A changing drainage network observed from remote sensing in the Sahel and its impact on groundwater

The African Sahel is considered one of the most endangered ecological zones worldwide. Land use in the African Sahel has changed dramatically along with rapid population increases since about World War II, resulting in increased pressures on land and water resources.\ud \ud Previous studies into the dynamics of the unconfined aquifer of South-West Niger have revealed that the watertable, despite ongoing drought and increased groundwater extraction, has risen in average by 3.5 m since the early 1960's, with an increase in the rate of rise during the past decade. These studies suggest that the water table rise could be due to changes in land- use, reasoning that intense vegetation clearing has modified the\ud hydraulic properties of the ground surface, resulting in increased surface runoff and indirect recharge.\ud \ud We used a time series of historical aerial photograph starting in the 1950’s and remote sensing techniques for accurate change detection of draina ge network. Mosaics of\ud historical aerial photographs provided good coverage over both large spatial and long temporal extents. They also provided high spatial resolution and enabled detailed changes in the drainage network to be mapped with unprecedented accuracy for the region. The density of the drainage network was found to have increased by 210% between 1950 and 1992. Profound modifications of the drainage network were also noticeable in the form of\ud better connectivity and creation of numerous ponds. The corresponding increase in groundwater levels indicates that the changes in land use and drainage network within the\ud last ~ 50 years has had a greater impact on water resources compared with on-going drought conditions