Modélisation

The NWSAS project has made it possible to improve geological and hydrogeological knowledge of the basin as a whole, thanks to recent surveys and new hydrogeological prospects, in particular, a historical fifty-year study (1950-2000) of the piezometry (the water level coast), water salinity, and its exploitation.

withdrawals by drilling in the NWSAS, in the year 1970 and 2000

The results of this sharp knowledge of the basin's hydrogeology is a schematization of the aquifers with a view to realizing a mathematical model. The Saharan basin is a large multilayered, sedimentary entity. The adoption of a simultaneous representation of the collective aquifers - aquiferous and semipermeable - makes it possible to account for the hydraulic and chemical connections and exchanges among all of the basin's aquifers and thus, the performance of the system in the medium and long terms.

Management with full factual knowledge of the Aquifer System assumes the availability of a mathematical model that allows for the carrying out of simulations and the formulation of predictions. The reference period chosen for the adjustment was the historical period 1950-2000, with the situation assessed in 1950, as an initial condition.

A reference pattern, named a scenario zero was defined. It consists in holding constant the withdrawals from drilled wells carried out in the year 2000, and calculating the system's corresponding evolution to the year 2050.

Many patterns are defined

• In Algeria, two patterns:
  • A so called "strong" representing an additional withdrawal of 101 m3/s, which would carry Algerian withdrawals from 42 to 143 m3/s between the year 2000 and the year 2030;
  • A so-called "weak" assumption for an additional withdrawals of 62 m3/s, which would carry the withdrawals from 42 to 104 m3/s.
• In Tunisia: the contemplated pattern anticipates that the savings realized from improvement in the efficiency of irrigation will compensate for the additional demand of the new irrigated perimeters, which corresponds to the maintenance of the present withdrawals.
• In Libya: the exploratory simulations concern two programmes of the Great Manmade River Project (GMRP): the pumping field of Ghadames-Derj, with an additional flow of
  90 km3 /year, and the collecting field of Djebel Hassaounah.

Results of the Exploratory Simulations:

Scenario Zero:

for the CI: the scenario zero will involve important drawdowns more than 40 meters within the Algerian Sahara lower part; in Tunisia, they are approximately 20 to 40 meters around the Chott Fedjej; in Libya, the drawdowns back are approximately 25 meters.

for the CT: in Algeria and in Tunisia, the drawdowns exceed 30 meters around the chotts; in Libya they reach 60 meters. The disappearance of all artesian flow in the Algerian-Tunisian chotts region, with the risk of waters intrusion from the chotts recharge into the CT's aquifer and probable salt contamination, is notable. From this perspective, the continuation of the current rhythm constitutes a major potential danger within the region.

Scenario "Strong hypothesis":
With respect to the CI, the drawdowns are 300 to 400 meters within the Algerian Sahara lower part, with the complete disappearance of artesian flow; Libya has not been affected by this pattern; with respect to Tunisia, the drawdowns are from 200 to 300 meters and the disappearance of artesian wells and the Tunisian outlet are notable. Concerning the CT, there is no effect in Libya, there are no important drawdown in Algeria, and the chotts are in the position of resupply.

Scenario "weak hypothesis":
In Algeria, as well as in Tunisia, the effects are very strong and quite unacceptable with respect to the CI, as well as the CT.

Libyan scenarios of the GMRP:
In Ghadames, the drawdowns in the CI are 100 meters to the collecting field, around 50 meters within the deep southern region of Tunisia and in Deb Deb, Algeria. As for the collecting field of the jebel Hassaounah, its impact on the CI remains negligible.

These exploratory simulations have highlighted the harmful effects and the risks to which water resources in this basin are exposed. Continued exploitation of the CI and CT aquifers will require the minimizing and management of these risks, which may be summarized, as follows:

1. the disappearance of artesian flow
2. excessive drawdowns in pumped wells
3. the drying up of Tunisian outlet
4. excessive interferences of drawdowns among countries
5. potential re-supply by the Chotts.

