Core Ideas: AMMA‐CATCH is a long‐term critical zone observatory in West Africa. Four sites sample the sharp ecoclimatic gradient characteristic of this region. Combined measurements of meteorology, water, and vegetation dynamics began in 1990. Intensification of rainfall and hydrological cycles is observed. The strong overall re‐greening may hide contrasted changes. West Africa is a region in fast transition from climate, demography, and land use perspectives. In this context, the African Monsoon Multidisciplinary Analysis (AMMA)–Couplage de l'Atmosphère Tropicale et du Cycle eco‐Hydrologique (CATCH) long‐term regional observatory was developed to monitor the impacts of global change on the critical zone of West Africa and to better understand its current and future dynamics. The observatory is organized into three thematic axes, which drive the observation and instrumentation strategy: (i) analyze the long‐term evolution of eco‐hydrosystems from a regional perspective; (ii) better understand critical zone processes and their variability; and (iii) meet socioeconomic and development needs. To achieve these goals, the observatory has gathered data since 1990 from four densely instrumented mesoscale sites (∼10 ⁴ km ² each), located at different latitudes (Benin, Niger, Mali, and Senegal) so as to sample the sharp eco‐climatic gradient that is characteristic of the region. Simultaneous monitoring of the vegetation cover and of various components of the water balance at these four sites has provided new insights into the seemingly paradoxical eco‐hydrological changes observed in the Sahel during the last decades: groundwater recharge and/or runoff intensification despite rainfall deficit and subsequent re‐greening with still increasing runoff. Hydrological processes and the role of certain key landscape features are highlighted, as well as the importance of an appropriate description of soil and subsoil characteristics. Applications of these scientific results for sustainable development issues are proposed. Finally, detecting and attributing eco‐hydrological changes and identifying possible regime shifts in the hydrologic cycle are the next challenges that need to be faced.

In 2017, diarrheal diseases were responsible for 606 024 deaths in Sub-Saharan Africa. This situation is due to domestic and recreational use of polluted surface waters, deficits in hygiene, access to healthcare and drinking water, and to weak environmental and health monitoring infrastructures.Escherichia coli(E.coli) is an indicator for the enteric pathogens that cause many diarrheal diseases. The links betweenE.coli, diarrheal diseases and environmental parameters have not received much attention in West Africa, and few studies have assessed health risks by taking into account hazards and socio-health vulnerabilities. This case study, carried out in Burkina Faso (Bagre Reservoir), aims at filling this knowledge gap by analyzing the environmental variables that play a role in the dynamics ofE.coli, cases of diarrhea, and by identifying initial vulnerability criteria. A particular focus is given to satellite-derived parameters to assess whether remote sensing can provide a useful tool to assess the health hazard. Samples of surface water were routinely collected to measureE.coli, enterococci and suspended particulate matter (SPM) at a monitoring point (Kapore) during one year. In addition, satellite data were used to estimate precipitation, water level, Normalized Difference Vegetation Index (NDVI) and SPM. Monthly epidemiological data for cases of diarrhea from three health centers were also collected and compared with microbiological and environmental data. Finally, semi-structured interviews were carried out to document the use of water resources, contact with elements of the hydrographic network, health behavior and condition, and water and health policy and prevention, in order to identify the initial vulnerability criteria. A positive correlation betweenE.coliand enterococci in surface waters was found indicating thatE.coliis an acceptable indicator of fecal contamination in this region.E.coliand diarrheal diseases were strongly correlated with monsoonal precipitation, in situ SPM, and Near Infra-Red (NIR) band between March and November. Partial least squares regression showed thatE.coliconcentration was strongly associated with precipitation, Sentinel-2 reflectance in the NIR and SPM, and that the cases of diarrhea were strongly associated with precipitation, NIR,E.coli, SPM, and to a lesser extent with NDVI. Moreover,E.colidynamics were reproduced using satellite data alone, particularly from February to mid-December (R²= 0.60) as were cases of diarrhea throughout the year (R²= 0.76). This implies that satellite data could provide an important contribution to water quality monitoring. Finally, the vulnerability of the population was found to increase during the rainy season due to reduced accessibility to healthcare and drinking water sources and increased use of water of poor quality. During this period, surface water is used because it is close to habitations, easy to use and free from monetary or political constraints. This vulnerability is aggravated by marginality and particularly affects the Fulani, whose concessions are often close to surface water (river, lake) and far from health centers.

