Abstract
In the Sahara and the Sahel, groundwater is a limited and indispensable resource for pastoral livestock farming. The daily life and work of the herders are organised around the location of the wells and the depth of the water table. To ensure the sustainable development of these regions, it is therefore essential to develop accurate piezometric maps, even in the areas that are most difficult to access.Thanks to high-resolution satellite images, the tracks made by cattle, goats and camels in the Sahara and Sahel could become a key indicator of the depth of the water table.In the northern Sahel, pastoralists water their livestock from deep wells. To draw water, they hitch oxen or camels to a rope whose length is an accurate measure of the depth of the piezometric surface of the water table. When pulling on this rope, the animals leave deep tracks on the ground that can be observed and measured on satellite images.We have developed a remote sensing technique that allows us to (a) identify pastoral wells, (b) isolate the tracks left by the animals used to draw water, and (c) use these animal tracks to estimate the water depth.After carefully calibrating the method, we were able to use open data (Landsat) and satellites images freely accessible data thanks to Google Earth Pro (SPOT and Worldview) to draw up, in just a few weeks, the piezometric map of a large aquifer (200,000km2) that is not easily accessible by other means due to the prevailing insecurity that has persisted in this part of the Sahel region for several years. This same method was then subsequently tested and validated on two other aquifers, one in Nigeria and one in Niger.
Highlights
Since 2015, we have supervised two major European Union-funded drilling programmes in the northern part of the Chadian Sahel: the project PAEPA (Programme d’Eau Potable et d’Assainissement - 1100 boreholes, including 264 in the study area) and the project RESTE (Résilience et Emploi au Lac Tchad - 200 boreholes in the study area)
The deeper wells are impressive structures of sometimes more than 100 m deep. These are the wells for which satellite images will be used to measure the depth of the water table
The low resolution images (30 m - Landsat 7 and 8 satellites) are useful for quickly locating some of the wells over very large areas. These images cannot be used to discern the tracks made by draught animals or to assess the depth of the piezometric surface
Summary
Since 2015, we have supervised two major European Union-funded drilling programmes in the northern part of the Chadian Sahel: the project PAEPA (Programme d’Eau Potable et d’Assainissement - 1100 boreholes, including 264 in the study area) and the project RESTE (Résilience et Emploi au Lac Tchad - 200 boreholes in the study area). It is difficult and costly to carry out measurement campaigns in these areas because they are remote, the trails through sand dunes are difficult to follow and some areas (Kanem, Lake) are extremely insecure because of Boko Haram attacks This lack of data raises difficulties for sustainable groundwater management because it is impossible to establish the water balance of the aquifer and assess its recharge without a good piezometric map. These are the wells for which satellite images will be used to measure the depth of the water table Most of these deep wells are dug in the loose sedimentary formations (sands) of the Pliocene and Quaternary periods.
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