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This paper is intended to show how to aggregate two different remote-sensing techniques carried out in the framework of the European-Union funded project EFEDA (European Field Experiment in a Desertification-threatened Area). One of these is based on satellite measurements, and the: other on aircraft flux measurements. The aim of putting together both methodologies was to determine regional evapotranspiration as a major component of the water balance in Castilla-La Mancha, a semiarid region in southeast Spain. The basis for the satellite approach is using temperature information collected by the Advanced Very High-Resolution Radiometer (AVHRR) sensor on board the National Oceanic Atmosphere Administration (NOAA) satellite to derive reference and actual evapotranspiration estimates: extending and applying results previously achieved by the present authors, The aircraft measurements make use of the principles of turbulent water vapour transport in the lowest layer of the atmosphere, processing information about the spatial variability of energy and evaporation fluxes along several flight paths across the area, allowing an evapotranspiration measurement useful to validate satellite estimates. The differences between these methods are below 1 mm/d, which is within the range of the accuracy of available methods for estimating surface fluxes on a regional scale. (C) 1998 Silsoe Research Institute.