The Congo River Basin (CRB), located in the central region of Africa, is of particular importance... more The Congo River Basin (CRB), located in the central region of Africa, is of particular importance for regional and global climate and carbon studies. Being the second largest river basin after the Amazon, it is also the one with the most free-flowing rivers. However, despite these important characteristics, it has not attracted as much attention among the scientific communities as the Amazon Basin or other large tropical rivers in the world. Because of the lack of comprehensive and maintained in situ data networks over time, large-scale monitoring of hydroclimatic variables has not been properly conducted. In this context, near real-time observations of the CRB, such as water surface elevation (WSE) and river discharge, as well as understanding the impacts of climate change in a spatiotemporally distributed manner across the basin present a major challenge. In the last few years, however, the scientific community, supported by the leading operational organism in the CRB (the CICOS),...
This article reviews the state of the art in the use of space-borne observations for analyzing ex... more This article reviews the state of the art in the use of space-borne observations for analyzing extreme rainfall and flood events in Africa. Floods occur across many space and timescales, from very localized flash flood events to slow propagation of discharge peaks in large rivers. We discuss here how satellite data can help us understand the genesis and impacts of these flood events, monitor their evolution, and better constrain prediction models, thereby improving early warning and population protection. To illustrate these topics, we reanalyze major flood events that occurred in Niger, Mozambique, Central African Republic and Ivory Coast, using satellite information.
La région sahélienne est caractérisée, depuis la moitié du XXème siècle, par un déficit pluviomét... more 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...
revanche, la variation du régime des pluies, tend à diminuer le ruissellement au cours du temps d... more 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.
Rural societies in the face of climatic and environmental changes in West Africa, 2017
Introduction The hydrology of the Sudano-Sahelian strip has evolved rapidly in recent decades. We... more Introduction The hydrology of the Sudano-Sahelian strip has evolved rapidly in recent decades. West Africa has experienced strong climatic and environmental changes that have deeply modified the runoff/infiltration/evapotranspiration balance and, as a result, the conditions of flow formation. The observations and scientific progress that have enabled better understanding of the impact of environmental changes on the hydrological cycle can be summarised in the form of several key stages. Durin..
In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last f... more 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.
Land surface processes play an important role in the West African monsoon variability. In additio... more Land surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff i...
Hydrology and Earth System Sciences Discussions, 2016
In the last decades the Sahel has witnessed a paradoxical increase in surface water despite a gen... more In the last decades the Sahel has witnessed a paradoxical increase in surface water despite a general precipitation decline. This phenomenon, commonly referred to as-the Sahelian paradox‖, is not completely understood yet. The role of cropland expansion due to the increasing food demand by a growing population has been often put forward to explain this situation for the cultivated Sahel. However, this hypothesis does not hold in pastoral areas 10 where the same phenomenon is observed. Several other processes have been suggested to account for this situation such as the degradation of natural vegetation following the major droughts of the 70ies and the 80ies, the development of crusted top soils, the intensification of the rainfall regime and the development of the drainage network. In this paper, a modeling approach is proposed to quantify and rank the different processes that could be at play 15 in pastoral Sahel. The KINEmatic EROSion model (KINEROS-2) is applied to the Agoufou watershed, in the Gourma region in Mali, which underwent a significant increase in surface runoff during the last 60 years. Two periods are simulated, the-past‖ case (1960-1975) preceding the Sahelian drought and the "present" case (2000-2015). Surface hydrology and land cover characteristics for these two periods are derived by the analysis of aerial photographs, available in 1956, and high resolution remote sensing images in 2011. The major changes 20 identified are: 1) a partial crusting of isolated dunes, 2) an increase of drainage network density, 3) a marked decrease in vegetation with the non-recovery of tiger bush and vegetation growing on shallow sandy soils and 4) important changes in soil properties with shallow soil being eroded and giving place to impervious soils. These changes were implemented independently and in combination in the KINEROS-2 model. The simulations results show a significant increase of annual discharge between the-past‖ and the-present‖ case (p value < 0.001) 25 despite a slight overestimation of the past discharge. Mean annual discharges are estimated at 0.51×106 m 3 and 3.29×106 m 3 for past and present respectively. Modification of soil properties and vegetation cover (grassland and tiger bush thickets) are found to be the main factors explaining this increase, with the drainage network development contributing to a lesser extent. These synergistic processes explain the Sahelian paradox in the absence of land use change and could play a role in 30 other Sahelian watersheds where runoff increase has been also observed.
