8 research outputs found
Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review
In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968â1990s âGreat Droughtâ period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity
Contribution Ă l'Actualisation des Normes Hydrologiques en relation avec les Changements Climatiques et Environnementaux en Afrique de l'Ouest
Water resources plays a key role in the social progress and economic development of west african countries. But the mobilization of water is hampered by climate and environmental changes that undergoes the region since 1970. Extremes parts of hydrological regimes are also impacted, but less studies have focus on their evolution, in relation with climate and environmental changes. The main objective of this thesis is to characterize hydrological extreme events in West Africa, we tried to answer the following questions:-What are the trends of maximum discharge in west Africa?-Are these trends due to climate changes or enviromental changes?-Which speculations can be made from these evolutions, according to future climate simulations?In the first part of the work, we analyzed the trends of floods over 14 watersheds of the region. This analysis allowed us to highlight a clustering behavior of flood according to the climatic region the catchments belong to. Increasing trends have been found on flood magnitude and flood frequency of the 3 sahelian catchments used, and decreasing trends of flood magnitude were found on three sudanian catchments. Finally, the remaining catchments did not showed significant trend in their flood regime...La mobilisation et la gestion de lâeau de surface constitue la clĂ© de voĂ»te du bien-ĂȘtre social et Ă©conomique des populations des zones arides dâAfrique de lâOuest. Cependant lâutilisation des outils de gestion et de prĂ©dĂ©termination hydrologique est entravĂ©e par les changements climatiques et environnementaux que connait lâAfrique de lâOuest depuis les annĂ©es 1970. Les impacts de ces changements sont assez bien documentĂ©s sur les caractĂ©ristiques moyennes des rĂ©gimes hydrologiques mais le sont moins lorsquâon sâintĂ©resse aux caractĂ©ristiques des extrĂȘmes. Cette thĂšse sâinscrit dans le cadre dâune contribution Ă un projet dâactualisation des normes hydrologiques en Afrique de lâOuest, et constitue le premier pas essentiel pour ce projet. Son objectif gĂ©nĂ©ral est de caractĂ©riser les rĂ©gimes de crues dans la rĂ©gion en rĂ©pondant aux questions suivantes : Quelles sont les Ă©volutions observĂ©es sur les rĂ©gimes des crues en Afrique de lâOuest ? Ces Ă©volutions sont-elles en rapport avec les changements climatiques et/ou les modifications environnementales ? Quels sont les scĂ©narii possibles de lâĂ©volution des crues au regard des simulations climatiques futures ? La difficultĂ© majeure de ce travail est liĂ©e Ă la disponibilitĂ© des donnĂ©es hydro climatiques et environnementales dans la rĂ©gion, aussi, lâanalyse sâest basĂ©e sur des outils reconnus robustes dans la littĂ©rature. Dans un premier temps, lâutilisation des donnĂ©es de 14 stations hydromĂ©triques de la rĂ©gion..
