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X Foreword

surface runoff from smali to medium watersheds in Niger (Desconnets et al.) and in Senegal (Albergel & Dacosta), and finally an analysis of a large hydrosystem, the Niger (Briąuet et al.). A second group of articles deals with the global approach to hydrological regimes through the FRIEND project of UNESCO in West and Central Africa (Aka et al.) and in South Africa (Bullock & Matondo). A third group of articles introduces a synthetic analysis of the regime of two large rivers, the Oubangui (Wesserlink et al.) and the Amazon (Molinier et al.).

-    The fourth part, with the title « Tropical hydrological processes », deals with the mechanism of runoff. The papers show that at smali scales, the surface features mainly determine the dynamie of the flood runoff in the arid and sub-arid regions (Casenave & Valentin, Lamachere & Puech). In the morę humid regions, geochemical techniąues permit better understanding of the water paths whose dynamics is particularly complex (Bariac et al.). At the large basin scalę the geodynamics of the Continental surface is portrayed, analysing the interactions between the geological components and the rivers that pick up and transport solids and dissolved materiał (Laraque & 01ivry, Censier, Bullock & McCartney) that are the markers of the evolution of the continents and the subject of natural and human actions and constraints.

—    The fifth part, « Data processing and hydrological modelling in the tropics », shows perspectives offered to users of hydrological information due to the rapid development of the Computer. Operational tools are identified (Servat et al., Bouvier et al.), the applications on particular geographical sets (Cappelaere et al., Lamagat et al.) and there are reflections on methods able to bring interesting results in the futurę (Sambou & Thirriot, Gresillon & Puech, Delclaux & Hilaire).

These 30 papers only give a partial view of the hydrological activities in the tropical world. But the references listed will provide the interested reader, the fanatic researcher or the efficient engineer further means of answering their questions. Jean Rodier was a person who fitted all three categories.

Pierre Chevallier & Bernard Pouyaud

Directors de Research at Orstom Montpellier and La Paz, May 1996