Impact of climate change on the Figuig aquifer using a numerical

Transcription

Journal of Biology
and Earth Sciences
ISSN: 2084-3577
EARTH SCIENCES
ORIGINAL ARTICLE
Impact of climate change on the Figuig aquifer
using a numerical model: Oasis of Eastern Morocco
Abdelhakim Jilali
Hydrogeology and Environmental Geology – Aquapôle, University of Liège,
Chemin des Chevreuils 1, Building B52/3, 4000 Liège, Belgium
ABSTRACT
Numerical groundwater modeling was applied to investigate the climate change impact on the
groundwater resources in the unconfined aquifer of Figuig basin of Moroccan eastern High Atlas. The
stresses imposed to the model were derived from the IPCC (Intergovernmental Panel on Climate
Change) emission scenarios and included recharge variation. The Figuig aquifer consists of carbonates,
alternating marl-limestone, sandstone and alluvial dated of Jurassic and Quaternary age respectively.
The limits of the model are topographic boundaries except for the western boundary which has been
truncated. The model is composed of a single layer representing the whole thickness of the aquifer. It
has been studied in steady state conditions. Modeling improves the understanding of the interactions
between groundwater and surface waters by measuring the exchange of water flow. The decline in
recharge due to climate change has a negative effect of groundwater resources, and therefore on the
Figuig oasis. Currently, the drawdown of the groundwater resources has affected both the number of
springs that supply the oasis.
Key words: Figuig; Groundwater; Numerical modeling; Climate change; IPCC scenario.
J Biol Earth Sci 201 4; 4(1 ): E1 6-E24
Corresponding author:
Abdelhakim Jilali
E-mail: [email protected]
Original Submission: 01 December 201 3; Revised Submission: 03 January 201 4; Accepted: 09 January 201 4
Copyright © 201 4 Author(s). Journal of Biology and Earth Sciences © 201 4 T.M.Karpiński. This is an open-access article distributed
under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
Journal of Biology and Earth Sciences, 201 4, Vol 4, Issue 1 , E1 6-E24
E1 6
Jilali Impact of climate change on the Figuig aquifer: Oasis in Eastern Morocco
INTRODUCTION
MATERIALS AND METHODS
The estimate of the potential impact of climate
change on water resources is one of the most difficult challenges to explore. Thus, several studies
have been the goal of this topic. The impact of climate change requires for groundwater systems a
reliable estimate of the balance (input and output
flow water on the aquifer). This depends at a good
estimate of recharge and hydrogeological models to
represent the observed phenomena.
The objective of this study is to construct a
model of groundwater flow across the watershed
Figuig (327.5 km 2). The applied climate change
scenarios are derived from IPCC [1 ] data. This data
reinforces the idea that climate changes predict an
increase in temperature and a decrease in rainfall,
depending on the scenario that was selected. This
causes an impact on groundwater resources. These
scenarios indicate an increase in the average temperature of 2.8°C and a decrease in rainfall of around 1 2% by 2099 [1 ].
According to the methodology and objectives,
several approaches have been applied, either by
studying surface water or groundwater, by using
simple or complex approaches. In addition to the
choice of the modeling approach and climate scenarios, studies have adopted scenarios of general,
regional circulation models [2-4] or IPCC scenarios
[1 , 5-6].
Fig. 1.
RESULTS AND DISCUSSION
Overall framework
Oasis of Figuig is situated at the extremity of the
Eastern High Atlas of Morocco (Fig. 1 ). This region
is characterized by a hot climate in summer and
cold in winter. The climate of this region is arid. The
study area is encircled by several mountains. Figuig
is known for its many springs that are used for irrigation. Tzadert, Tighzert, Tajamalt and Maghni are
the most important springs (springs are captured by
Study area and boundary conditions.
E1 7
Khettaras). Khettara is groundwater draining gallerie. It’s a long gallery (several kilometers) leading
gravity drainage water from the groundwater to the
soil surface for the purpose of irrigation of the oasis
and palm trees.
The average rainfall calculated between 1 935
and 2002 is 1 24 mm. The maximum rainfall occurs
in October and the minimum in July. Rainfall is very
irregular from one year to another; it never rains in
the same quantity or in the same month. The catchment area of Figuig can receive 2 to 3 storm a year
shortly but violently and may cause flooding. The
frequency of drought is very variable. The wadis are
temporary. The temperature calculated between
Fig. 2.
