La région sahélienne est caractérisée, depuis la moitié du XXème siècle, par un déficit pluviométrique important marqué par de fortes sécheresses en 1972-73 et en 1983-84 qui ont eu des effets considérables sur l'écosystème, les ressources et la population locale. Les réponses hydrologiques induites par ce déficit se traduisent par des effets contrastés suivant la zone géographique considérée. Si dans la zone soudano-guinéenne, une baisse des écoulements de surface a été observée, la zone sahélienne, paradoxalement, a connu une augmentation de ces écoulements durant la même période. La plupart des études visant à mieux comprendre cette évolution paradoxale ont été effectuées en milieu sahélien cultivé, où l'augmentation du ruissellement a souvent été attribuée au changement d'usage des sols suite aux besoins de la population croissante. Toutefois, ce même phénomène est aussi observable dans les zones majoritairement pastorales où les modifications d'usage des sols ne suffisent pas à expliquer les modifications hydrologiques observées. Les objectifs de cette thèse consistent à étudier, au travers de la modélisation hydrologique, l'évolution conjointe des différents processus régissant l'hydrologie sahélienne et de hiérarchiser leurs impacts sur l'évolution du ruissellement de surface sur les 50 dernières années en milieu pastoral. Pour ce faire, le modèle de ruissellement KINEROS2, a été appliqué au bassin versant d'Agoufou. Ce bassin de type endoréique est situé dans la région du Gourma, au Mali et est instrumenté par le service d'observation AMMA-CATCH qui fournit une partie des données nécessaires au forçage et à l'évaluation du modèle. La première partie de ces travaux a été dédiée à la quantification des changements survenus au cours des 50 dernières années sur ce bassin ainsi qu'au niveau de son exutoire, le lac d'Agoufou. Une cartographie des unités paysagères identifiées à partir d'images satellite et de photographies aérienne, a été réalisée pour les années 1956 et 2011. Cette cartographie a mis en évidence l'évolution des états de surface, marquée par l'érosion des sols peu profonds et la dégradation de la végétation de type brousse tigrée en faveur de l'extension des surfaces de sols nus (+35 km² entre 1956 et 2011) ainsi que par l'augmentation de la densité du réseau de drainage (×1.5) et par celle de la superficie du lac (534 m² en 1956 pour 2×106 m² en 2011). L'évolution du débit au cours du temps a été quantifiée à partir des variations de volume du lac établies via une relation entre les surfaces estimées par télédétection et les hauteurs d'eau mesurées in-situ. Cette relation a été couplée à une équation de bilan d'eau, prenant en compte les précipitations, l'évaporation et l'infiltration sur le lac pour quantifier les apports d'eau à l'exutoire. Nous avons ainsi mis en évidence l'évolution nette du coefficient de ruissellement du bassin qui est passé de 0% dans les années 1970 à 5.5 % dans les années 2000. La deuxième partie de cette thèse a été consacrée à la modélisation hydrologique et à la réalisation de simulations de références et d'attribution des changements observés à différents facteurs. L'augmentation des surfaces de sols nus associée à une diminution de la végétation explique largement l'évolution de l'hydrologie de surface du bassin depuis les années 1960. Le développement du réseau de drainage et l'encroûtement des dunes jouent des rôles secondaires dans cette évolution. En revanche, la variation du régime des pluies, tend à diminuer le ruissellement au cours du temps de plus de 34%. Les mécanismes identifiés sur le bassin d'Agoufou montrent l'importance de la dynamique du système couplé végétation/érosion/réseau hydrique et de leurs rôles sur d'autres bassins Sahéliens où l'augmentation du ruissellement a été également mise en évidence. / Since the mid-twentieth century, the Sahel is characterized by a significant rainfall deficit marked by severe droughts in 1972-73 and 1983-84 that have significantly impacted ecosystems, resources and local population. The responses induced by this deficit result in opposite effects according to geographical area. If, in the Sudano-Guinean zone, a reduced surface runoff was observed, the Sahel experienced a paradoxical increase of surface runoff during the same period. Most of the studies aiming at understanding this paradoxical situation have been performed in crop-dominated areas, where the runoff increase has been attributed to land use change following population growth. However, the same phenomenon is also observed in pastoral areas where land use change cannot explain the observed hydrological changes. The objective of this thesis is to study the different processes governing Sahelian hydrology and prioritize their impact on the surface runoff evolution in pastoral areas by means of hydrological modeling. To do this, the KINEROS2 runoff model was applied to the Agoufou watershed. This endorheic watershed is located in the Gourma region, in Mali. It is instrumented by the AMMA-CATCH observatory, which provides most of the necessary data for forcing and evaluating the model. The first part of this work was dedicated to the quantification of the changes occurred over the last 50 years on the Agoufou watershed and its outlet, the Agoufou lake. Land cover maps derived