Remote Sensing and GIS : Applications for Groundwater Potential Assessment in Eritrea

Abstract: An integrated approach with remote sensing, GeographicInformation Systems (GIS) and more traditional fieldworktechniques was adopted to assess the groundwater potential inthe central highlands of Eritrea. Digitally enhanced colorcomposites and panchromatic images of Landsat TM and Spot wereinterpreted to produce thematic maps such as lithology andlineaments. The potential of the Advanced Spaceborne ThermalEmission and Reflection Radiometer (ASTER) data forlithological and lineament mapping was evaluated. Topographicparameters such as surface curvature, slope and drainagesystems were derived from digital elevation models and used tomap landforms. Digital elevation models (DEM) derived fromcontours and acquired in the Shuttle Radar Topographic Mission(SRTM) were compared in relation to location, drainage networksand lineament extraction. Fracture patterns and spacing weremeasured in the field in different rock types and compared withlineaments. Selected springs and wells were visited to studytheir topographic and hydrogeological setting. Well logs,pumping tests, water table depth in dry and wet season as wellas location of wells were collected. All thematic layersincluding hydrogeological data were integrated and analysed ina geographic information system. A groundwater potential mapwas generated and compared with yield data. Groundwaterrecharge was estimated based on water level fluctuations inlarge dug wells and chloride mass-balance method.Principal component analysis for rock type mapping providedbetter results with ASTER than with Landsat TM data. DEM datapermitted to create detailed landform maps useful torgroundwater potential assessment. DEM derived from SRTM dataare better for detection of drainage systems and linearfeatures than those derived from contours. Most of the fracturesystems corresponding to lineaments are either extensionalrelated to normal faults and dykes, or shear fractures relatedto strike-slip faults. N-S, NW-SE, WNW-ESE, NE-SW and ENE-WSWare dominant fracture orientations with often very densespacing. High yielding wells and springs are often related tolarge lineaments and corresponding structural features such asdykes. Typically wells and springs in basaltic areas havehigher yields mainly due to primary joints. Young alluvialsediments with high permeability and deeply weathered rocklayers are important for water supply especially in hydraulicconnection with fracture systems in crystalline bedrock.Groundwater potential zones demarcated through the model are inagreement with bore well yield data. The spatial distributionof groundwater potential zones shows regional patterns relatedto lithologies, lineaments, drainage systems and landforms.Recharge rates of 10 - 50 mm were estimated in this region. Theresults demonstrate that the integration of remote sensing,GIS, traditional fieldwork and models provide a powerful toolin the assessment and management of water resources anddevelopment of groundwater exploration plans.Key words:Remote sensing, Geographic InformationSystems, groundwater, geomorphology, Digital elevation model,lithology, hard rock, lineament, structures, hydrogeology,Eritrea