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J. Bio. & Env. Sci.
2016
165 | Bennani et al.
RESEARCH PAPER OPEN ACCESS
Variability of surface water resources and extreme flows under
climate change conditions in arid and mediterranean area: Case
of Tensift watershed, Morocco
Oumaima Bennani1, Yassine Ait Brahim2, Mohamed El Mehdi Saidi*1,
Fatima Fniguire3
1Laboratory of Geosci. & Environ. & LMI TREMA, Cadi Ayyad University, Marrakech, Morocco
2Laboratory of Applied Geology and Geo-Environment, Ibn Zohr University, Agadir, Morocco
3Laboratory GEOHYD, Cadi Ayyad University, Faculty of Sciences, Marrakech, Morocco
Article published on October 30, 2016
Key words: Tensift watershed, Climate change, Discharge variability, Floods
Abstract
Within the context of climate change and increasing demographic pressures, problems of water resources
variability have become particularly crucial. The management of arid watersheds, which are highly exposed to
droughts and floods, needs to be supported by a thorough understanding of their susceptibility to these hazards.
The watershed of Tensift in Morocco includes a high mountainous area and a large alluvial plain. In the
mountainous part, the bedrock has a low permeability and steep slopes, while the main valleys are narrow and
deep. The important rainfall events, which are usually short but intense, are favoured by high elevation and good
exposure to oceanic disturbances. Furthermore, predictions of climate change consequences on several socio-
economical fields in Morocco are very alarming. For example, climate trends and future climate projections at
Marrakech show a clear trend towards higher temperatures and lower rainfall. Consequently, surface water
resources are directly affected and the flows of Tensift river have fallen significantly. For the extreme events, the
conjunction of physical and climatological factors in the mountainous basins, is at the origin of violent floods,
which are characterized by the highest velocities and peak flows. The frequency analysis of floods shows that
these extreme events are repetitive with a variable intensity. The rising time of floods is very short, and human
and material damages can often be substantial. plain's floods are slower, but their water volumes are very
important and can submerge wide areas around the rivers as the case of the flooding of November 30, 2014.
*Corresponding Author: Mohamed El Mehdi Saidi m.saidi@uca.ma
Journal of Biodiversity and Environmental Sciences (JBES)
ISSN: 2220-6663 (Print) 2222-3045 (Online)
Vol. 9, No. 4, p. 165-174, 2016
http://www.innspub.net
J. Bio. & Env. Sci.
2016
166 | Bennani et al.
Introduction
Freshwater resources are vital to the support of
livelihoods and most countries are developing
institutional frameworks for the implementation of
measures to protect the quantity and quality of water
resources. These measures are often based on indices
quantifying the variability of these resources. These
indices are determined by crossing a lot of
information (Lei et al. 2007) related to the physical
characteristics of the environment (rainfall,
temperature, climate moisture, surface flow... etc.). In
the countries with arid or semi-arid climate, surface
water resources are becoming more limited and more
variable. These resources are conditioned by the very
irregular precipitations in time and space and the
large variability of flood flow.
Extreme flows have always been one of the most
important natural hazards with a lot of material
damage, as in the case of the famous flood disaster of
Oued Ourika on August 17, 1995 (Saidi et al., 2003).
Otherwise, the use of water in arid or semi-arid
environment depends on the availability of this
resource and its regularity. In the absence of this
regularity, a good water resources management is
needed. Arid and semi arid areas have rightly a
reputation for having a variability of these resources,
and the Tensift Watershed doesn't make exception to
this rule. Its plain is an agricultural area whose
activities and demand for water continues to increase.
The study of the variability of water resources and
climate and their hydrological consequences are
paramount. They will allow to make the right decisions
to better manage water scarcity. Some studies have
already begun to identify the variability of rainfall and
temperatures in the region (Riad et al., 2006; Khomsi
et al., 2013; Zamrane et al., 2016). They all reported the
obvious climate variability, but the hydrological
consequences, especially on extreme flows still need to
be investigated. For this purpose, the aim of this study
is to understand the variability of the precipitation and
temperatures in the Tensift watershed and to
determine how their fluctuations are expressed in the
river discharges and in the extreme flows.
Keeping above in consideration, this research
presents informations on hydroclimatic conditions in
an arid watershed with examples of flash floods
related to climate change.