1 937 and 1 967 is -2 to 1 °C in December, January
and February, and up to 48°C in June, July and August.
Geology
Structurally, the study area is formed by anticlines of Jbels of Grouz, Haimeur and Maiz. Anticlines
are separated by synclines of Tisserfine and Figuig.
Anticlines are formed by Triassic red clay and carbonates of the Middle and Lower Lias. Synclines
are composed of limestones of the Upper Lias, alternations of marl-limestones and sandstones of
Bajocian (Fig. 2 and Fig. 3). The Quaternary formations (alluvial silts, eolian sand and travertine) are
Geology of the study area [1 2].
Fig. 3.
Simplified cross-section of the study area (see location in Fig 2).
E1 8
present in the slopes and plains [7-1 3]. Various
geological and geophysical studies [1 3-1 5] show
three main fault directions: EW, NE-SW and NWSE. In the synclines, faults are hidden by recent
Quaternary deposits. Tectonics plays an important
role in the hydrogeology of the area.
Hydrogeology
Hydrogeological units of Figuig may be as follows: (1 ) fluvial terrace, silts and eolian sands; (2)
marl-limestone and fractured carbonates formations. Carbonates and alternations of marl-limestone are considered essential to the aquifer in this
region. The fault directions in the area are oriented
ENE-WSW and ESE-WNW with a subvertical dip.
Fracturing plays a very important role in the movement of groundwater. A piezometric map that shows
the main flow directions is presented in [1 4, 1 6].
The pumping test conducted in the study area
provide hydraulic conductivity values between
1 ,31 x1 0 -5 and 2,55x1 0 -3 m/s [1 6].
The average flow rate of the springs measured
in: 1 975 (Official Bulletin No. 3292), 1 977 [1 7], 2004
[1 6] and 201 0 (Study outstanding) is shown in
Fig. 4. The decrease in the volume of water of the
springs is due to both: (1 ) a decrease in flow rates
of some springs, such as the flow of Tzadert spring
(decrease of 25 l/s between 1 994 to 2011 , in Fig. 5),
with the exception of 2009-201 0 which was exceptional and it was very wet. It will have a positive effect on the flow rates of the spring; (2) obstruction
or collapse of some others. Therefore, a numerical
model was developed to study the possible impact
of climate change on the basin of Figuig.
Numerical model
The proposed model is a single layer (aquifer
consists of carbonates, alternating marl-limestone,
sandstone and alluvial dated of Jurassic and Quaternary age respectively) and it was studied in steady state conditions. The surface of the modeled
area is 327.5 km 2. The boundary conditions of the
model have been defined based on the following
arguments (Fig. 1 ):
- In the upstream limit (limit West) and downstream (between Jbels Melias and Sidi Youssef) of basin, Piezometric head were imposed (Dirichlet
condition). This type of condition allows an exchange flow (incoming or outgoing) in the model (Fig. 1 ).
The inflow calculated by the model is compared
with that estimated from the rest of the basin; it is
calculated by multiplying the area surface by the
recharge zone;
- The north and south boundaries of the model
correspond to topographic boundaries (Neumann
condition). The finite difference mesh is a layer of
rectangular cells of 250 meters. This network enables us to represent the geometry of the geological
formations. The top of the aquifer is assumed to be
the topographic surface. The bottom of the aquifer
has been simplified in the light of the limited number
of boreholes data (Fig. 6);
- Piezometry of 2008 (81 wells) was taken as the
initial reference value set in MODFLOW (is developed in Fortran by USGS. this version is running in
GMS "Groundwater Modeling System". The calculations are based on the principles of hydraulic in
underground porous media by finite difference method). Piezometric levels measured at the observation points were used for calibration of the model.
The faults introduced into the model are 1 0. These
Fig. 4.
Fig. 5.
Average flow rate of springs.
Flow rate of Tzadert spring.
E1 9
Fig. 6.
Spatial discretization of the modeled area.
faults allow rapid drainage in their neighborhood;
- Model calibration was done by comparing the
computed values to the observed data. Hydraulic
conductivity and conductance streams were adjusted in the model to better reproduce the measurements. Recharge (effective infiltration), calculated
by different authors [1 3, 1 6-1 8], gave the following
results: R = 0.1 *P (with: R: Recharge and P: Rainfall).
