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sensible and the latent heat fluxes between the surface and the atmosphere. 
- The investigation of the changes in surface fluxes as a) the land-use changes from one area to the 
other, including accompanying fetch phenomena, and b) the weather changes from wet to dry conditions. 
- The testing and validation of algorithms to deduce relevant information on land-surface 
characteristics from aircraft and satellite remote sensing data. 
In order to meet the objectives and to minimize experimental problems arising from complex 
topography, a part of the relatively flat and homogeneous area of about 10 4 km 2 of Castilla-La Mancha 
(Spain) was selected as EFEDA study area. June, 1991, was selected for the intensive observation period 
because during this month in the average the surface is drying and some crops are ripening. 
The underlying question that makes this research relevant for the CEC beyond the climate issue is to 
explore where the limits are for the sustainable development of such semi-dry areas and which methods are 
generally applicable to investigate this problem in other endangered regions as well. 
The intensive field experiment period gained its major thrust from the simultaneity of measurements 
at three sites representing the major land-use practices in the area: dry farming (Tomelloso), irrigated 
farming (Barrax) and semi-"natural", or matorral vegetation (Rada de Haro). As far as possible, all 
important parameters were measured, which are necessary to study land-surface processes and their 
development in time under aridifying conditions at scales from single plants to 10 4 km* and their synthesis 
by models. 
Satellite data were used to determine surface albedo, the diurnal change in surface temperature and 
their variation during the vegetation period. The vegetation index is related to the biological activity of 
vegetation. Vegetation types and their development can be classified by high-resolution multispectral data 
from satellite and NASA aircraft remote-sensing data (NASA/JPL DC-8 Aircraft Synthetic Aperture Radar 
(AIRSAR) and the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) on the NASA ER-2). 
To validate data obtained from satellites and their processing, mobile measurements were made with 
transportable equipment of the spectral transmittance of the atmosphere, atmospheric moisture content 
(from radiosonde ascents), spectral measurements of soil and vegetation reflectance and emissivity, spectral 
albedo over all essential cover types, surface temperature measurements and surface soil moisture 
measurements. The role of vegetation was additionally assessed by random sampling surveys on leaf area 
index (LAI), photosynthetic active radiation (PAR), stem flow, stomatal resistance and biomass production. 
The authors of this paper recognize the difficulty of summarizing in a few pages the large amount of 
work carried out by the whole EFEDA Technical Working Group on Remote Sensing and Radiation 
Properties of the Surface. A first summary has already been edited by Bolle and Streckenbach (1993) for the 
preparation of the Final Report to the CEC. This reference should be consulted by all means. Although the 
whole work carried out so far could perfectly well fit in the scope of this Symposium, the authors of this 
presentation, for the obligation of brevity, have made a big effort to select the aspects more directly related 
to the topic guidelines of the Symposium on Physical Measurements and Signatures in Remote Sensing. 
2 - LAND-SURFACE RADIATION PROPERTIES 
The radiation and remote sensing component of EFEDA includes the testing and application of 
algorithms by which information about land-surface characteristics is inferred from measurements made in 
space. The goal is to develop these methods to a degree that they become applicable in the context of 
"Global Change" studies. The requirements for this use of satellite data can shortly be summarized as 
follows: it must be possible to interrelate quantitatively data obtained at different locations and at different 
times, they must be linked to those quantities at the ground, which are used to characterize the state of the 
surface, and they must be available in temporal sequences that allow to distinguish long term changes from 
the short term variability of the signal. This includes the solution of problems arising from insufficient 
absolute calibration of space borne instruments, degradation of sensors, interference of the atmosphere and 
dependence of the signal on the measuring geometry. The need for multitemporal measurements rules out 
the use of high resolution imagery for global studies, since the frequency of cloudfree observations is too 
low in many parts of the world, and the amount of data to be analysed would become too large. This 
limitation to medium resolution data like the NOAA-AVHRR immediately causes additional interpretation 
problems due to the mixture of surface types present in pixel sizes of 1.2 km x 1.2 km (Billing et al., 
1993b). 
For global studies the interaction is important between the land-surface and the atmosphere at scales 
compatible with the grid width of global models (10 4 - 10 5 km 2 ). To establish and check the relationships 
between surface characteristics and remote sensing data, control measurements are inevitable. 
2.1 - Surface Optical Properties 
The first step in the characterization of a given area for interpreting satellite images begins with ground 
measurements and the analysis of the factors that influence the processes under study at the ground level.