An international team has worked on the edge of Sahara to gather data on the ground and in the air, to be compared with imagery of the same region acquired by ESA satellites. The results will be used in support of an ambitious project to apply satellite remote sensing to improve monitoring and management of vast water aquifers concealed beneath the desert. High-resolution radar as well as hyperspectral optical imagery was acquired during flights across two test areas in southern Tunisia. Meanwhile ground teams precisely documented ground vegetation and terrain at sampling sites within these test areas, with samples taken to local laboratories for detailed analysis. And ESA’s Envisat, ERS-2 and Proba satellites acquired images of these sites around the same time.
The aim was to scale up the findings from the ground, and at the same time to use this ‘ground truth’ to calibrate satellite imagery with reality on the sandy arid ground – as well as seeing what can be learnt about the water beneath it. The Sahara has altered through the ages: during the last Ice Age, 10 000 years ago, there was savannah here with rivers, lakes and plentiful rains. That landscape has vanished now, but the rains from that period progressively percolated beneath the ground to be collected in layers of water-bearing rock known as aquifers.
Working with partners including African water agencies, ESA has commenced a project called Aquifer to develop satellite-derived products and services to support the sustainable management of ground water. Planned products include land-use and land-cover maps, change maps, surface water extent and dynamics, digital terrain models and estimates of water consumption and extraction. These required products were identified by the involved water agencies, in Tunisia specifically the Direction Générale des Ressources en Eau (DGRE) under the Ministère de l’Agriculture.