Approche géographique des "Déserts" dans les communes de Sao Francisco de Assis et Manuel Viana, état du Rio Grande do Sul, Brésil
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Abstract EN:
In the south of brazil, in the western part of the state of rio grande do sul, the identification of sandy deposits in the agricultural domains, from the 1970s onwards, has been associated with the desert or with the process of desertification. The coupling of sandy deposits with these latter was justified by the existence of an observable erosion linked to aeolian deflation that produces dunes similar to those of a desert. However, analysis of the average rainfall over a period of several years indicates precipitation of greater than 1200 mm. In-depth study of the atmospheric conditions, on a monthy and daily scale, brings to light the existence of dry periods which cause aeolian deflation and of spells of heavy rain causing overland flow and intensive gullying. Besides the climatic constraints we list those of a structural nature linked to the mineral potential of the rocks which have a fundamental influence of the formation of the soils. A pedological examination reveals the existence of friable, acidic soils and the presence of mineral deficiencies and toxicity capable of determining the growth of vegetation, largely herbaceous (campos limpos), which adapts itself to siliceous soils. Traditionally this vegetation and these soils are subjected to extensive farming involving (pastoral) burning and, more recently, the development of agro-industrial cultivation. These two formsof social exploitation of the area have an undeniable impact on the fragility of the environment. Study of hydrological system enables us to bring to light the relationships between the various constituents of the ecological potential, and between the latter and the crisis it is going through. This approach shows us the types of reaction of the hydrological structure to periods of atmospheric dryness and of heavy rainstorms. It is principally the phase of overland flow that is highlighted. From the cartographic analysis of the terrain and the dynamics of the environment we can distinguish the terrain families and the most effective processes of morphogenesis. Finally, the system approach to a large ravine enables us to recognise : the relative position of the rocks in the stratigraphic scale, the geomorphological evolution of the landforms and the sequence of processes of morphogenesis. The study of concret examples indicates several courses that we might pursue in order to rethink the
Abstract FR:
Au sud du bresil, dans la partie occidentale de l'etat du rio grande do sul, l'identification de depots sablonneux dans les domaines agricoles, a partir des annees 1970, a ete associee au processus de desertification ou au desert. L'association des depots sablonneux a ce dernier se justifiait par le constat d'une erosion liee a la deflation eolienne qui genere des dunes similaires a un desert. Cependant, l'analyse de la pluviometrie moyenne interannuelle signale de precipitations superieures a 1200 mm. L'approfondissement de l7etude des conditions atmospheriques locales, a l'echelle du mois etdu jour, met en evidence l'existence de periodes seches generatrices de deflation eolienne et d'episodes pluvieux generateurs de ruissellement direct et de ravinement. A part les contraintes d'ordre climatique nous avons repertorie les contraintes d'ordre structurel liees au potentiel mineral des roches qui ont une influence fondamentale sur le developpement des sols. L'examen pedologique revele l'existence de sols friables, acides et la presence de carences et de toxicites minerales capables de determiner le developpement d'une vegetation silicicole en grande partie herbacee (campos limpos). Cette vegetation et les sols sont soumis a un elevage extensif seculaire pratiquant les brulis et, plus recemment, au developpement des cultures agro-industrielles. Ces deux formes d'utilisation sociale de l'espace provoquent un impact indeniable sur la fragilite du milieu. L'etude du systeme hydrologique nous permet de decouvrir les liaisons entre les composants du potentiel ecologique, entre ce dernier et la crise qu'il traverse. Cette demarche nous montre les types de reponses de la structure hydrologique aux periodes de secheresse atmospherique et a la puissance des phenomenes pluvieux. C'est principalement la phase de ruissellement direct qui est mis en evidence. A partir de la cartographie des terroirs et de la dynamique des milieux nous distinguons les familles de terroirs et les processus morphogenes les plus efficaces. A la fin, l'etude systemique d'un grand ravin nous permet de reconnaitre : la place relative des lithologies dans l'echelle stratigraphique, l'evolution geomorphologique des modeles et l'enchainement des processus morphogenes. L'etude des cas concrets nous signale l'existence des quelques voies a exploiter pour repenser l'economie agricole locale et en part