沙漠研究 19-1, 323- 326 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Water Management and Salinisation in Irrigated Areas-A Case Study of Sidi Sallem Oasis in the Southeast of TunisiaCherifa FDHIL1, Abdessatar HATIRA2, Tahar GALLALI2, Houcine TAAMALLAH1
Abstract: This research aims to study soil salinisation in irrigated lands of Sidi Sallem oasis(Southeast Tunisia). This zone covers about 146 ha, irrigated by an artesian drill exploiting the deep
water of Djeffara (Q = 75 l/s and Total Dissolved Salts = 3.5 g/l).
In this study, morphological and chemical characteristics of 10 soil profiles samples wereidentified. The oasis has an alluvial soil dominated by fine sand with a particular structure andgypsum crusts and crusting at groundwater levels. The salt accumulation in the soil was recorded in
the first layer of the majority of soil profiles and essentially near the water table level. Because of thetopography, we identify that the highest salinity was recorded in northern sector. Also we studied thehydrological and chemical characteristics of 29 piezometers. The ground water level was very close
to the soil surface in northern part of the oasis (Z < 1 m), whereas in the Southern and Western part, it
was further than 2 m. The highly mineralized water table located at shallow depth increased the riskof salinisation. The high soil salinity was attributed to the poor quality of irrigation water and saline
ground water very close to the soil surface mainly during the summer period when evaporative
demand was high.
The soil and groundwater properties, coupled with the data related to irrigation and drainage water,
can facilitate the comprehension of the soil salinity in this oasis in order to minimize by consequence
the severe climatic conditions in this region. Also they help to estimate the necessary leaching
fraction of the soil and control the network of drainage for better evacuation of salts.
Key words: Drainage, Groundwater, Irrigation, Oasis, Soil salinity

1. Introduction
The problem of salinity and water shortage in agricultural areas is of increasing concern in the arid
regions of southern Tunisia which has a wide variation in the climatic conditions, salinity and crops types
and where there is little scientific information on the occurrence of this problem. Indeed, climate in thisregion is characterised by a low yearly precipitation (150-200 mm), yearly evaporation exceeding 1300
mm and diurnal evapotranspiration higher than 8 mm during the summer. Capillary rise under these
conditions can be extremely important which increase the salinity problem in the soil because of the low
salts leaching especially in dry years (Ould Ahmed, 2006). In addition, the level of salinity in the water orthe accumulation of soluble salts in the soil is transient conditions since it varies during the growing
seasons and may also aggravate in dry years and impairing traditional systems of cultivation in the oasis
with consequent risks of soil degradation (Brahim, 2002). Thus, permanent care and proper control actions
are required. Adequate soil and water conservation practices based on a comprehensive soil or landdegradation assessment can provide an "early warning system" as a tool for efficient salinity control.
2. Materials and methods
2.1. Study area description
The oasis of Sidi Sallem is approximately 25 km southwestof the city of Gabes (Southern Tunisia - Northern Africa) (Fig. 1).
It covers about 146 ha, distributed in 70 plots (4.58 to 23 ha).
2.2. Soil study Thirty soil profiles covering the whole area ofthe oasis were identified. The morphological and chemical
characteristics of the soil profiles were studied from thesurface layer until groundwater levels (98-300 cm).
The
description of the profiles was conducted during the summer.
12Gabes
Sidi
SallemMedenine
Fig. 1. Location of the study area.

Eremology and Combating Desertification Laboratory, Institute of Arid Area 4119 Medenine - Tunisia).
UR. Pedology. Department of Geology. FST. Universities Campus 2092. Manar II. Tunisia.

