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

AlgeriaLibya

TunisiaTotal

Complexe Terminal

0

10

20

30

40

50

1950 1960 1970 1980 1990 2000

Déb

it en

m3 /s

Continental Intercalaire

0

5

10

15

20

25

30

1950 1960 1970 1980 1990 2000

Déb

it en

m3 /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 4.5Recharge 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 0.0Evaporation in the sabkhas 0.0 0.0 0.0

Gulf of Sirte 0.0 0.0 0.0Tunisian 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 2050Inflows (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 3.0Outflows (m3/s) 0.0 0.0 0.0Evaporation 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

Rési

du s

ec (m

g/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