Modflow utilization for the Saharan aquifers management M. Zammouri September 2013

Modfow is a modular finite difference groundwater model code developed by the US Geological Survey's (McDonald and Harbaugh, 1988) Based on the Modflow code, several software (Visual Modflow; Processing Modflow, GMS etc.) were developed:  to make Modflow convivial  Integration of pre- and post-processing  Other codes were coupled to Modflow : MT3D for solute transport modeling,  PEST for optimization etc. - Development of groundwater flow : the North Western Sahara Aquifer System (SASS) - Development of transport model : Complexe Terminal in Fefzawa oases region (Tunisian South)

Système Aquifère du Sahara septentrional SASS (OSS, 2003) Groundwater resources of this basin are shared by three countries

Groundwater withdrawals evolution Algeria Libya Tunisia Total In 2000 : 2.2 Billion m 3 /y In 2000 : 0.9 Billion m 3 /y CT : 1.3 Billion m 3 /y Algeria: 49% Tunisia: 34% Libya: 17% Algeria: 78% Tunisia: 10% Libya: 12%

Negative effects from groundwater mining are nowadays observed : - Strong drawdown reaching 100 m and 40 m in the CI and CT aquifers, respectively - Vanishing of the natural springs - Degradation of the C.T groundwater quality A numerical groundwater flow model of the whole basin (SASS model) was developed to asses the impact of the long-term ( ) application of existing and planned extraction projects on drawdown. The SASS modeling study is piloted by the “Observatoire du Sahara et du Sahel (OSS)” in Calibration for the period was carried out in order to adjust geological and hydrological system parameters. Since the model calibration was considered satisfactory, the groundwater simulation model was extended to the year 2050 with various management alternatives modeled according to the planned extraction projects of Algeria, Libya and Tunisia

CI : 55 Potential pumping fields Volume (Billion m 3 /year)2000SIM-8total Algeria Tunisia0.1 Libyea Total

CT : 33 potential pumping fields Volume (Billion m 3 /year)2000SIM-5total Algeria Tunisia Libya Total

Inflows (Billion m 3 /year) Reservoir depletion Recharge0.3 Leakage from the Cambro- Ordovician 0.1 Exchange with the Turonian aquifer Total Outflows (Billion m 3 /year)0.0 Evaporation in the sabkhas0.0 Gulf of Sirte0.0 Tunisian outlet (outflow to Tunisian Jefara plain) Foggaras Pumping Exchange with the Turonian aquifer Total

Drawdowns in

Inflows (Billion m 3 /year) Reservoir depletion Recharge0.6 Exchange with the Turonian aquifer Total Outflows (m 3 /s)0.0 Evaporation in the chotts and sabkhas Gulf of Sirte0.0 Djerid, Nefzawa and Ain Tawergha springs Pumping Exchange with the Turonian aquifer 0.1 Total

Drawdowns in

In Tunisia, a rise in the groundwater salinity has been observed as consequence of increasing abstraction from the CT aquifer during the last few decades.

Distribution of TDS increase (mg/l) over the period

Conceptualization of the three different salinization processes: infiltration of brine from Chott Upconing of water from the CI/Turonian aquifer salinization by agricultural drainage water

A transport model was built by using the transport model MT3DMS. The aim was twofold: to crossvalidate conceptualization by the comparison of the historical salinity development with that calculated. to get an understanding of the future relevance of the three salinization mechanisms. The simulation of planned extraction projects predicts a worsening of the present situation. Maintenance of the present abstraction regime will not reduce or stop the salinity increase. The transport model was calibrated over the period After calibration, Control measures to limit CT salinization would include the following. Planned extraction projects in the south of the Nefzawa region have to be revised. Increasing irrigation efficiency Implementation of effective drainage measures