Regional hydrogeology and groundwater budget modeling in the arid Middle Drâa Catchment (South-Morocco)

Abstract

This study deals with the regional hydrogeology in Middle Drâa Catchment (MDC) in South-Morocco that features 15,000 km² of semi-arid to hyper-arid conditions and a heterogeneous geological setting. The MDC covers the southern flank of the central Anti-Atlas Mountains reaching the southerly adjacent Saharan Foreland. Pasture and migration are of significant meaning for existence. Important agriculture and settlement concentrate mainly in six date palm oases along the Wadi Drâa relying on both stream flow and uncontrolled groundwater pumping for irrigation. The dominant groundwater exploitation taps the most important alluvial Drâa aquifers that relate to the six Drâa oases. The Drâa aquifers form an interrupted chain of shallow groundwater reservoirs embedded in a hard rock aquitard system veined by a tributary wadi network. <br /> The pressure of climate and global change particularly demands the analysis of the groundwater system, the quantification of water availability and scenario projections to derive options of adaptation and mitigation. Accordingly, results of the analysis of groundwater level data, hydrogeochemical and hydrogeological information lead to the characterization of the current state of the groundwater system and the development of the groundwater budget model BIL. The BIL model simulates the lumped annual groundwater availability and response at the Drâa aquifers.<br /> Lithological information from mapping surveys and bore log descriptions form the basis of a hydrofacies framework refining the existing concept of the aquifer system in the MDC. The groundwater level response of the Drâa aquifers relates to the re-interpreted distribution of specific yield values. The recent recharge of the Drâa aquifers depends mostly on transmission losses from the regulated inflow from the Upper Drâa Catchment to the Wadi Drâa. Indirect re-charge from floods generating after intense rainfall within the MDC is another source of aquifer replenishment. The analysis of inorganic groundwater composition and stable isotope signature verifies the interpretation of the aquifer system and the main groundwater flow paths. The distribution of the hydrochemical facies and the state of hydrogeochemical evolution hint on significant influence of groundwater pumping for irrigation. <br /> Based on the preceding analysis, items of the groundwater balance are pre-processed for each Drâa aquifer individually considering groundwater discharge from one aquifer to another. Ac-cordingly, the BIL model assesses the annual groundwater budget of the Drâa aquifers for the 33 year period 1974-2006. The model results are highly sensitive to changes in indirect recharge from stream flow infiltration, aquifer properties and irrigation-related parameters. The plausibility tests of the model results reveal satisfying accordance with observed piezometric data as available. <br /> So, scenarios of climate and global change are analyzed using the BIL model. As climate change has a significant impact on the groundwater availability of the Drâa aquifers and global change even worsens the situation, options of groundwater management are derived from the hydro-geological analysis and the groundwater budget modeling.