Small water bodies play a pivotal role in Sahel, as a critical source of water for livestock and people, and providers of important ecosystems services. Monitoring, modeling, and better understanding their hydrological behavior is, therefore, a key issue. The future Surface Water and Ocean Topography (SWOT) satellite mission will bring a resolution and spatial coverage breakthrough, allowing the estimation of water levels and volumes in small water bodies worldwide. This paper assesses the potential of SWOT for monitoring water volumes in Sahelian ponds and lakes. This is done by analyzing SWOT-like synthetic data produced using a SWOT simulator developed by NASA-JPL. For the Agoufou lake, water levels were retrieved with an accuracy better than 4 cm, while slightly worse results were obtained for the Zalam-Zalam lake, that has a more elongated shape. In addition, data from the global precipitation mission dual-frequency precipitation radar have been also employed to investigate the backscattering coefficient variability in the same radar frequency band (Ka-band) as SWOT. We have found that, in the study region, the contrast between water and land, dependent on soil type, soil moisture, and wind conditions, is sometime quite small which can be challenging for water masks estimation. Overall, the first application of the SWOT simulator over the Sahel has shown the good potential of SWOT for monitoring the seasonal variability of water levels and volumes in this region.

The Sahelian region is characterized by significant variations in precipitation, impacting water quantity and quality. Suspended particulate matter (SPM) dynamics has a significant impact on inland water ecology and water resource management. In-situ data in this region are scarce and, consequently, the environmental factors triggering SPM variability are yet to be understood. This study addresses these issues using remote sensing optical data. Turbidity and SPM of the Agoufou Lake in Sahelian Mali were measured from October 2014 to present, providing a large range of `values (SPM ranging from 106 to 4178 mg/L). These data are compared to satellite reflectance from Landsat (ETM+, OLI) and MODIS (MOD09GQ, MYD09GQ). For each of these sensors, a spectral band in the near infrared region is found to be well suited to retrieve turbidity and SPM, up to very high values (R² = 0.70) seldom addressed by remote sensing studies. The satellite estimates are then employed to assess the SPM dynamics in the main lakes and ponds of the Gourma region and its links to environmental and anthropogenic factors. The main SPM seasonal peak is observed in the rainy season (June to September) in relation to precipitation and sediment transport. A second important peak occurs during the dry season, highlighting the importance of resuspension mechanisms in maintaining high values of SPM. Three different periods are observed: first, a relatively low winds period in the early dry season, when SPM decreases rapidly due to deposition; then, a period of wind-driven resuspension in January‒March; and lastly, an SPM deposition period in April–May, when the monsoon replaces the winter trade wind. Overall, a significant increase of 27% in SPM values is observed between 2000 and 2016 in the Agoufou Lake. The significant spatio-temporal variability in SPM revealed by this study highlights the importance of high resolution optical sensors for continuous monitoring of water quality in these poorly instrumented regions.

Au cours des années 1970 et 1980, le Burkina a connu plusieurs sécheresses. Ces dernières se sont déroulées dans le contexte d’une explosion démographique. Les régions les plus touchées ont été celles du Nord sous climat sahélien. Des situations de famine ont été observées, des troupeaux ont été décimés. Les populations, principalement peules, n’ont pas eu d’autre choix que de migrer en direction d’espaces moins peuplés où leurs troupeaux disposeraient de pâturages en quantité suffisante. Ils se sont déplacés vers le sud du Plateau Central et l’Ouest du Burkina Faso. Leur arrivée a eu pour conséquence, d’une part, de créer une concurrence dans l’accès à la terre en pleine période de Réforme Agraire et Foncière (années 1980) et, d’autre part, de voir l’apparition de conflits principalement lors de l’hivernage après que les champs aient été semés et récoltés. Au vu de l’amplification des conflits et des problèmes de gestion de la ressource forestière, les autorités se sont trouvées dans l’obligation de réfléchir à la mise en place de solutions. L’une d’entre elles est la délimitation d’espaces à vocation pastorale. Ainsi, la zone pastorale de la Doubégué a été créée pour répondre à ces difficultés. À partir de cet exemple, il est intéressant de s’interroger d’une part sur la pertinence de l’instauration de ce type d’espace, et d’autre part sur l’atteinte des objectifs (résolution des conflits et préservation de l’environnement).