Materials and methods
Description of study area
Tensift area lies between latitudes 30°50' and 32°10'N
and longitudes 7°25' and 9°25' W (Fig. 1). The catchment
occupies an area of 18500 km2 with a perimeter of about
574 km. The altitude varies between 53m at Talmest and
4167m at Mount Toubkal (Fig. 2) with an average
altitude of 1028 m. The basin is elongated with an
important time of water concentration. The climate
spatial variability in this area is related to its extension
and its relief. The climate is actually semi-arid and
influenced by the cold ocean current of the Canaries in
the coastal zone, semi-arid and warm in the Jbilet and
continental arid in the Haouz. (Fniguire et al., 2014).
The basin comprises two major morphological
entities: (i) a very high mountainous set and (ii) a vast
alluvial plain. The strong mountain range of the High
Atlas of Marrakech is a real water tower and a
provider of normal and extreme flows towards the
plain of Tensift. This part of the catchment receives
the largest amounts of rainfall, thanks to its high
altitude and Northwest exposure.
Fig. 1. Geographic position of Tensift catchment.
J. Bio. & Env. Sci.
2016
167 | Bennani et al.
Fig. 2. Hypsometry of the Tensift basin.
Climatic context of Tensift catchment
The climate in the area of Tensift catchment is
characterized by a high aridity in the plain zone, for
which the intensity is controlled by the low altitude, the
subtropical latitude and the continental effect.
However, the mountainous areas are characterized by
high rainfall amounts, thanks to their high altitudes
and their wind exposure, which slightly reduces the
aridity degree. The seasonal contrast is very important
and most of the rain events occur in the autumn and
the winter. These rains are usually irregular and
sometimes intense and violent. During the rest of the
year, drought occurs mainly in the lowland areas where
temperatures and evaporation are elevated. Moreover,
the annual thermal amplitudes are quite important: the
minimum winter and maximum summer temperatures
range between 5 and 45°C respectively.
For the rainfall study in Tensift area, we used rainfall
datasets from six stations in the mountains and four
stations in lowlands (Fig. 3). The records of the
lowland stations in Talmest, Chichaoua, Marrakech
and Abadla indicated annual heights between 200 and
250 mm, while the mid-mountain stations recorded
more important heights, especially in Aghblaou with
550 mm (Table 1). These rainfall heights are even
greater in the upstream part of the catchment, beyond
2000 m of altitude, where the annual average
precipitation may exceed 600 mm.
Fig. 3. Location of the main rain gauge stations in
Tensift catchment.
Table 1. Annual rainfall amounts from the main stations of Tensift basin.
Station
Elevation (m)
Annual rainfall (mm)
Talmest
53
286
Chichaoua
340
185
Abadla
250
176
Marrakech
460
250
Sidi Bouathmane
820
356
Imin Elhammam
770
375
Tahannaout
925
357
Aghblaou
1070
537
Taferiat
760
352
Sidi Rahal
690
348
The rainfall data analysis has shown great spatial and
temporal variability of annual precipitation. This
variability is illustrated by the alternation of dry years
and wet years (Fig. 4). Spatially, the lowland station of
Abadla and the station of Aghbalou (located at 1070 m
of altitude) indicate the greatest rainfall gaps.
J. Bio. & Env. Sci.
2016
168 | Bennani et al.
Temporally, all the stations exhibit quite high annual
and monthly variation coefficients. Thus, the
variability is significant within annual and seasonal
scales. The months of May, June, July and September
are generally the less rainy.
Fig. 4. Annual precipitation trends (mm) in
Marrakech from 1961 to 2014.
Observed climate change in Tensift area
Drought has always been present in the history of
Morocco, but during the recent decades, it has forcefully
become a structural element of the country climate.
Morocco is currently experiencing the longest dry period
in its modern history, which is characterized by a
decrease in precipitation and a clear trend of rising
temperatures (Stour and Agoumi 2008). This fact is also
confirmed by the fourth report of the Intergovernmental
Panel on Climate Change, which also predicts a decrease
of up to 20% in rainfall by the end of this century in
Morocco. The increase in temperature is expected to
reach 2.5°C to 5.5°C under the same scenarios (IPCC,
2007, IPCC, 2013). For this purpose, our study is focused
on the observed climate trends and variability at
Marrakech station. The aim of this part is to characterize
the recent climate change that affected this city since
1961 and the eventual impacts on natural resources
through analyses of temperature and rainfall evolutions
and calculation of climatic and extreme indices.
Temperature and precipitation
Temperature shows an irregular character in Marrakech
with important amplitude between minimum and
maximum temperatures (Fig. 5).
It thus confirms the continental character of this city.
