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Summary: Despite the strong reduction in rainfall observed after 1968, the water table of some endorheic areas in the Sahel has been found to be rising over the last several decades. It has been previously demonstrated that this is due to land use changes which have led to a severe increase in runoff and erosion. In such areas, the excess in runoff causes a strong increase in the number of ponds, their sizes and thus, their duration. Ponds have been identified as the main zones of deep infiltration of water. The aim of this study was to investigate whether other areas of the Sahelian region could also be defined as deep infiltration ones as well, and then, whether they were contributing to aquifer recharge. Soil water content was surveyed for five consecutive years (2004–2008) by implementing a set of measurement devices at different depths. The hydrologic water balance was monitored at stream flow gauge stations located upstream and downstream of two small endorheic catchments. The observed replacement of bush vegetation by crops and fallow areas led to the appearance of extended bare soil areas due to both aeolian and hydric erosion, triggering a strong reduction in soil infiltrability under millet fields and fallow lands as well as in the soil water holding capacity. It also resulted in the formation of a great number of gullies and sand sediment deposits in the endorheic areas. Measurements showed that sandy deposits correspond in fact to large areas of deep infiltration: tens of thousands of cubic meters of water infiltrated catchments of less than 1km2. Runoff decreased by up to 50% in the sandy deposit areas, while infiltration (close to 1300mmh−1) was observed up to depths of 10m. These factors would raise the water table and significantly modify the surface and sub-surface components of the water cycle. [Copyright &y& Elsevier]

Summary: Abundant information is available on West African drought and its hydrological and environmental impacts. Land-use and climatic changes have greatly modified the conditions of Sudanian and Sahelian hydrology, impacting the regime and discharge of the main rivers. Human pressure on the environment (significant increase in crops and disappearance of natural bushes and landscapes, for example) has led to severe soil crusting and desertification throughout Sahelian regions. Despite recent increases in rainfall, the drought has not ended, resulting in two different hydrological evolutions. In the Sudanian areas, stream flows have been reduced, sometimes as much as twice the rainfall reduction rate. In the Sahelian regions, runoff coefficients have increased to such a degree that discharges are increasing, in spite of the reduced rainfall. The main goal of this paper is to synthesize the recent advances in the Sahelian and Sudano-Sahelian West African hydrology. The other objectives are two fold: First, to discuss the “Sahelian Paradox” (the increase in runoff in most of the Sahel during the drought, at least during the 1968–1995 period, as described in the 1980s) and paradox of groundwater highlighted in the square degree of Niamey (the rise in water table levels in some endorheic areas during the same drought, evidenced in the 1990s), and second, to attempt to define the application of their respective geographical areas. The land-use changes act as a general factor of hydrological evolution of soils and basins, while some spatial factors explain the great variability in the response to environmental evolution, such as endorheism, geological context, latitudinal climate gradient, and local hydrodynamic behaviour of environment. This paper is literature-based, and incorporates current research advances in the field, as well as a prospective focused on resources and socio-economic impacts. [Copyright &y& Elsevier]

Summary: Among the three sites distributed along the West African latitudinal gradient in the AMMA-CATCH observation system, the experimental setup in the Niamey area of south-west Niger samples the cultivated Sahel environment, for hydrological, vegetation and land surface processes. The objective is to investigate relationships between climate, land cover, and the water cycle, in a rapidly changing semiarid environment. This paper first presents the main characteristics of the area, where previous research, including the EPSAT and HAPEX-Sahel experiments, had evidenced a widespread decadal increase in water resources, concurrently with severe drought conditions. The specifics of AMMA-CATCH research and data acquisition at this site, over the long-term (∼2001–2010) and enhanced (∼2005–2008) observation periods, are introduced. Objectives and observation strategy are explained, and the main characteristics of instrument deployment are detailed. A very large number of parameters – covering rainfall, vegetation ecophysiology, phenology and production, surface fluxes of energy, water vapour and CO2, runoff and sediment, pond water, soil moisture, and groundwater – were monitored at local to meso scales in a nested structure of sites. The current state of knowledge is summarized, connecting processes and patterns of variation for rainfall, vegetation/land cover, and the terrestrial hydrologic cycle. The central role of land use and of its spectacular change in recent decades is highlighted. This paper provides substantial background information that sets the context for papers relating to the south-west Niger site in this AMMA-CATCH special issue. [Copyright &y& Elsevier]