REMOTE SENSING CONTRIBUTION TO THE STUDY OF SURFACE
RADIATIVE PARAMETERS IN THE EFEDA AREA
E. LOPEZ-BAEZA & J. MELIA
Remote Sensing Unit. Faculty of Physics. University of Valencia
Burjassot. 46100 Valencia (Spain)
ABSTRACT:
The ECHIVAL Field Experiment in Desertification-threatened Areas (EFEDA) was the first major activity
within the European International Project on Climatic and Hydrological Interactions between the
Vegetation, the Atmosphere and the Land-surface (ECHIVAL), which was developed by members of the
European research community jointly with the Commission of the European Communities, Directorate
General XII, within the European Programme on Climate and Natural Hazards (EPOCH). The EFEDA field
experiment was complemented by the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEXSAHEL)
(HAPEXSAHEL) which took place in Niger, 1992.
The EFEDA project was developed primarily to study the changes of the land-surface - atmosphere
interactions and their dependence on the sub-surface water regime in the course of an aridification process,
thus improving the understanding of causes, mechanisms and manifestations of desertification. This
knowledge is indispensable for the development of stategies to combat this type of land degradation.
During the development of this project, a lot has been learned about the quantitative information
content and applicability of remote sensing data. Thanks to the detailed information about the landuse, the
classification method could be validated. The availability of radiosonde and atmospheric spectral
transmittance data allowed the computation of correction functions for atmospheric effects. Long-term
AVHRR data series over for some time almost constant surface conditions made it feasible to analyse the
aspect angle effect. Direct intercomparisons between albedo and reflectance measurements at the surface
and directional reflectance values measured by satellite instruments contributed to the validation of the
information content inferred from remote sensed quantities. Due to the dry and warm weather, there was a
marked decrease observed for most of Spain from 20/21 May to 11/12 July in the NDVI derived from
AVHRR data. The interpretation and relationship to the precipitation regime and to the water cycle in
general of this apparent drop in productivity was one major challenge for the EFEDA remote sensing group.
Finally, the scaling problem could be studied by intercomparing high and medium resolution satellite data.
With this knowledge about the representativeness and accuracy of the remotely sensed data now the
problem can be approached on how accurate algorithms are, that have been developed to estimate fluxes
with the help of satellite data.
KEYWORDS'. Desertification, EFEDA, Land-surface climatology 1
1 - INTRODUCTION
The ECHIVAL Field Experiment in Desertification-threatened Areas (EFEDA) was the first major
activity within the European International Project on Climatic and Hydrological Interactions between the
Vegetation, the Atmosphere and the Land-surface (ECHIVAL), which was developed by members of the
European research community jointly with the Commission of the European Communities, Directorate
General XII, within the European Programme on Climate and Natural Hazards (EPOCH).
The EFEDA project was developed primarily to study the changes of the land-surface - atmosphere
interactions and their dependence on the sub-surface water regime in the course of an aridification process,
thus improving the understanding of causes, mechanisms and manifestations of desertification. This
knowledge is indispensable for the development of stategies to combat this type of land degradation.
Consequently, EFEDA addresses the following research areas (Bolle et al., 1993):
- Interrelationship of sub-surface water fluxes, water transfer in soils, and impact of water use on
groundwater as well as latent heat fluxes.
- Coupling between land-surface and atmosphere by exchange of energy, heat, momentum and
moisture, and by other susbstances such as carbon dioxide and particles.
- Role of vegetation in these processes under relatively dry conditions.
- Upscaling from local measurements to mesoscale by using the capability of remote sensing
measurements to extend short term regional studies to larger scales both in space and time.
The field experiment in 1991 is regarded as a pilot study and its specific emphasis was on:
- The experimental determination and modelling of area averages of the available energy, the