Moreover, the trend lines indicate increasing
temperatures. Average annual temperatures have
actually increased by 1,5°C between 1961 and 2014. Also,
simulations predict for the city, a temperature increase of
about 2.3°C by the year 2050 and about 4.7°C by the year
2099 (Kouraiss and Ait Brahim, 2010).
Fig. 5. Annual temperature trends (°C) in Marrakech
from 1961 to 2014.
On the other hand, precipitations are even more
variable than temperatures. Morocco is highly
vulnerable to extreme precipitation events (Tramblay
et al., 2012). This irregularity depends on atmospheric
circulations, including the movement of the Azores
High towards South-West and the installation of
depressions on the Moroccan offshore, which would
produce interesting rainy disturbances over the
country (Jury and Dedebant, 1925).
During the last decades, there is a strong tendency
towards a decrease of precipitation totals and wet
days together with an increase in the duration of dry
(Tramblay et al., 2013). This decrease varies, by
region, between 3% and 30% (Sebbar et al., 2011). At
Marrakech city, analysis of the annual rainfall data
revealed that this climatic parameter is highly variable
and irregular from one year to another (Fig. 4).
Furthermore, there is an overall trend toward
decrease in these annual rainfall heights. According to
the trend line, the rainfall decline is about 65 mm over
the period 1961-2014.
J. Bio. & Env. Sci.
2016
169 | Bennani et al.
The Standardized Precipitation Index
The Standardized Precipitation Index (SPI) is a tool,
which was developed primarily for defining and
monitoring droughts. It allows a characterization of
rainfall deficits for a given period for any rainfall
station with historic data (Wu et al., 2005; Wu et al.,
2007). It can also be used to determine periods of
anomalously wet events. In fact, the SPI reflects the
impact of drought on water resources availability. It is
particularly calculated when the precipitation is not
normally distributed (Hayes et al., 1999).
The SPI can be calculated with the formula: SPI = (Pi
- Pm)/, with: Pi: Precipitation of year i; Pm: Average
precipitation of the whole study period; : Standard
deviation. As shown in table 2, a classification of
drought levels can be established thanks to SPI values
(Mckee et al., 1993).
Table 2. SPI classification.
SPI ≥ 2
Extreme humidity
1,5 ≤ SPI < 2
High humidity
1 ≤ SPI < 1,5
Moderate humidity
-1 < SPI < 1
Normal
-1,5 < SPI ≤ -1
Moderate drought
-2 < SPI ≤ -1,5
High drought
SPI ≤ -2
Extreme drought
In our case, SPI values are calculated for 44 years and
indicate a downward evolution for Marrakech city.
The trend line is therefore suggesting an evolutionary
trend towards drought (Fig. 6). Although SPI values
are highly irregular, it is also clearly noticeable that
the occurrence of negative SPI values became
increasingly more frequent after the year 1980. This
observation demonstrates longer periods of annual
drought and an increase of their persistence.
Fig. 6. SPI trends in Marrakech from 1961 to 2014.
Results and discussion
Variability of surface water resources
The types of river discharge on Tensift catchment differ
from the mountains to the plains. On mountainous sub-
basins, the flow is usually permanent despite its high
temporal variability. The annual average can reach 5m3/s
as in Ourika, at Aghbalou station, in an area of 503 km²,
or in N'Fis at Imin El Hammam (1100 km²). In the plain
zone, the flow is neither regular nor permanent; it is
intermittent and characterized by a high spatiotemporal
heterogeneity. This is because of scattered rains in a few
weeks, but mainly because of the long plain course that
favours infiltration and evaporation. Otherwise, changes
in precipitation conditions and evapotranspiration,
owing to the climate change, had visible impacts on the
river discharge. Much of the inter-annual flow variability
is explained by the influence of temperature and
especially the precipitation.
The analysis of four decades of annual flows in the
Ourika basin at Aghbalou for example (Fig. 7). Shows
a clear downward trend and the deficits are important
especially from 1992-1993. Before that date, the
interannual mean was 6,15 m3/s and dropped to 3,44
m3/s afterwards (a decrease of 44%).
Fig. 7. Trend and variability of annual discharge at
Aghbalou station (1070 m).
On monthly and seasonal scales (Fig. 8), the
discharges decrease is even clearer. From the
hydrologic year 1969/70 to 1991/92, the monthly
average flows were relatively important, especially
those of spring. But starting from 1992/93, the flows
of March, April and May, for example, dropped
respectively by 42%, 49% and 59,5%.