Research of New Patterns:

At the completion of the exploratory simulations, the adopted principle has been to dismiss the research of development patterns based solely upon predictions of the demand for water and to seek the building of patterns having a hydraulic basis, founded upon NWSAS output capacities and minimizing the identified risks of harmful effects, at the sites as close as possible to the places where present or future demand might be expressed, without refraining from the prospecting of favorable sectors that would be distant but could prove to be favorable for exportation. The first stage for such a process has consisted in making inventory of all the potential sites for pumping. The NWSAS Digital Model, which is committed to such a function, has been used to simulate the newly identified patterns

Control of the risks and durable management of the NWSAS:

The simulations concerning the future and that have been carried out on the NWSAS Model have highlighted the most vulnerable regions:

1. with respect to the Artesian Basin of the CI aquifer, the anticipated additional drawdowns will exceed 100 meters; verification is easy there: deep drillings and very few in number.
2. the flow, present and future, of the CI's Tunisian Outlet is of great importance, as it contributes to the supply of the Djeffara coastal aquifer which itself has been heavily exploited. now, this flow from the Tunisian outlet will have to be decreased, even if the withdrawals at their current level must be blocked.
   
   
   
   
   
   
3. the CI aquifer, the Ghadames Basin presents risks of excessive folding back, drawdowns of approximately 200 meters; however, these risks occur in the medium term, and from a technical standpoint, their controlling is easy.
4. sector most exposed is the Algerian-Tunisian chotts basin in the CT. This is the region where this aquifers most vulnerable. It is there where the strongest density in population can be found, where the pressure on resources will be the strongest. The calculations made on the Model have clearly shown that the simple continuation of the present rates of withdrawals, by the year 2050, would bring about additional drawdowns of approximately 30 to 50 meters on each of the two aquifers, with respect to all of the four interdependent sectors - the Oued Rhir, the Souf, the Djerid, and the Nefzaoua. Such a situation would be unacceptable for the Complex Terminal: the risk of the chott's water percolation toward the layer would be inevitable for the latter in terms of salinity. The simple continuation of the existing rates, at least within the CT, thus, would be completely unacceptable for the Chotts region. There, the reduction of withdrawals must be seriously contemplated as a plausible pattern and preparation for the same must be made going forward.
5. On the Khoms/Zliten coast, if the simulated pattern of filling shortages satisfies the need had to be carried out, the risk of sea invasion by the year 2050 would be seriously harmful to the Complex Terminal.
6. At Ferjan, in the CT aquifer, additional drawdowns are anticipated to exceed 50 meters.
7. Moreover, one of the results of the investigations performed has allowed for checking as to the possibility of bringing the level of exploitation through drilling of the NWSAS up to 7.8 billion m3/year by the year 2050. The reaching of such a level of development can be done only at the cost of dispersing new fields of exploitation: 80% of the additional withdrawals will have to be done within distant areas: the CI's Western Basin and the CT's Oued Mya in Algeria. This will provide a total exploitation, by country, of 6.1 billion m3/year in Algeria, 0.72 billion m3/year in Tunisia, and 0.95 billion m3/year in Libya.
   
   This possibility would cause exploitation of the NWSAS to climb to a level equivalent to eight times its renewable resources. Such an operation is realizable only by considerable drawing upon the system's reserves. Nonetheless, the necessity of confirming the results obtained must be stressed: in spite of the progress realized by the NWSAS project, uncertainties remain as to knowledge of the system, which uncertainties will require the undergoing of new investigations

The combined exercise of hydrogeological knowledge and the model makes it possible to reach realistic conclusions as to the capacities of the NWSAS to supply appreciable quantities of water while minimizing risks concerning the resource. The results obtained show that it is advisable to manage this resource jointly. The intention of planning this joint use has been advocated by the OSS since the launching of the project: to promote a basin consciousness and to implement a "dialogue mechanism."

Three regional models are being developped:

• Djeffara
• Occidental Basin
• North of Chotts