from satellite images and aerial photographs for 1956 and 2011 allowed estimating the changes in surface characteristics. Erosion of shallow soil and degradation of the tiger bush vegetation in favor of the bare soil extension (35 km² between 1956 and 2011) as well as increases in drainage density (× 1.5) and in lake area (534 m² in 1956 to 2 x 106 m² in 2011) were the main changes observed. The discharge evolution over time was quantified from changes in lake's volume, computed via a relationship between the lake's surface estimated by remote sensing and in-situ water height data. This relationship was coupled to a water balance equation, taking into account precipitation, water evaporation and lake's bottom infiltration to quantify the water inflow to the lake. The runoff coefficient of the watershed was shown to increase from 0 % in the 1970s to 5.5% in the 2000s. The second part of this thesis was dedicated to hydrological modeling in order to rank the impact of the surface changes highlighted above on runoff. Series of reference and attribution simulations were performed. The increase in bare soil surfaces associated with a decrease in vegetation largely explains the hydrological changes of the watershed since the 1960s. The development of the drainage network and the crusting of sandy dunes play a less important role on this evolution, while the evolution of daily precipitation reduces runoff over time by more than 34%, which contradicts the observed trend. The mechanisms identified in the Agoufou watershed highlight the pivotal role of the dynamics of the coupled vegetation/erosion/drainage network system and their role in other Sahelian regions, where increased runoff was also observed.

Made available in DSpace on 2015-05-14T12:09:17Z (GMT). No. of bitstreams: 1 parte2.pdf: 1283524 bytes, checksum: 82414a2866e0921004f86255c205b056 (MD5) Previous issue date: 2006-03-31 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / The urban floods are constituted one of the most important impacts on the society. Those impacts could happen due to the urbanization or to the natural flood of the riverside area. In the current context, several prevention and protection measures against urban floods are adopted, in general, measures of corrective character. The measures can be structural (when they modify the fluvial system and reduce risks of flood damages) and non-structural (a better coexistence of the population with the floods). This project has as main aim the elaboration of a plan of measures that contemplates the prevention aspects against the occupation of the spaces with flood risks, through structural and non-structural measures, associated or not, for the municipal districts located within Mamanguape river basin in Paraíba State, prone to floods since almost all of them occupy the flood plains of the main water courses. Several problems could be associated with the occurrence of those events: inadequate occupation of the river beds; lack of resources for dwelling and waste water system; lack of planning for the occupation of the urban space; and lack of systems for catastrophe alert and for evacuation of the flooded areas. Therefore, it is presented practical and efficient solutions for the best use of the risk areas, which would lead to a better life quality for the urban areas. The effective participation of the public government and the responsible department for the environmental and urban administration in the implementation of a politics of environmental education and elaboration of a management plan that prioritizes the flood control in the urban areas is indispensable / As inundações urbanas constituem-se num dos mais importantes impactos sobre a sociedade. Esses impactos podem ocorrer devido à urbanização ou à inundação natural da várzea ribeirinha. No contexto atual, diversas medidas de prevenção e proteção contra enchentes urbanas são adotadas, em geral, medidas de caráter corretivo. As medidas podem ser estruturais (quando modificam o sistema fluvial evitando os prejuízos decorrentes das enchentes) e não-estruturais (melhor convivência da população com as enchentes). Este projeto tem como abordagem principal a elaboração de um plano de medidas que contemple os aspectos de prevenção contra a ocupação dos espaços de riscos de enchentes, através de medidas estruturais e não-estruturais, associadas ou não, para os municípios localizados na Bacia do Rio Mamanguape/PB, que apresentam tendências a cheias por ocuparem, na sua maioria, as planícies de inundação de importantes cursos d água. Diversos problemas podem estar associados à ocorrência desses eventos: ocupação inadequada dos leitos dos rios; falta de recursos para moradia e saneamento; falta de planejamento de ocupação do espaço urbano; e inexistência de sistemas de alerta em catástrofes e de planos de desocupação de áreas atingidas. Portanto, são apresentadas soluções práticas e eficientes para o melhor aproveitamento das áreas de risco, o que possibilitará uma melhor qualidade de vida no meio urbano. A participação efetiva do poder público e dos órgãos responsáveis pela gestão ambiental e urbana na implementação de uma política de educação ambiental e na elaboração de um plano diretor que priorize o controle de inundação no meio urbano deve ser imprescindível