324

2.3. Water study
The irrigation water of the oasis was pumped from artesian drilling water exploiting the deep
groundwater of Djeffara. The water which has a Total Dissolved Salts (TDS) about 3.44 g/l was stored intank upstream of the oasis and distributed by irrigation network (seguias and PVC pipes) with the flow rate of75 l/s and a time pump operation of 10 h/day. The net area of the oasis is about 146 ha. The applied
irrigation water supply was about 54 mm which was under the estimation of the Agriculture Department ofGabes (88mm). The saline groundwater very close to the surface (>1m), imposed a soil drainage either by
natural or artificial drainage. Sampling and analysing of drainage and irrigation water have been carried out
in January 2006. To monitor ground water depth and salinity, a network of 29 piezometers (D=8 cm, L=3 m)
was installed (Fig. 2). To ensure better water percolation into the piezometers, holes were drilled each 10cm
along the first 100 cm. The ground water Salinity and depth were measured in August and January.
3. Results and discussion
3.1. Soil study
The oasis of Sidi Sallem has an alluvial soil dominated by fine sand (Table 1). All profiles werefilled of gypsum (Gy=1-69%) with an increasing gradient top-down profiles (Fig. 2). Generally, the rate oflimestone (TL) was low and almost invariant along the soil profiles but in some profile correlate with therate of gypsum such as the profile P8 where they were inversely proportional with (Gy=0.59%, TL=16% or
Gy=27.3%, TL=0.8%).
The P4 profile had high proportions of gypsum (Gy = 69.91%). Its situation near the water channel(Wadi El Maleh) where the water circulates especially in rainy periods inhibited the formation of gypsumcrusts (Fig. 3). The salt accumulation in the soil surface was recorded in the first centimetres of the
majority of soil profiles «salt efflorescence» and near the water table levels. The P1 profile (centre of theoasis) had a moderate saline profile (EC = 3.47 - 4.4 dS/m). In addition P5 profile was not very
concentrated in salt (EC = 8.91 dS/m), only near water table level (EC= 61.2 dS/m) whereas the P4 profile
Table l. Morphological characteristics of the four representative soil profiles in the oasis of Sidi Sallem.
P1

P4P5

Northern sector
Thin layer of salt
Abandoned plot
Natural Halophytes
species

Soil structure

Eastern sector
Wind recovery
Cultivated plot
-Fodder (alfalfa)- Shrub (pomegranates)- Vegetables (peppers)- Particular- Massive (105-150 cm)

Soil texture

Sandy clay soil (FS =52%, Gy =24%),rich in gypsum nodules and in pinksand in low profile (130-150 cm)

CorpsW(P5)02502035558075130

130265300

(P4)

v

180

-240VVVSouthern sector
Sand + Manure residue
Cultivated plot
Trees (pomegranatesand apricots)- Particular- Slightly massive(75- 35 cm)- Fine sandy soil
(77%) at surface- Sandy clay soil
Gy = 14%) + coarse
and gravel (1m)Fine sandy soil
(FS = 82%) at surface
gradually sandy claysoil at 3 m(FS = 68%, Gy = 18%)

- Fine sandy soil
(FS = 68%)
- Gypsum crust(III)
0(P1)

65

Fine sandPink sandvvSlime
Diffuse Gypsum

E3733800

3733600

3733400

3733200

Gypsum Clusters
Claystt(I)N

P10Pz29Pz1
Pz2Pz28
Pz27
Pz26
SP9
Pz13 P5
Pz7 P1
Pz24
Pz14
Pz25
Pz12
P3 Pz5
P6Pz15 Pz9
Pz23
Pz18P2
Pz24 P8Pz22
Pz16 Pz17
Pz28 Pz19
OuedPz21
P7Pz20
PZ29

PZ1

W

EPz11
P4 Pz3

PZ28

PZ2

PZ27

PZ3

PZ26

PZ123733000

PZ7

PZ13

PZ14

PZ4

PZ25

PZ10PZ5PZ9PZ153732600

PZ23PZ18135185205

Z (cm)

Particular + gypsum
crust at the intermediate
layersPZ113732400

PZ16

PZ22

PZ24

PZ8

PZ17

PZ21

PZ19

PZ203732200

300

Z (cm)

Z (cm)

0103070(P8)3732800

105
130150250

LegendP8

Western sector
Anthropogenic surface.
Cultivated plot
-Trees (peaches, figsand pomegranates)- Forage
Particular, gradually
Massive along the profile,rich in pink sandEl Melah3732000
60960050275

Z (cm)
Gypsum crust
Hydromorphic Signs

609800610000610200610400610600610800611000611200611400SAltitude (m)Location
Surface state
Current use

611600N(II)Water table
P7P6 P5P3

P2P1 P4 P8 P9Soil profiles

P10Fig. 2. Location of the soil profiles (P) and piezometers (Pz) on the oasis map (I); Location of soil profiles based
on topography (II); 4 representatives soil profiles of the oasis (III).

