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A number of studies have dealt with the assessment of potential and actual desertification risk using composite indices. The Environmental Sensitivity Areas (ESA) approach, developed in the framework of MEDALUS project funded by the European Community, is one of the most used procedures to monitor land vulnerability to degradation in the Mediterranean region. The final output of this procedure is an index (ESI) composing four indicators of climate, soil, vegetation, and land management based on 14 elementary variables. Although applied to a number of case studies throughout southern Europe, northern Africa and the Middle East, the performance of this monitoring system has never been assessed. The present study evaluates the robustness of the ESI through an original procedure incorporating sensitivity analysis and data cost analysis. For each variable, the standard error of the estimate, the correlation coefficient with the ESI, the sensitivity score, and the estimated costs of data collection and handling were calculated in order to evaluate the stability of the final index and the relative importance of each composing variable. The overall performance of the ESI was computed by averaging the score of the four indicators. Variables such as vegetation cover, climate aridity, rainfall, and the degree of land protection provided the largest contribution to the ESI. The illustrated approach is suited to evaluate the overall performance of a set of variables composing a synthetic index. Moreover, to our knowledge, this is the first attempt to consider explicitly the monetary costs of data collection and handling within a composite index evaluation procedure.
