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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. SUMMARY(1) It has been claimed that deep wells drilled in the Sahel to provide water for cattle cause vegetation degradation by concentrating animals, and may cause desertification.(2) Satellite measurements of red and near infra-red reflectance from vegetation, when combined into the normalized difference vegetation index (NDVI), are correlated with the absorbed fraction of photosynthetically active radiation, and hence to primary production. In this study, NDVI measurements from the advanced very high resolution radiometer on the NOAA satellites were used to estimate the total vegetation production during 1987 on transects from twenty deep wells in the North-Ferlo region of Senegal.(3) Regression analysis indicated no consistent relationship between primary production and proximity to a well at a resolution of 1 1 km.(4) Most of the variance in the data was explained by the differing mean productivities at the different wells correlated with a gradient of increasing rainfall southward. Distance effects fitted to model the change in productivity with distance from the wells produced both positive and negative slope coefficients which are probably associated with factors other than grazing (e.g. soil type, relief and local rainfall patterns).

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Large power system disturbances and their consequences, represent a challenging problem for the power industry and especially for the network system operators, the ones who are responsible for the network operation at large, its security and reliability. This fact has reinforced interest in the new generation of monitoring systems with the aim to gain if possible a precise and maximum knowledge about those disturbances and their impact. Most of these severe events with impact on the entire system typically occur in or near the transmission network and are therefore best monitored at transmission system level. This puts great demands on the equipments and in general restricts the use of the data since it becomes property of the transmission system operator. Synchronized Phasor Measurement System has emerged as a powerful tool to improve power systems performance and reliability and fault localization possibility. Typical example of fault event that occurred in the transmission system and which was well registered by Phasor Measurement Units