Hydrus workshop Prague 28 Mars 2008

Context

Field data and modelling of salt
transport in an irrigated coastal plain

 Salinisation in coastal aquifers under semiarid climate : seawater intrusion vs irrigation
practises Lack of studies treating with salt recycling
under irrigation
 The unsaturated zone role is rather missknown (soil scientists and hydrogeologists)

F. Slama1,2, R. Bouhlila1, and J. Tarhouni3

1: National Engineering School of Tunis, Tunisia
2: Centre of hydrogeology, university of Neuchâtel, Switzerland
3: Institut National Agronomique (INAT), Tunisia

12Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

IRRIGATED PLAIN OF KORBA NORTHEASTERN TUNISIA (CAP BONPENINSULA)

Purposes Understand salinisation processes of soil andgroundwater in semi arid regions
 Scales : Profile, cross section, entire domain
 Sustainable management of soil andgrounwater resources3

 Agricultural region withseveral irrigated areas A variety of crops consuming
important amounts of water The annual mean precipitation
is 450 mm while mean annual
evapotranspiration is 1100mm. Effective infiltration isabout 10% of the annual
rainfall (Ennabli, 1980)
 More than 6000 equippedwells

41Hydrus workshop Prague 28 Mars 2008

Methodology

Materials et Methods

Hydro-Geo-Chemestry

Experimental

 Korba coastal aquifer S=438 km²
 Two geological units
1. Pliocene formation is sandstone withalternating marl units mean thickness of 85m.
The Quaternary alluvium is composed of
detrital sediment (sand, gravel, and silt) withthin clay lenses and has a thickness that varies
between 20m and 25m.
2. Miocene marl formation underlying the firstunit (Paniconi et al., 2001)

Modellingstudy
2D

2 Transects• 3 plots in each transect• Soil sampling•ERT measurements• Infiltration tests•Groundwater sampling inobservation wells

Densitydependant
Variablysaturated model(Feflow)

1DGEODENS
2D Density
dependant variably
saturated andmulti-species
geochemical
model in porous
media (Bouhlila,1999)

Hydrus1D
thirdversion

5Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

Soil Sampling of the entire unsaturated zone (4-20 m)
after end of irrigation and after wet season in 3 plotsalong the transect (August 2006 and April 2007)

Lebna damTransectTransect AA’
Dry drilling
Plot 3

Plot 2Plot 1

Sea level

The most touched area by salinisation

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Hydrus workshop Prague 28 Mars 2008

Electrical Resistivity
Tomographyz Wenner-Shlumberger

Câble

Lebna dam Monitoring sites:
private wells,abandoned wells andpiezometers(Measurements GW level,Ec, Ec profiles)

Transect32 électrodes

5m

Résistivimètre

9Hydrus workshop Prague 28 Mars 2008

Modelling : steps andprocedureModelling Step 1. Homogeneous Profile without transport
 with transport
 with transport, root water uptake, (Tomato data parameters)
 with transport, root water uptake, stress with transport, root water uptake, stress, root growth IC. Hydraulic head or water content
 BC. Up: Atmospheric condition

15m

Down : ???????
 Tfinal = 395 time, step= 1 day
 Profile depth : 15 m
 Tests .VG
 Tests Disp Step 2. Layered Profile Same thing11

 Step 3. Layered Profile with chemistry using HP1

Plot 2 : drip
irrigated tomato
123Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

Modelling : steps andprocedure

The layered Profile
tethartethasalphanKs (m/j)

 Atmospheric boundary conditions in Hydrus.


Input : potential evaporation potential transpiration and precipitation ierain + irrigation
ETo was calculated using daily meteorological (Tmin, Tmax, Rhmin,Rhmax, wind speed and sunshine hours) data from Oued souhil(Nabeul) and using Eto calculator free software of UCL (based on FAO
modified Penman Monthieth equation)
To split ETo into evaporation and transpiration we used the equation
proposed by Ritchie (1974). To estimate the LAI, a polynomial function
was generated based on literature review (hevelink, 1994).

0.05880.375921.3588

0.236

0.04590.379331.8186

1.39

0.05410.3744

3.17

3.198270.0492

0.37733.42

2.02720.5

0.05230.37543.26

2.96594.83

0.04530.3806

3.62

1.53280.65050.0526

0.3767

3.3

3.0912

6.53

VG parametres using Rosetta neural network prediction using SSC
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Hydrus workshop Prague 28 Mars 2008

Measured hydraulic head along the transect AA’ in the months of June,
August and December 2006.

Results : Experimental Data

Cumulative distance from the sea (m)
5000

4000

3000

2000

1000

SEA0-2

Piezometric depression

H_jun_2006
H_aug_2006
H_dec_2006-4-6Hydraulic head (m)0-8-1015164

Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

Measured grounwater electrical conductivity along the transect AA’ in themonths of June, August and December 2006.

