ABSTRACT. Mounting evidence from satellite observations of a re-greening across much of the Sahel and Sudan zones over the past three decades has raised questions about the extent and reversibility of desertification. Historical ground data that could help in interpreting the re-greening are scarce. To fill that void, we tapped into the collective memories of local land users from central and western Senegal in 39 focus groups and assessed the spatial association between their perceptions of vegetation changes over time and remote sensing-derived trends. To provide context to the vegetation changes, we also explored the land users' perspective on the evolution of other environmental and human variables that are potentially related to the greening, using participatory research methods. While increases in vegetation were confirmed by the study participants for certain areas, which spatially corresponded to satellite-observed re-greening, vegetation degradation dominated their perceptions of change. This degradation, although spatially extensive according to land users, flies under the radar of coarse-resolution remote sensing data because it is not necessarily associated with a decrease in biomass but rather with undesired changes in species composition. Few significant differences were found in the perceived trends of population pressure, environmental, and livelihood variables between communities that have greened up according to satellite data and those that have not. Our findings challenge the prevailing chain of assumptions of the satellite-observed greening trend indicating an improvement of environmental conditions in the sense of a rehabilitation of the vegetation cover after the great droughts of the 1970s and 1980s, and the improvement of environmental conditions possibly translating into more stable livelihoods and greater well-being of the populations. For monitoring desertification and rehabilitation, there is a need to develop remote sensing-based indicators that better reflect the changes in the biophysical environment that matter most to the land users.

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[1] The usefulness of stochastic methods to efficiently quantify uncertainties in computational models of electromagnetic interactions is illustrated. A refined study of the second-order moments of a complex-valued Thévenin model, which represents the coupling between a wire structure and a time-harmonic electromagnetic field, is presented. The configuration of a stochastically undulating thin wire illuminated by a stochastic incident plane wave is investigated in detail. Three computational methods are used to evaluate the mean values and covariance coefficients of the observable: a straightforward Cartesian-product quadrature method, a Monte-Carlo method, and a space-filling-curve method. The underlying patterns of the randomness are revealed by analyzing the covariance matrix' principal components. The study of this interaction configuration shows some general characteristics, which are expected to show up in any stochastic electromagnetic interaction problem. In particular, the results indicate that fluctuations in self-interaction coefficients (impedances) have distinct features and are quite different from the coefficients describing the interaction with externally generated fields (voltages).

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