Les sociétés rurales face aux changements climatiques et environnementaux en Afrique de l’Ouest
Introduction Une conséquence inattendue de la sécheresse multi-décennale qui affecte le Sahel dep... more Introduction Une conséquence inattendue de la sécheresse multi-décennale qui affecte le Sahel depuis les années 1970 a été l’augmentation des écoulements d’eau en surface, conduisant à différents phénomènes rassemblés sous le terme de « paradoxe sahélien ». Ce paradoxe, qui peut se résumer par la formule lapidaire « moins de pluies, mais plus d’eau dans les rivières », est décrit par Descroix et al. (chap. 7, ce volume). La plupart des observations de ce paradoxe ont été effectuées en milieu ..
International Journal of Applied Earth Observation and Geoinformation, 2016
Monitoring turbidity and Surface Suspended Sediment Concentration (SSSC) of inland waters is esse... more Monitoring turbidity and Surface Suspended Sediment Concentration (SSSC) of inland waters is essential to address several important issues: erosion, sediment transport and deposition throughout watersheds, reservoir siltation, water pollution, human health risks, etc. This is especially important in regions with limited conventional monitoring capacities such as West Africa. In this study, we explore the use of Moderate Resolution Imaging Spectroradiometer data (MODIS, MOD09Q1 and MYD09Q1 products, red (R) and near infrared (NIR) bands) to monitor turbidity and SSSC for the Bagre Reservoir in Burkina Faso. High values of these parameters associated with high spatial and temporal variability potentially challenge the methodologies developed so far for less turbid waters. Field measurements (turbidity, SSSC, radiometry) are used to evaluate different radiometric indices. The NIR/R ratio is found to be the most suited to retrieve SSSC and turbidity for both in-situ spectoradiometer measurements and satellite reflectance from MODIS. The spatio temporal variability of MODIS NIR/R together with rainfall estimated by the Tropical Rainforest Measuring Mission (TRMM) and altimetry data from Jason-2 is analyzed over the Bagre Reservoir for the 2000-2015 period. It is found that rain events of the early rainy season (February-March) through mid-rainy season (August) are decisive in triggering turbidity increase. Sediment transport is observed in the reservoir from upstream to downstream between June and September. Furthermore, a significant increase of 19% in turbidity values is observed between 2000 and 2015, mainly for the July to December period. It is especially well marked for August, with the central and downstream areas showing the largest increase. The most probable hypothesis to explain this evolution is a change in land use, and particularly an increase in the amount of bare soils, which enhances particle transport by runoff.
A large part of the Sahel consists of endorheic hydrological systems, where reservoirs and lakes ... more A large part of the Sahel consists of endorheic hydrological systems, where reservoirs and lakes capture surface runoff during the rainy season, making water available during the dry season. Monitoring and understanding the dynamics of these lakes and their relationships to the ecohydrological evolution of the region is important to assess past, present and future changes of water resources in the Sahel. Yet, most of Sahelian watersheds are still ungauged or poorly gauged, which hinders the assessment of the water flows feeding the lakes and the overall runoff over their watershed. In this paper, a methodology is developed to estimate water inflow to lakes for ungauged watersheds. It is tested for the Agoufou lake in the Gourma region in Mali, for which in situ water height measurements and surface areas estimations by remote sensing are simultaneously available. A Height-Volume-Area (HVA) model is developed to relate water volume to water height and lake surface area. This model is combined to daily evaporation and precipitation to estimate water inflow to the lake, which approximates runoff over the whole watershed. The ratio between annual water inflow and precipitation increases over the last sixty years as a result of a significant increase in runoff coefficient over the Agoufou watershed. The method is then extended to derive water inflow to three other Sahelian lakes in Mauritania and Niger. No in situ measurements are available and lake surface areas estimation by remote sensing is the only source of information. Dry season surface area changes and estimated evaporation are used to select a suited VA relationship for each case. It is found that the ratio between annual water inflow and precipitation has also increased in the last 60 years over these watersheds, although trends at the Mauritanian site are not statistically significant. The remote sensing approach developed in this study can be easily applied to recent sensors such as Sentinel-2 or Landsat-8, to quantify the evolution of hydrological systems in ungauged Sahelian regions.
Contact: Gal Laëtitia – laetitia.gal@get.obs-mip.fr – Géosciences Environnement Toulouse (GET) Ob... more Contact: Gal Laëtitia – laetitia.gal@get.obs-mip.fr – Géosciences Environnement Toulouse (GET) Objectives : The Sahel is subject to a real hydrological paradox (decrease in rainfall but increase in surface water observed over the last 50 years). Our study focuses on a model-based approach (KINEROS 2) to better understand the hydrological mechanisms operating in non-cultivated Sahel and the changes that affected this area over the last century.