ERT measurements
 Point section 1 à 700 m de la mer

12

Ecw_aug_06
Ecw_dec_06

10864High salinities reaching 10 mS/cm

20

5000

4000

300020001000

Electrical conductivity (mS/cm)

Ecw_jun_06

Low resistivity : seawater/freshwater interface is at 6m depth at 0.5 Km far fromthe sea

0Cumulative distance from the sea (m)SEA
1718Hydrus workshop Prague 28 Mars 2008

Results : Plot 1 at 0.6 km from the sea

Results : Plot 2 at 2 km from the sea
PLOT24060801000

10203040500

200 400 600 800 1000

012320406080

100

0100

200

300

Cl- (mg/l)
400
0400 800 1200 1600 2000 0Ks (m/s)

EC (mS/cm)
2461E-006

81E-005

0.0001045014812162EC (mS/cm)

Loam

3048121604
Loam + clay

April 07sept 06

April 07sept 06

Depth bsl (m)Depth below soil level (m)20EC (mS/cm)

Cl- (mg/l)

Depth bsl (m)Granular fraction (%)

00NO3-(mg/l)

NO3-(mg/l)

Granular fraction (%)

PLOT1481216195

Hydrus workshop Prague 28 Mars 2008

Results : Plot 3 at 4.2 km from the sea

Results : Modelling

PLOT3

NO3-(mg/l)

Granular fraction (%)
20406080 100
0100200300

0400400 800 1200 1600 20000

EC (mS/cm)
12345048121620SandLoam + clay

April 07sept 06

2221Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

Some results : after 162 days
(September 06)

Some results : Final time simulations(April 2007)

TDS (g/l)
0510TDS (g/l)
1520002468101202Obs

2

Homogeneous_Disp=2m_Head IC

44obs

6
Homogeneous_Disp=6m _ Head IC

68Depth bsl (m)Depth bls (m)Depth below soil level (m)0Cl- (mg/l)

Layered_Disp=3m_water content IC

10p1212p141416

disp=2 m
disp=6 m8

disp=310

disp=1.5

12

Layered_Disp=3m_water content IC
Leaching profile

1418162318246

Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

Some results : after 10 years

Some preliminary conclusions

Profile Information: Concentration
0T0

T6

T12

T1

T7

T13-10

T2

T8

T14-15

T3

T9

T15

T4

T10

T16

T5

T11

T17

 The model is most sensitive to :-5-20

dispersion, initial conditions both flow and solute (we arenow using hot-start initial conditions) Lower boundary conditions both flow and solute Heterogeneity
 Peaks are only visible within long term
simulations-25
0102030405060

708090Conc [g/l/m3]
25

Hydrus workshop Prague 28 Mars 2008

26

Hydrus workshop Prague 28 Mars 2008

Further work…..
 Try to :
 Simulate 10 or 20 years prior to our sampling
campaign to reproduce the picks (we needstochastic modelling of irrigation water amounts…)
 Make stochastic modelling of heterogeneity (opensource software SGEMS) to study its impact onsolute transport

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Hydrus workshop Prague 28 Mars 2008

Hydrus workshop Prague 28 Mars 2008

GEODENS 2D density dependant variably saturated and multi-species

Further work…..

geochemical model in porous media (Bouhlila, 1999) which solves
Equations (1) and (2) using Finite Element Method

Fluid conservation Equation

 Include solute recycling (define a time step atwhich we interrupt simulations and use the resulting
groundwater concentration as an input
concentration for irrigation; develop a script….)
 Simulate the other two profiles
 Estimate flow and solute flux reaching groundwater Compare HP1 to GEODENS(1)⎞ ⎞ ⎧⎛∂S ⎞ ∂p
∂ ⎛ kij kr ⎛ ∂p
⎛ ∂ρ ⎞ ∂C ⎫− ρ g ⎟ ⎟ − ⎨⎜ ρ Ss Sw + φρ w ⎟ + φ Sw ⎜⎜ρ⎜⎟ ⎬ + Qm = 0⎜⎟⎜⎟ ⎩⎝∂xi ⎝∂∂∂C ⎠ ∂t ⎭μ ⎝ ∂x jpt⎝⎠⎠⎠
Darcy’s velocity

Multi-species non conservatif Transport(2)∂ ( ρφ Sw C i )∂ ⎛∂C i ⎞∂C i+ φ S w f i + Qm C i =⎜⎜ ρφ Sw Dij⎟⎟ − ρφ Sw ui∂xi ⎝∂x j ⎠∂xi∂tGeochemical Process29Hydrus workshop
Prague 28 Mars 2008
Numberof Precipitatedgypsum (CaSO4,2H2O) moles
after 100 days of evaporation

30Hydrus workshop Prague 28 Mars 2008

Moles/m3051015202530354045

500-2

THANKYOU

Depth (m)-4-6After 100 days

-8-10-12-14

Roman well Skalba (Korba plain)-16

Compare HP1 toGEODENS

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