A contribution to the update of hydrological standards in West Africa : impacts of climate and Environmental changes on hydrologic extremes
La mobilisation et la gestion de lâeau de surface constitue la clĂ© de voĂ»te du bien-ĂȘtre social et Ă©conomique des populations des zones arides dâAfrique de lâOuest. Cependant lâutilisation des outils de gestion et de prĂ©dĂ©termination hydrologique est entravĂ©e par les changements climatiques et environnementaux que connait lâAfrique de lâOuest depuis les annĂ©es 1970. Les impacts de ces changements sont assez bien documentĂ©s sur les caractĂ©ristiques moyennes des rĂ©gimes hydrologiques mais le sont moins lorsquâon sâintĂ©resse aux caractĂ©ristiques des extrĂȘmes. Cette thĂšse sâinscrit dans le cadre dâune contribution Ă un projet dâactualisation des normes hydrologiques en Afrique de lâOuest, et constitue le premier pas essentiel pour ce projet. Son objectif gĂ©nĂ©ral est de caractĂ©riser les rĂ©gimes de crues dans la rĂ©gion en rĂ©pondant aux questions suivantes : Quelles sont les Ă©volutions observĂ©es sur les rĂ©gimes des crues en Afrique de lâOuest ? Ces Ă©volutions sont-elles en rapport avec les changements climatiques et/ou les modifications environnementales ? Quels sont les scĂ©narii possibles de lâĂ©volution des crues au regard des simulations climatiques futures ? La difficultĂ© majeure de ce travail est liĂ©e Ă la disponibilitĂ© des donnĂ©es hydro climatiques et environnementales dans la rĂ©gion, aussi, lâanalyse sâest basĂ©e sur des outils reconnus robustes dans la littĂ©rature. Dans un premier temps, lâutilisation des donnĂ©es de 14 stations hydromĂ©triques de la rĂ©gion...Water resources plays a key role in the social progress and economic development of west african countries. But the mobilization of water is hampered by climate and environmental changes that undergoes the region since 1970. Extremes parts of hydrological regimes are also impacted, but less studies have focus on their evolution, in relation with climate and environmental changes. The main objective of this thesis is to characterize hydrological extreme events in West Africa, we tried to answer the following questions:- What are the trends of maximum discharge in west Africa?- Are these trends due to climate changes or enviromental changes?- Which speculations can be made from these evolutions, according to future climate simulations?In the first part of the work, we analyzed the trends of floods over 14 watersheds of the region. This analysis allowed us to highlight a clustering behavior of flood according to the climatic region the catchments belong to. Increasing trends have been found on flood magnitude and flood frequency of the 3 sahelian catchments used, and decreasing trends of flood magnitude were found on three sudanian catchments. Finally, the remaining catchments did not showed significant trend in their flood regime..
The Multiscale TROPIcal CatchmentS critical zone observatory MâTROPICS dataset I: The Nyong River Basin, Cameroon
International audienceSimultaneously acquiring time series of climate, hydrology and hydrochemical data over decades on river systems is pivotal to understand the complex interactions involving rock, soil water, air and biota in the Critical Zone, to build integrated modelling and to propose predictive scenarios. Among the Critical Zone Observatories (CZOs) implemented in the past 25 years, only a few are located in the humid Tropics despite the importance of these regions in terms of population density, fast-changing land use, biodiversity hotspots, biomass stock on continents, size of river systems, etc. Since 1994, weathering and erosion processes and fluxes have been investigated at both local (experimental watershed) and regional scales in the Nyong River Basin (Cameroon) which belongs to the Critical Zone Observatories network named Multiscale TROPIcal CatchmentS (M-TROPICS). The data shared by M-TROPICS in Cameroon are: (1) rainfall; (2) air temperature, air relative humidity, wind speed and direction, and global radiation; (3) stream and river water level; (4) pH, electrical conductivity, water temperature and suspended particulate matter (SPM) concentration; (5) major ion, alkalinity and dissolved organic carbon (DOC) concentrations. The dataset already contributed to describe the water partitioning in these tropical humid watersheds, to better understand the factors controlling chemical weathering and physical erosion in tropical ecosystems, particularly the role of organic matter. The dataset also contributed to calculate elemental weathering fluxes and saprolite production rate and to propose denudation rates on tropical cratonic landscapes. Hydrological modelling allowed quantification of the geographical water sources contributing to streamflow. DOC data were used to determine greenhouse-gas emissions and carbon budgets from African inland waters. However, long-term solute concentrations at the outlet of a small tributary of the Nyong River exhibit non-stationary behaviour over the last 26 years. The processes governing those fluctuations are not yet fully understood and might be related to changes in the hydrological regime, land-cover and land-use. The latter highlights the need for longer time-series and continued support for CZOs particularly in the humid tropics