0
4
8
12
16
20
1969-70
1971-72
1973-74
1975-76
1977-78
1979-80
1981-82
1983-84
1985-86
1987-88
1989-90
1991-92
1993-94
1995-96
1997-98
1999-00
2001-02
2003-04
2005-06
2007-08
2009-10
Discharge (m3/s)
J. Bio. & Env. Sci.
2016
170 | Bennani et al.
All the other months recorded more or less important
lessening, except October and August. This summer
month recorded increasingly stormy localized
phenomena. Those phenomena trigger flash floods, as
it was the case in August 1995, August 2006 and
August 2010 (El Alaoui El Fels and Saidi, 2014; Saidi
et al., 2010).
Fig. 8. Variability of monthly discharge at Aghbalou
station during the periods 1969/70 to 1991/92 and
1992/93 to 2009/2010.
At the Tensift Plain, there is roughly the same
conclusion at Abadla station where the downward
trend is even more pronounced at an annual scale
(Fig. 9). Here, the interannual average discharge was
approximately 5,85 m3/s before 2000 and then
declined to an average of 2,92 m3/s after the year
2000 (a decrease of 50%).
Fig. 9. Trend and variability of annual discharge at
Abadla station.
The overall decrease of water resources has rightly
been observed by global institutions. For example, the
World Resources Institute has combined twelve
indicators for calculating the risk of water stress in
each country of the world (Gassert et al., 2013).
According to his report, most of the Moroccan
territory is now on a "high stress" alert. Some regions
are even classified as "extremely high stress", like that
of Marrakech. Furthermore, the water quality is
closely dependent to its quantity. The quantity
decrease also induces a quality decrease because of
the increasing saline concentration.
The adjustment of new hydro climatic conditions is
therefore required in the Tensift catchment. Hydraulic
water storage structures must be enhanced as well as
a rational use of the available resources.
The extreme flows and the rivers floods in Tensift
watershed
The floods of different rivers in Tensift catchment are
exclusively from a rain origin. They usually result
from heavy rains, which can be localized in space.
Actually, the physiography of Tensift watershed offers
a conducive environment to the development of
strong floods for several reasons: (i) the wind
exposure is favorable to precipitation, (ii) the slopes
are strong, (iii) the bedrock has a low permeability,
(iv) the vegetation cover is weak and discontinuous
and (v) the runoff network is hierarchical. This
morphological configuration contributes to amplifying
the observed peak flows at the catchment outlets
(Saidi et al., 2003; Saidi et al., 2012).
The hydrographs related to floods are often
individualized with quite short base and rise times.
The time of flooding ranges from few hours for the
short floods to two days for the long ones (Fig. 10).
The hydrographs are usually quite sharp with fairly
high peak flows. However, the most dreadful
character of flood events in the High Tensift is their
suddenness. Several floods had a rise time of 1 to 4
hours, while other events lasted 4 to 10 hours, which
is the most common class.
0
2
4
6
8
10
12
14
16
18
Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug
Discharge (m3/s)
1969-70 to 1991-92 1992-93 to 2009-10
0
2
4
6
8
10
12
14
16
18
20
22
24
26
1969-70
1971-72
1973-74
1975-76
1977-78
1979-80
1981-82
1983-84
1985-86
1987-88
1989-90
1991-92
1993-94
1995-96
1997-98
1999-00
2001-02
2003-04
2005-06
2007-08
2009-10
Discharge (m3/s)
J. Bio. & Env. Sci.
2016
171 | Bennani et al.
These periods are relatively short and risky for the
local residents and tourists who are often surprised by
the rapid and sudden rises of water levels and flow
rates, due to the difficulty to launch evacuation alerts
in time.
Fig. 10. Example of floods hydrograph of Ourika at
Aghbalou, Ghdat at Sidi rahal and N’Fis at Imin
elhammam.
The flood of August 17th, 1995 for example is one of
the most deadly and devastating floods in the modern
history of Morocco. It was the result of violent storms
that broke out in a summer afternoon in the upstream
part of the catchment in the areas between 2000 and
3000 m of altitude. The flood lasted only 3 hours in
Ourika catchment at Aghbalou station and the rise
time was particularly short (barely 15 minutes). The
peak flow and the mobilized water reached 1030 m3/s
and 3,3million m3 respectively during the three hours
of flooding (Fig 10). Moreover, the flood of October
28, 1999 was similar to that of August 17, 1995 with
exceptional peak flows. These flows beat a record in
the N'Fis River at Imin Elhammam station where the
recorded peak flow is 1575 m3/s and the rise time is of
9 hours. These violent floods always transport blocks,
pebbles, sand, silt and branches on their way. These
materials usually form dams, which give way under
the pressure of water. The result is a flood of mud,
armed with fine and coarse sediment load, sweeping
fields and taking trees, roads, bridges and houses
(Aresmouk, 2001).
