Showing papers by "Abdou Moumouni University published in 1997"

The diurnal and seasonal cycles of wind-borne dust over Africa North of the Equator

Abstract: This article presents a study of the diurnal and seasonal cycles of dust over North Africa, using surface visibility as an indicator of dust. The diurnal cycle shows a reduction of visibility during the daytime hours in the areas where dust is generated, a consequence of the elimination of the nocturnal inversion. The annual cycle reveals that, at latitudes from 5° to 16°N, there is a latitudinal increase in the duration of the presence of aerosols over the course of the year. The presence of aerosols dimishes in the latitudes from 20° to 35°N, indicating that the aerosol content of the Saharan air is lower than that over the semiarid sub-Saharan zones, such as the Sahel. A comparison of three periods, 1957–61, 1970–74, and 1983–87, shows a continually increasing presence of dust, particularly in the western Sahel. The interannual variability of the dust and its annual cycles in these three periods throughout North Africa bear a strong relationship to rainfall fluctuations in the Sahel. Overall, ...

Étude des écoulements souterrains dans le bassin de Bilma-Djado à l'aide des isotopes de l'environnement

Abstract: Resume Les isotopes stables 18 O et 2 H et radioactifs 3 H et 14 C ont permis d'identifier un ecoulement general d'est en l'ouest du Tibesti au bassin du Djado et, dans une moindre proportion, nord-sud selon la direction de la flexure de Bilma (axe du synclinal). Une autre composante existe du nord-est vers le sud-ouest a la faveur d'infero-flux d'oueds descendant le Hoggar. Les principales aires de recharge des aquiferes sont les massifs du Tibesti et la bordure orientale du Hoggar. La presence du tritium permet d'envisager le phenomene de drainance : jusqu'a 2,5 ± 1,0 UT dans le Cambrien, 1,8 ± 1,0 UT dans le Viseen, 2,9 ± 1,0 dans le Continental intercalaire CI. Le Cambrien alimente par drainance le Carbonifere et le CI. L'exutoire principal est caracterise par l'evaporation des eaux des aquiferes excepte celles du Cambrien ; en effet, ces dernieres sont proches de la droite meteorique locale des regions arides et semi-arides δ 2 H = 8 δ 18 O + 5. Les variations piezometriques saisonnieres sont la consequence, a la fois d'un transfert de masse des precipitations sur les massifs et de la charge dans les aquiferes (drainance).

An Inequality About the Largest Roots of a Polynomial

Abstract: We present an upper bound for the product of the k largest roots of a monic polynomial of degree n with complex coefficients. For that, we use the theory of compound matrices and the localization of the eigenvalues of matrices.

The CFAF Devaluation, Naira Parallel Exchange Rate and Niger's Competitiveness

Abstract: Niger's competitiveness with Nigeria decreased significantly during the last decade, and the recent CFAF devaluation did not actually result in a sustained real and nominal depreciation of this currency versus the naira in the parallel market. To understand this behavior of the naira/CFA exchange rate, this article builds upon a simple model of arbitrage taking into account two main motives for foreign currency demand by Nigerians (dollar and CFAF): the international trade-via-transit motive and the capital transfer motive. Econometric results from vector error correction model estimates show that the CFAF and the dollar are viewed by Nigerians as very close substitutes, and that this continued even in the aftermath of the August 1993 decision to limit CFAF convertibility and the January 1994 devaluation. Such results, which seem to reflect the substantial role played by the demand for transit import by Nigerians via such CFA countries as Niger, are also used to explain the continuous decline in Niger's competitiveness with regards to Nigeria in terms of locally produced goods. Copyright 1997 by Oxford University Press.