The Multiscale TROPIcal CatchmentS critical zone observatory MâTROPICS dataset I: The Nyong River Basin, Cameroon
Simultaneously acquiring time series of climate, hydrology and hydrochemical data over decades on river systems is pivotal to understand the complex interactions involving rock, soil water, air and biota in the Critical Zone, to build integrated modelling and to propose predictive scenarios. Among the Critical Zone Observatories (CZOs) implemented in the past 25?years, only a few are located in the humid Tropics despite the importance of these regions in terms of population density, fast-changing land use, biodiversity hotspots, biomass stock on continents, size of river systems, etc. Since 1994, weathering and erosion processes and fluxes have been investigated at both local (experimental watershed) and regional scales in the Nyong River Basin (Cameroon) which belongs to the Critical Zone Observatories network named Multiscale TROPIcal CatchmentS (M-TROPICS). The data shared by M-TROPICS in Cameroon are: (1) rainfall; (2) air temperature, air relative humidity, wind speed and direction, and global radiation; (3) stream and river water level; (4) pH, electrical conductivity, water temperature and suspended particulate matter (SPM) concentration; (5) major ion, alkalinity and dissolved organic carbon (DOC) concentrations. The dataset already contributed to describe the water partitioning in these tropical humid watersheds, to better understand the factors controlling chemical weathering and physical erosion in tropical ecosystems, particularly the role of organic matter. The dataset also contributed to calculate elemental weathering fluxes and saprolite production rate and to propose denudation rates on tropical cratonic landscapes. Hydrological modelling allowed quantification of the geographical water sources contributing to streamflow. DOC data were used to determine greenhouse-gas emissions and carbon budgets from African inland waters. However, long-term solute concentrations at the outlet of a small tributary of the Nyong River exhibit non-stationary behaviour over the last 26?years. The processes governing those fluctuations are not yet fully understood and might be related to changes in the hydrological regime, land-cover and land-use. The latter highlights the need for longer time-series and continued support for CZOs particularly in the humid tropics
Evolution of Surface Hydrology in the Sahelo-Sudanian Strip: An Updated Review
In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last five decades. The first paradox was observed during the 1968–1990s ‘Great Drought’ period, during which runoff significantly increased. The second paradox appeared during the subsequent period of rainfall recovery (i.e., since the 1990s), during which the runoff coefficient continued to increase despite the general re-greening of the Sahel. This paper reviews and synthesizes the literature on the drivers of these paradoxical behaviors, focusing on recent works in the West African Sahelo/Sudanian strip, and upscaling the hydrological processes through an analysis of recent data from two representative areas of this region. This paper helps better determine the respective roles played by Land Use/Land Cover Changes (LULCC), the evolution of rainfall intensity and the occurrence of extreme rainfall events in these hydrological paradoxes. Both the literature review and recent data converge in indicating that the first Sahelian hydrological paradox was mostly driven by LULCC, while the second paradox has been caused by both LULCC and climate evolution, mainly the recent increase in rainfall intensity
Are the Fouta Djallon Highlands Still the Water Tower of West Africa?
International audienceA large share of surface water resources in Sahelian countries originates from Guinea's Fouta Djallon highlands, earning the area the name of "the water tower of West Africa". This paper aims to investigate the recent dynamics of the Fouta Djallon's hydrological functioning. The evolution of the runoff and depletion coefficients are analyzed as well as their correlations with the rainfall and vegetation cover. The latter is described at three different space scales and with different methods. Twenty-five years after the end of the 1968-1993 major drought, annual discharges continue to slowly increase, nearly reaching a long-term average, as natural reservoirs which emptied to sustain streamflows during the drought have been replenishing since the 1990s, explaining the slow increase in discharges. However, another important trend has been detected since the beginning of the drought, i.e., the increase in the depletion coefficient of most of the Fouta Djallon upper basins, as a consequence of the reduction in the soil water-holding capacity. After confirming the pertinence and significance of this increase and subsequent decrease in the depletion coefficient, this paper identifies the factors possibly linked with the basins' storage capacity trends. The densely populated areas of the summit plateau are also shown to be the ones where vegetation cover is not threatened and where ecological intensification of rural activities is ancient