The Congo River Basin (CRB), located in the central region of Africa, is of particular importance... more The Congo River Basin (CRB), located in the central region of Africa, is of particular importance for regional and global climate and carbon studies. Being the second largest river basin after the Amazon, it is also the one with the most free-flowing rivers. However, despite these important characteristics, it has not attracted as much attention among the scientific communities as the Amazon Basin or other large tropical rivers in the world. Because of the lack of comprehensive and maintained in situ data networks over time, large-scale monitoring of hydroclimatic variables has not been properly conducted. In this context, near real-time observations of the CRB, such as water surface elevation (WSE) and river discharge, as well as understanding the impacts of climate change in a spatiotemporally distributed manner across the basin present a major challenge. In the last few years, however, the scientific community, supported by the leading operational organism in the CRB (the CICOS),...
This article reviews the state of the art in the use of space-borne observations for analyzing ex... more This article reviews the state of the art in the use of space-borne observations for analyzing extreme rainfall and flood events in Africa. Floods occur across many space and timescales, from very localized flash flood events to slow propagation of discharge peaks in large rivers. We discuss here how satellite data can help us understand the genesis and impacts of these flood events, monitor their evolution, and better constrain prediction models, thereby improving early warning and population protection. To illustrate these topics, we reanalyze major flood events that occurred in Niger, Mozambique, Central African Republic and Ivory Coast, using satellite information.
La région sahélienne est caractérisée, depuis la moitié du XXème siècle, par un déficit pluviomét... more 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...
revanche, la variation du régime des pluies, tend à diminuer le ruissellement au cours du temps d... more 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.
Rural societies in the face of climatic and environmental changes in West Africa, 2017
Introduction The hydrology of the Sudano-Sahelian strip has evolved rapidly in recent decades. We... more Introduction The hydrology of the Sudano-Sahelian strip has evolved rapidly in recent decades. West Africa has experienced strong climatic and environmental changes that have deeply modified the runoff/infiltration/evapotranspiration balance and, as a result, the conditions of flow formation. The observations and scientific progress that have enabled better understanding of the impact of environmental changes on the hydrological cycle can be summarised in the form of several key stages. Durin..
In the West African Sahel, two paradoxical hydrological behaviors have occurred during the last f... more 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.
Land surface processes play an important role in the West African monsoon variability. In additio... more Land surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff i...
Hydrology and Earth System Sciences Discussions, 2016
In the last decades the Sahel has witnessed a paradoxical increase in surface water despite a gen... more In the last decades the Sahel has witnessed a paradoxical increase in surface water despite a general precipitation decline. This phenomenon, commonly referred to as-the Sahelian paradox‖, is not completely understood yet. The role of cropland expansion due to the increasing food demand by a growing population has been often put forward to explain this situation for the cultivated Sahel. However, this hypothesis does not hold in pastoral areas 10 where the same phenomenon is observed. Several other processes have been suggested to account for this situation such as the degradation of natural vegetation following the major droughts of the 70ies and the 80ies, the development of crusted top soils, the intensification of the rainfall regime and the development of the drainage network. In this paper, a modeling approach is proposed to quantify and rank the different processes that could be at play 15 in pastoral Sahel. The KINEmatic EROSion model (KINEROS-2) is applied to the Agoufou watershed, in the Gourma region in Mali, which underwent a significant increase in surface runoff during the last 60 years. Two periods are simulated, the-past‖ case (1960-1975) preceding the Sahelian drought and the "present" case (2000-2015). Surface hydrology and land cover characteristics for these two periods are derived by the analysis of aerial photographs, available in 1956, and high resolution remote sensing images in 2011. The major changes 20 identified are: 1) a partial crusting of isolated dunes, 2) an increase of drainage network density, 3) a marked decrease in vegetation with the non-recovery of tiger bush and vegetation growing on shallow sandy soils and 4) important changes in soil properties with shallow soil being eroded and giving place to impervious soils. These changes were implemented independently and in combination in the KINEROS-2 model. The simulations results show a significant increase of annual discharge between the-past‖ and the-present‖ case (p value < 0.001) 25 despite a slight overestimation of the past discharge. Mean annual discharges are estimated at 0.51×106 m 3 and 3.29×106 m 3 for past and present respectively. Modification of soil properties and vegetation cover (grassland and tiger bush thickets) are found to be the main factors explaining this increase, with the drainage network development contributing to a lesser extent. These synergistic processes explain the Sahelian paradox in the absence of land use change and could play a role in 30 other Sahelian watersheds where runoff increase has been also observed.