The result of the hydrodynamic calibration piezometry is shown in Fig. 7. Nevertheless, the simulated levels are slightly lower than the levels
observed. The flow is facilitated by the presence of
faults. Groundwater discharged at the surface contributes to the flow of springs and the waters of the
Oued Zouzfana. The general shape of the potentiometric surface calculated by the model is similar to
that observed (Fig. 8). Hydraulic conductivities adjusted are heterogeneous. In contrast, the adjusted
hydraulic conductivity in sandstone and faults is
greater by a factor of 1 00 as introduced. These adjusted values facilitate the exchange of groundwater
between the formations and allow a fair correlation
between the calculated and the measured hydraulic
head. They also facilitate the transfer of larger quantities of flow water in the neighboring faults. Significant hydraulic conductivities of faults constitute
of important corridors for evacuation and transfer of
groundwater to the east. The conductance value
beds of rivers (3.1 0 -5 m 2/s) were applied to all stre-
Fig. 7.
Observed values vs Computed values.
ams and at the springs.
Global flow rates exchanged as calculated by
the model are larger than those calculated by the
HBAM (Hydraulic Basin Agency of Moulouya) on
the entire basin. This difference remains after the
reduction of inflow through the upstream during the
calibration of hydraulic conductivities in this area
border. The quantities of entering water calculated
by the model are 37661 .76 m 3/day (Table 1 ). They
come from the recharge of aquifers of 48% and of
the West (upstream part of the basin of Figuig) to
52%. As for the water outflow system, 45.6% are
extracted by pumping wells for drinking water,
1 2.1 % corresponds to the drains (wadis) and
E20
Fig. 8.
Groundwater level contours.
springs and, 42.1 % contributes to the stock of groundwater and flows out of the model to the southern
border.
IPCC scenarios
The scenarios of the IPCC (Intergovernmental
Panel on Climate Change) were imposed for an estimate of changes in temperature and precipitation
until the year 2099. The scenarios of the IPCC
selected are: (1 ) A1 B reference scenario of the
IPCC, (2) B1 scenario of the minimal variation, and
(3) A1 F1 , scenario of maximum variation. Table 2
shows an example of the A1 B scenario with a 1 2%
decrease in rainfall. Recharge calculated is proportional to rainfall and indicates the same percentage
decrease (Table 3).
Fig. 9 shows the results of differences in piezometry of the various scenarios of the IPCC applied
to the aquifer of Figuig. Generally, it is demonstraTable 1.
ted that the effect of the decrease in the recharge
process involves a decrease of the groundwater level. The decrease in the groundwater level is in the
range of 0.07 to 6.2 meters, with a major drawdown
at the oasis of Figuig (springs area) by 1 .1 , 2.4 and
6.2 meters for B1 , A1 B of A1 F1 respectively. In
other words, renewable freshwater resources are
greatly diminishing.
CONCLUSIONS
A numerical model was used to study the predictions of climate change by 2099 derived from the
IPCC data. It is not in any case providing definitive
answers, nor the tools considered as a management strategy. Even if the models are a very good
efficiency (in terms of data and calibration), they are
still debatable. Simulations applied provide general
trends in climate change projections. Simulation
Flow balance.
E21
Variation of groundwater level in 2099, compared with the actual, imposing
scenario B1 , A1 B and A1 F1 of IPCC data.
Fig. 9.
E22
A1 B scenario of precipitation of the study area
(30N to 48N coordinates, 1 0O to 40E) derived from the
IPCC data [5].
Table 2.
4.
5.
6.
Table 3.
Climate scenario introduced in the model.
7.
scenarios A1 B, B1 and A1 F1 were chosen in order
to find possible variations of groundwater level in
the area of Figuig. The area is currently in agricultural expansion (increase drilling) due to lower flow
rates of springs and high salinity at the oasis area.
This can increase the stress of the aquifer. According to the simulations, the drawdown can reach 6
meters at Oasis of Figuig for the extreme scenario,
due to the decrease in recharge. The decrease of
water resources and increasing water requirements
may affect the socio-economical aspect of the region.
TRANSPARENCY DECLARATION
8.
9.
1 0.
11 .
The author declares no conflicts of interest.
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Impact of climate change on the Figuig aquifer using a numerical

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