源中心

The dehesa, the most widespread agroforest ecosystem in Europe (approximate to 3 million ha), is recognized as an example of sustainable land use and for its importance in rural economy ([1] [2]). It is characterized by widely-spaced oak trees (mostly Quercus Ilex L.), combined with crops, pasture and shrubs in the sub-canopy region. The estimation of the ecosystem evapotranspiration (ET) using remote sensing may assist the monitoring of its state from local to regional scales, improving the management and the conservation of the ecosystem. Thermal-based energy balance techniques which distinguish soil/substrate and vegetation contributions to the radiative temperature and radiation/turbulent fluxes have proven to be reliable in the estimation of the energy surface fluxes, and therefore in the estimation of ET. In particular, the two-source energy balance (TSEB) model of Norman et al. ([3]) and Kustas and Norman ([4]) has shown to be robust for semi-arid sparse canopy-cover landscapes. With the objective of evaluating the model over this environment, an energy flux measurement system has been used. It was installed in a dehesa located in Southern Spain (38 degrees 12' N; 4 degrees 17' W, 736 m a.s.l) with 1 km homogeneous fetch in wind direction. The quality of the measured data fluxes has been tested with the energy-balance closure criterion ([5] [6]) yielding an average closure of 86% which is within the error range found in similar studies. The TSEB model was evaluated in the area for 2012 summer season, using images from MODIS (Moderate Resolution Imaging Spectroradiometer) sensor and ground measured meteorological data. The half-hourly estimates were compared with the flux tower measurements, obtaining a RMSD between modeled and measured energy fluxes within the closure balance error.