325% of Gy and TL
80

60

40

EC (dS/m)% of Gy and TL

200(P4)

020

40

60

0-3580(P8)- 50-150Z (cm)-100
Total Limestone (TL)

- 200

Gypsum (Gy)

- 250

50

100150-65

P1-95

P4-130

P8-150P5-300- 300

Z (cm)

Fig. 3. Gypsum content and total limestone in soil (P4 and P8).
meq/l
600 500 400 300 200 100

Fig. 4. 4 representative Saline Profiles of the oasis.

meq/l0(P4)0100 200

300 400

500

600

600

500

meq/l
400 300

200

100

00005050
100
150200
250
300

100
150

SO4200250
300

Cl2-Ca2+
Mg2+Na+K+-HCO3-

meq/l300 400 500

100 200600

Z (cm)

Z (cm)

Fig. 5. Ionic profiles of two representative soil profiles of the study area (P4 and P8).
Table 2. chemical analysis of irrigation and drainage water (January).
Chemical analyses(meq/l)Irrigation water
Drainage water

Ca2+Mg2+K+Na+

SO42-

Cl-

HCO3-SAR

EC (dS/m)

17.6
29.2

18.4
86

0.37
2.18

16.6
150

24.5
109

23
166

3.4
8.85

3.91
19.76

3.44
23.6

TDS(mg/l)
3000
17000

pH
7.4
8.27

(northern oasis) had a very high salinity in all layers (EC = 85 -130.9 dS/m) and it was very affected byhydromorphy which could be explained by its location at the downstream near the principal drain of theoasis (Fig. 4). The salt accumulation in surface soil layer could be explained by irrigation with saline
water and the high evaporative demand.
The ionic profiles in P8 (eastern oasis) showed that the ion concentration was greater in the soil
surface layer (Fig. 5). The sodium adsorption ratio (SAR=Na+/√(Ca2++Mg2+)/2) is an index quantifying
the proportion of sodium (Na+) to calcium (Ca2+) and magnesium (Mg2+) ions in a sample. It was lower
and invariable along the profile (6.68 to 9.58). The major cations are very similar; Na+/Mg2+ andCa2+/Mg2+ ratios are close to 1. Cl-/SO42- was somewhat variable along the profile and the proportions ofthese two major anions were very close. The P4 profile (northern oasis) was the most concentrated in ions.
Its ions concentration increases, mainly in the intermediates layers. The SAR ranged from 8.62 to 21.77;
the highest was recorded in the intermediate levels where the Na+ concentration was maximum. SO42dominated for most levels.
3.2. Water study
3.2.1. The irrigation water (drilling of Sidi Sallem)
Its chemical analysis showed an electrical conductivity of 3.44 dS/m, neutral pH (7.4) and a Joint
sulphated facies with a very high salinity risk and a medium risk of alkalinity (C4-S2) because its highcontents on sodium and chloride (Table 2).
3.2.2. The drainage water
It had a chloride sulphated sodium facies. These waters eliminated especially sodium, chlorides andsulphate (Table 2). Its drainage flow was about 1.6 l/s which eliminated only 31.8% of all salts provided
by irrigation water (CRDA Gabes). The salinity of drainage water was closely related to the injected water
volume from rainfall.
3.2.3. The groundwater
The Chemical analysis was carried out during August and rainy period (January). The groundwater had
chloride sodium sulphated facies. The maximum of ions has been recorded in the piezometer Pz11 withhigh concentration in Cl- and Na+. Only some piezometers showed a variation in their chemicalcomposition (Pz2: Σcations = 179-1097 meq/l and Σanions = 185-1061 meq/l). This seasonal and spatial326

variation is closely linked to rainfall, irrigation supply and soil topography (Fig. 6). The oasis includes two
major sectors: an up sector (altitude: 50 m) composed by the centre and the west of the oasis and a down
sector of Oued El Maleh. The depth Changed between the dry season “August” when the minimum wasabout 97 cm in Pz3 (N) and the maximum was 3.2 m in Pz16 (SE) and the wet season “January”(Zmin =45 cm in Pz3). Salinity changed. In the summer, two main axes of the highest water salinity were N-S andE-W. The maximum salinity was recorded in the northern oasis at Pz11 (EC = 152.5 dS/m). The highsalinity could be explained by an increase of the irrigation water supply to compensate the loss of water by
evapotranspiration and the gathering of drainage waters in these areas. In rainy period, the minimum
salinity was observed along E-W axis (ECmin = 25 dS/m). The downstream areas kept high salinity (Pz11:
EC = 110 dS/m) (Fig. 7). As irrigation was insignificant during this period only storm water had flowed inand diluted the ground water.
SO42-

Concentrations (meq/l)

1400
1200
1000
800
600
400
2000Cl-2000

HCO3-Ca2+Mg2+ K+ Na+3734000

3734000Août 2003
August3733800

(IA)

1500

(IB)

Z Prof
(cm)
(cm)

PZ29

PZ1

300

3733400

PZ123733000

250

PZ7200

PZ253732800

PZ6150

PZ23

Frg

1303732600

OuedElMaleh

3732200

3732000

PZ18

1500

10
500

610000610200610400610600610800611000611200611400611600611800150

PZ23Frg

1303732600

PZ22

PZ24

PZ8

PZ18100
100

PZ21

PZ17PZ163732400

PZ19PZ2070

503732200

40609600

612000EC(dS/mPZ29CE (mS/cm)

