Identification and observation of desertification processes with the aid of measurements from space: Results from the European Field Experiment in Desertification-threatened Areas (EFEDA).

The ECHIVAL Field Experiment in Desertification-threatened Areas (EFEDA) addresses the question of desertification from the viewpoint of changing interactions between the land surface and the atmosphere under varying climatic conditions. The basic tool to improve our understandin of these processes are Soil-Vegetation-Atmosphere-Transfer (SVAT) and climate models. In testing techniques for deriving the needed input data from observations from space, EFEDA requires high-precision data sets that can be used to aggregate desertification-related land-surface characteristics into the scale up to the grid width of global climate models (10(4)-10(5) km(2)). In this context schemes have been developed to infer from satellite measurements fluxes at the surface. To validate the information inferred from observations in space, ground measurements have been performed on 2500 km(2) of Castilla-La Mancha, Spain, during the drying periods of the summers of 1991 and 1994. Ultimately EFEDA aims to determine cumulative fluxes over longer periods to allow discrimination between natural variability and trends over large areas, such as the land around the Mediterranean. To recalibrate and adjust the algorithms used to infer information about the land surfaces from satellite measurements, "anchor stations" are proposed for critical areas to provide collateral information and continuous quality control of inferred information.