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 large confined aquifers of well-known sedimentary basins such as those of the Sahara desert (North Africa) and Australia (Great artesian basin).
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, it is surprisingly in such precipitation-poor environments that isotopic tools have proven their efficiency in tackling groundwater issues. Arid zone isotope hydrology has gained more and more importance during the last three or four decades.
The present paper review the contribution of this methodology as applied to the groundwaters occurring in the north western part of the Sahara sedimentary basin (North Africa). 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, Ouargla, El-Oued 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 evaporation and recharge rates obtained for a 10 m unsaturated soil profile varied between 1 and 40 mm/y. The main objective of the present study, is to gather all these data and to examine how they may be interpreted in terms of recharge rate, evaporation losses can help the water managers of the involved countries to develop or refine appropriate models. This should facilitate the implementation of a trans-boundary integrated management of the shared resources.
Keywords: stable isotopes, chloride, aquifer, modelling, Evaporation, Recharge, Sahara