PZ1

PZ27

PZ3

PZ26
PZ123733000

PZ7100PZ4PZ13PZ14PZ25
PZ103732800

PZ6PZ550

PZ9PZ15

PZ23Frg3732600

PZ18

PZ22PZ24

PZ810

PZ21

PZ17
PZ163732400

160
150140130120
1101009080706050403020181614121086420150

PZ28

PZ2PZ11

PZ19PZ20

1 2 3 1113 17 1819 20 21 242628 29160

PZ15609800

610000610200610400610600610800611000611200611400611600

6118006120003734000

Aout 2003
August3733200

4(IC)0Jan 2004
Januar3733800

EC(dS/m)

PZ29

CE (mS/cm)

3733600

PZ11503733400

PZ28

PZ2
PZ113733200

PZ27

PZ3100

PZ26
PZ123733000

PZ7

PZ4

PZ13
PZ14

PZ25
PZ103732800

PZ6

PZ550

PZ9
PZ15

PZ23Frg
3732600

PZ18

PZ22

PZ24

PZ810

PZ21

PZ17
PZ163732400

PZ19PZ203732200

4(IID)

0160
15014013012011010090807060504030201816141210864203732000

3732000

1 2 3 1113 17 18 19 20 21 24 26 28 29190

PZ53731800

6098003733400

0PZ63732000

3733600

3732200

PZ10403734000

500

PZ2570PZ20(IA)6096001000

220200PZ4PZ9100

PZ21
PZ193731800

(IID)

PZ7

PZ13
PZ14

PZ22PZ24

PZ8

PZ17
PZ163733800(IIC)

3733000

160

PZ153732400

PZ263732800

PZ9250

PZ27

PZ3190

PZ5280250

PZ113733200

PZ12220

PZ4

PZ13
PZ14
PZ101600
1400
1200
1000
800
600
4002000300

PZ28

PZ2250PZ27

PZ3PZ2602000

PZ13733400

280PZ28PZ23733200500(cm)Z Prof(cm)300

PZ29300

PZ111000

Jan 2004
Januar3733800

3733600

3733600

3731800

3731800
609600609800610000610200610400610600610800611000611200611400611600611800612000609600609800610000610200610400610600610800611000611200611400611600611800612000Fig. 7. The groundwater depth (A and B) andsalinity (C and D), in August (I) and inFig. 6. Anions (A and C) and cations (C and D) Concentrations
January (II).
of the water table in August (I) and January (II).
N of piezometers

4. Conclusion
The oasis of Sidi Sallem has an alluvial soil dominated by fine sand. This texture is favourable for an
active dynamic of water in the unsaturated zone which was manifested by water capillary rise to the surface
during the dry season causing salt accumulation in the soil surface (Ben Hassine and Mtimet, 1999). The
northern sectors were the most affected by salinisation. Thus, hydromorphy affect most of soils in these
sectors and becomes permanent in some places. The analysis of the soil saturation extracts shows a highproportion of sodium and chloride. The salt accumulation was recorded in the first cm of the majority ofsoil profiles. Gypsum was concentrated in the form of gypsum crust and crusting in the middle of soil
profiles, close to water table level. In the northern sector of the oasis, the highly mineralized water tablelocated at shallow depth increased the risk of salinisation. The high soil salinity was attributed to the poor
irrigation water quality and the saline groundwater very close to the soil surface mainly during the summer
period where evaporative demand is high. Under these conditions, the excessive use of irrigation water by
farmers causes to the increasing level of the groundwater that in some places. The results of our studies
confirm the existence of an outstanding salinisation in the soil surface layers, particularly in northern oasis.
The intensity of salinisation process is not depended only on water quality but also on the type andproperties of soils. In order to avoid soil salinisation by capillary rise, it is necessary to check theinstallation of the drainage system and ensure regular maintenance.
References
Ould Ahmed B. (2007): The impact of saline water irrigation management options in a dunes and on available soil water and itssalinity. Agricultural water management Journal, 88: 63–72.
Brahim A. (2002): Contribution à la compréhension du processus de salinisation des sols irrigués en zones arides cas de l’oasis
Segdoud. Thèse 3ème ENIT. 168p.
Ben Hassine H., Mtimet A. (1999): Suivi de l’évolution de la salinité des sols dans les périmètres irrigués de Tunisie - Distribution
géographique et approche adoptée. Es n°309,9p.