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Modflow utilization for the Saharan aquifers management
M. ZammouriSeptember 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)
Morocco
AlgeriaTunisia
Libya
600 0 600 1200 Kilometers
Groundwater resources of this basin are shared by three countries
Groundwater withdrawals evolution
Algeria
Libya
Tunisia
Total
Complexe Terminal
0
10
20
30
40
50
1950 1960 1970 1980 1990 2000
Déb
it e
n m
3
/s
Continental Intercalaire
0
5
10
15
20
25
30
1950 1960 1970 1980 1990 2000
Déb
it e
n m
3
/s
In 2000 : 2.2 Billion m3/y
In 2000 : 0.9 Billion m3/y
CT : 1.3 Billion m3/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 (2000-2050) 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 2003.
Calibration for the 1950 - 2000 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 m3/year) 2000 SIM-8 total Algeria 0.7 3.7 4.4Tunisia 0.1 0.1 0.1Libyea 0.1 0.3 0.4Total 0.8 4.1 4.9
CT : 33 potential pumping fields
Volume (Billion m3/year) 2000 SIM-5 total Algeria 0.6 1.0 1.7Tunisia 0.4 0.1 0.5Libya 0.2 0.4 0.6Total 1.3 1.5 2.8
0
1000
2000
3000
4000
5000
6000
1950 2000 2050
Volu
me
(106
m3 /
year
)
Scenario CI8
Pumping
Recharge
0
50
100
150
200
250
1950 2000 2050
Volu
me
(106
m3 /
year
)
Scenario CI8
Algerain springs "Foggaras"
Outflow to Jefara
1950 2000 2050Inflows (Billion m3/year)
Reservoir depletion - 0.7 4.5
Recharge 0.3 0.3 0.3Leakage from the Cambro-Ordovician
0.1 0.1 0.1
Exchange with the Turonian aquifer
0.0 0.0 0.1
Total 0.4 1.1 5.0Outflows (Billion m3/year) 0.0 0.0 0.0
Evaporation in the sabkhas 0.0 0.0 0.0
Gulf of Sirte 0.0 0.0 0.0
Tunisian outlet (outflow to Tunisian Jefara plain)
0.1 0.0 0.0
Foggaras 0.1 0.1 0.0Pumping 0.0 0.8 4.9Exchange with the Turonian aquifer
0.1 0.1 0.0
Total 0.4 1.1 5.0
Drawdowns in
0
20
40
60
80
100
120
1950 2000 2050
Volu
me
(106
m3/
year
)
Scenario CT5
Pumping
Recharge
0
500
1000
1500
2000
2500
3000
3500
4000
1950 2000 2050
Volu
me
(106
m3/
year
)
Scenario CT5
Pumping
Recharge
1950 2000 2050
Inflows (Billion m3/year)
Reservoir depletion - 0.8 1.9Recharge 0.6 0.6 0.6Exchange with the Turonian aquifer
0.2 0.2 0.4
Total 0.7 1.6 3.0
Outflows (m3/s) 0.0 0.0 0.0
Evaporation in the chotts and sabkhas
0.3 0.1 0.0
Gulf of Sirte 0.0 0.0 0.0Djerid, Nefzawa and Ain Tawergha springs
0.1 0.1 0.0
Pumping 0.2 1.3 2.8Exchange with the Turonian aquifer
0.1 0.1 0.1
Total 0.7 1.6 3.0
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.
El Hsay
2500
3500
4500
5500
6500
7500
06/ 23/ 71 05/ 01/ 81 03/ 10/ 91 01/ 16/ 01
Rés
idu
sec
(mg/
l)
1500
1900
2300
2700
3100
3500
06/ 23/ 71 05/ 01/ 81 03/ 10/ 91 01/ 16/ 01
Rési
du s
ec (m
g/l)
Kébili West Dar el Gaied
Ras el Ain
0
500
1000
1500
2000
Ras el Ain Bazma Scast Mtouria Douz 2b El Hsay
Na+ Ca++ Mg++
0500
1000150020002500300035004000
Ras el Ain Bazma Scast Mtouria Douz 2b El Hsay
Cl- SO4-- HCO3-
20 Kilomètres0 10
Douz
Atilet
Tembib
Telmine
Menchila
Ras El Ain
Fatnassa
Klibia
Msaid
BazmaGuataya
Nagga
Gueliada
El Faouar 2
Ghidma
Grad
Ben Ghilouf
Jemna
Bechni
Ouled Touati
NouielEl Golaa
Zaafrane
Stiftmi
Dargine
Mtouria
El Hsay
Tarfayet Elma
Kelwamen
Limagues
Oued Nekhla
Mazraa Neji
Saidane250
Chott Djérid
20 Kilomètres0 10
Douz
Atilet
Tembib
Telmine
Menchila
Ras El Ain
Fatnassa
Klibia
Msaid
BazmaGuataya
Nagga
Gueliada
El Faouar 2
Ghidma
Grad
Ben Ghilouf
Jemna
Bechni
Ouled Touati
NouielEl Golaa
Zaafrane
Stiftmi
Dargine
Mtouria
El Hsay
Tarfayet Elma
Kelwamen
Limagues
Oued Nekhla
Mazraa Neji
Saidane250
20 Kilomètres0 10
Douz
Atilet
Tembib
Telmine
Menchila
Ras El Ain
Fatnassa
Klibia
Msaid
BazmaGuataya
Nagga
Gueliada
El Faouar 2
Ghidma
Grad
Ben Ghilouf
Jemna
Bechni
Ouled Touati
NouielEl Golaa
Zaafrane
Stiftmi
Dargine
Mtouria
El Hsay
Tarfayet Elma
Kelwamen
Limagues
Oued Nekhla
Mazraa Neji
Saidane250
Chott Djérid
Distribution of TDS increase (mg/l) over the period 1980 - 2000
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 1950-2000. 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