Les sociétés rurales face aux changements climatiques et environnementaux en Afrique de l’Ouest
Introduction Une conséquence inattendue de la sécheresse multi-décennale qui affecte le Sahel dep... more Introduction Une conséquence inattendue de la sécheresse multi-décennale qui affecte le Sahel depuis les années 1970 a été l’augmentation des écoulements d’eau en surface, conduisant à différents phénomènes rassemblés sous le terme de « paradoxe sahélien ». Ce paradoxe, qui peut se résumer par la formule lapidaire « moins de pluies, mais plus d’eau dans les rivières », est décrit par Descroix et al. (chap. 7, ce volume). La plupart des observations de ce paradoxe ont été effectuées en milieu ..
International Journal of Applied Earth Observation and Geoinformation, 2016
Monitoring turbidity and Surface Suspended Sediment Concentration (SSSC) of inland waters is esse... more Monitoring turbidity and Surface Suspended Sediment Concentration (SSSC) of inland waters is essential to address several important issues: erosion, sediment transport and deposition throughout watersheds, reservoir siltation, water pollution, human health risks, etc. This is especially important in regions with limited conventional monitoring capacities such as West Africa. In this study, we explore the use of Moderate Resolution Imaging Spectroradiometer data (MODIS, MOD09Q1 and MYD09Q1 products, red (R) and near infrared (NIR) bands) to monitor turbidity and SSSC for the Bagre Reservoir in Burkina Faso. High values of these parameters associated with high spatial and temporal variability potentially challenge the methodologies developed so far for less turbid waters. Field measurements (turbidity, SSSC, radiometry) are used to evaluate different radiometric indices. The NIR/R ratio is found to be the most suited to retrieve SSSC and turbidity for both in-situ spectoradiometer measurements and satellite reflectance from MODIS. The spatio temporal variability of MODIS NIR/R together with rainfall estimated by the Tropical Rainforest Measuring Mission (TRMM) and altimetry data from Jason-2 is analyzed over the Bagre Reservoir for the 2000-2015 period. It is found that rain events of the early rainy season (February-March) through mid-rainy season (August) are decisive in triggering turbidity increase. Sediment transport is observed in the reservoir from upstream to downstream between June and September. Furthermore, a significant increase of 19% in turbidity values is observed between 2000 and 2015, mainly for the July to December period. It is especially well marked for August, with the central and downstream areas showing the largest increase. The most probable hypothesis to explain this evolution is a change in land use, and particularly an increase in the amount of bare soils, which enhances particle transport by runoff.
A large part of the Sahel consists of endorheic hydrological systems, where reservoirs and lakes ... more A large part of the Sahel consists of endorheic hydrological systems, where reservoirs and lakes capture surface runoff during the rainy season, making water available during the dry season. Monitoring and understanding the dynamics of these lakes and their relationships to the ecohydrological evolution of the region is important to assess past, present and future changes of water resources in the Sahel. Yet, most of Sahelian watersheds are still ungauged or poorly gauged, which hinders the assessment of the water flows feeding the lakes and the overall runoff over their watershed. In this paper, a methodology is developed to estimate water inflow to lakes for ungauged watersheds. It is tested for the Agoufou lake in the Gourma region in Mali, for which in situ water height measurements and surface areas estimations by remote sensing are simultaneously available. A Height-Volume-Area (HVA) model is developed to relate water volume to water height and lake surface area. This model is combined to daily evaporation and precipitation to estimate water inflow to the lake, which approximates runoff over the whole watershed. The ratio between annual water inflow and precipitation increases over the last sixty years as a result of a significant increase in runoff coefficient over the Agoufou watershed. The method is then extended to derive water inflow to three other Sahelian lakes in Mauritania and Niger. No in situ measurements are available and lake surface areas estimation by remote sensing is the only source of information. Dry season surface area changes and estimated evaporation are used to select a suited VA relationship for each case. It is found that the ratio between annual water inflow and precipitation has also increased in the last 60 years over these watersheds, although trends at the Mauritanian site are not statistically significant. The remote sensing approach developed in this study can be easily applied to recent sensors such as Sentinel-2 or Landsat-8, to quantify the evolution of hydrological systems in ungauged Sahelian regions.
Contact: Gal Laëtitia – laetitia.gal@get.obs-mip.fr – Géosciences Environnement Toulouse (GET) Ob... more Contact: Gal Laëtitia – laetitia.gal@get.obs-mip.fr – Géosciences Environnement Toulouse (GET) Objectives : The Sahel is subject to a real hydrological paradox (decrease in rainfall but increase in surface water observed over the last 50 years). Our study focuses on a model-based approach (KINEROS 2) to better understand the hydrological mechanisms operating in non-cultivated Sahel and the changes that affected this area over the last century.
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