Présentations Colloques

Due to constant demographic growth and agricultural growing demand for irrigation water, the inescapable use of groundwater has continuously increased during the last forty years in North African arid and semi-arid regions. Some of these areas are known to comprise worldwide and in many locations huge underground water reserves. This is the case of The North-Western Sahara Basin (NWSAS). **For the sake of integrated groundwater resource management, hydrogeologists and water managers have constantly been worried by the same question that is- what are the evaporative losses and the recharge rates of those huge aquifers?**In addition to the technical and the logistic constraints related to the nature of these regions, they are also characterised by extreme climatic conditions. Whereas conventional techniques often failed, isotopic tools have proven their efficiency in tackling groundwater issues. **The present paper examines the contribution of this methodology as applied to this basin. Water transfers through the unsaturated zone were investigated in order to compute steady-state groundwater recharge rates and evaporative losses.**Many sites have been investigated during the last twenty years- Béni-Abbès, Chott Chergui (semi arid area), Ouargla, El-Oued and Hassi Messaoud (arid areas) in Algeria and Tozeur, Dissa in Tunisia within the framework of separate studies. These investigations aimed at estimating evaporation rates based on unsaturated zone stable isotopes and chloride profiles. This was implemented making use of the deterministic model developed by Barnes and Allison (1982). The recharge and evaporation rates obtained for a 10 m unsaturated soil profile varied between 1 and 30mm a. These results were found in the same range as those obtained elsewhere in other arid and semi-arid locations of the world. The extrapolation of these rates to the scale of the whole Basin is sought as possible and can help the water managers of the involved countries to develop or refine appropriate models. **