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Delineation of superficial alluvial aquifers using DEM geoprocessing : application to semi-arid northeast Brazil

Congress: 2008
Author(s): Pascal Kosuth, Guilhem Daly(1), Christian Feau(2), Julien Burte (3), Edouardo Savio Martins(3)
(1) Cemagref, UMR TETIS (Cemagref – CIRAD – ENGREF AGROPARISTECH), Maison de la Télédétection, 500 rue Jean-François Breton, 34093 Montpellier, Cedex 5 France ; Tel. (33) 04 67 54 87 54 ; Fax (33) 04 76 54 87 00 ; email : pascal.kosuth@teledetection.fr;

Keyword(s): Aquifer, DEM, geoprocessing, Brazil, hydrology, alluvial
AbstractWater stored in superficial alluvial aquifers of northeast Brazil (Nordeste Brasileiro) represent valuable resources for rural populations : it can be used for community and family water supply, for cattle water supply and for small farm irrigation. However, this water resource is still under-used and poorly known both in terms of its geographic distribution, extension and storage capacity, and in terms of its spatio-temporal dynamics under the coupled influence of natural processes (climate, hydrodynamics) and human activities (water extraction, upstream storage in reservoirs, land cover change). The objective of this work was to develop a method for the delineation of these superficial alluvial aquifers, in order to provide a reliable basis for their inventory, characterization, analysis and further management. The method is based on the geoprocessing of Digital Elevation Models (DEM) guided by the geomorphological properties of these aquifers : mainly flat areas along the river network. It has been designed, developed, tested and tuned on a pilot area, the rio Forquilha river basin (221 km²; 5°17’ S, 39°30’ W) and further implemented on the overall rio Banabuiu river basin (14244 km²; 5°20’ S, 38°55’ W). Geoprocessing of the DEM allows to delineate the basin hydrographic network, and establish a buffer zone around it, whose size depends, for each river network segment, on the upstream watershed area and on the river longitudinal and lateral slopes. Superficial alluvial aquifer limits are then extracted from within the buffer zone based on a maximum slope threshold. The method allowed to quantify the pilot area aquifer (Forquilha river alluvial aquifer : area 0.59 km²) with a spatial accuracy that was evaluated as very satisfactory taking into account both the uncertainty on the data (SRTM DEM; in situ delineation) and the objective of the study (aquifer inventory at regional scale). Implementation on the Banabuiu river basin led to a primary estimation of the superficial alluvial aquifer area of 60 km², distributed amongst more than 60 different aquifers. The corresponding water storage capacity was estimated to be ~36 Mm3, which can be compared to the mean annual surface runoff (36mm/year, i.e. 513 Mm3) and the reservoir storage capacity (2000 Mm3) on the basin. Current limitations of the method appear in flat lowland areas where aquifer area tends to be overestimated. Further works are addressing the use of remote sensing both to help in a more precise delineation of these aquifers and to analyse associated water and land use.
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