The exceptional hydrological event of november 2014
From November 20 to 30, 2014, the basin of Tensift
has experienced an important rainfall sequence (Saidi
et al., 2015). The stations recorded precipitation
amounts outstanding from usual standards. Two
stations in upstream Ourika, for example, have
recorded 291 and 519 mm of rainfall during 11 days.
After these heavy rains, all the rivers of the basin have
responded with strong hydrological swelling and
increasingly large flows downstream. The stations in
the foothills of Atlas Mountains in Marrakech, like
Aghbalou, Taferiat and Sidi Rahal, which control the
sub-catchments of Ourika, Zat and Ghdat, recorded
respective peak flows of 347, 442 and 340 m3/s. These
are peak flows for which the return period intervals
are between 10 and 20 years. However, the most
spectacular flows, with a very rare frequency, were
recorded in lowland stations, collecting the runoff
from mountainous sub-catchments, especially in the
stations of Abadla and Talmest. According to
observers, the water levels and flood flows of
November 2014 have never been seen before. Thus, no
less than 1597 m3/s was recorded in Abadla and even
3500 m3/s in Talmest station. These new flood peaks
have challenged engineers and researchers who built
their studies and forecasts from a sample, for which
the maximum reaches to 1022 m3/s in Abadla
(recorded on November 3, 1987) and 1275 m3/s in
Talmest (recorded on November 10, 1988). Today,
with the new peaks of 1597 and 3500 m3/s, all the
frequency studies of floods in Tensift have to be
updated. Using a statistical tool that uses a series of
mathematical laws of extreme flows, forecasts and
flood occurrence probabilities in Abadla before the
floods of November 2014 attributed a return period of
500 years; while with the integration of the same
floods of 2014 in the statistical adjustments, the peak
flood of November 30, 2014 (1597 m3/s) would have a
return period of only 200 years. For Talmest station,
the peak of 3500 m3/s which had a return period of
900 to 1,000 years would only have a return period of
200 to 250 years.
J. Bio. & Env. Sci.
2016
172 | Bennani et al.
Updating the hydrological forecasting models by
inserting the recent and exceptional flood peaks will
prompt to update all the estimates about the return
periods of extreme hydrological events. Thus, the
estimates of decennial or centennial floods for
example will be revised and the hydraulic structures
of the future should be established based on the new
hydrological thresholds.
The hydrological explanation of the very strong peak
flows and water volumes observed in Talmest is based
on the exceptional fact that all the sub-catchments of
Tensift (Fig. 11). Contributed to swelling the flow in
the main stream. The recorded flows, from east to
west, in the rivers of Lagh, Ghdat, Zat, Ourika, Issil,
Gheraya, N'fis, Assif Elmal, Chichaoua and Mramer
were all important. The rain disturbances responsible
for these floods had a temporal and most importantly
geographical extension.
These floods have caused significant infrastructure
damages, the remoteness of many communities and
the submersion of agricultural lands, compounded by
the intrusion of water from the river beds into public
water resources. Several damages were reported at
certain hydrometric stations, dams and some
groundwater recharge thresholds of Haouz aquifer
(Hydraulic Agency of Tensift Basin, 2014).
The updating of knowledge and studies about the
floods in the region is, for this purpose, required in a
climate change situation and repetition of extreme
hydro-rainfall events.
Fig. 11. Sub-catchments of the Tensift basin,
contributing to the flow of the main river.
Conclusions
The Tensift river basin is a semi-arid hydro system of
Morocco. It consists of two global morphological
entities: a very high mountainous system and a large
alluvial plain. Rainfall can be heavy on the high
mountains of Atlas, but they are weaker on the Tensift
Plain. However, annual rainfall tends to decrease in
the plain as in the mountains. The trend lines sag
slightly indicating a gradual evolution towards more
droughts. Furthermore, the temperature trends in
Marrakech shows a clear trend towards the increase.
Those evolutions indicate warming and drought
trends, which negatively impacted the surface water
resources in the watershed. In recent years, the rivers
runoffs in the mountain and in the plain have declined
significantly either at annual, seasonal or monthly
scales. Some monthly discharges fell by about 40 to
60% over the past two decades. Climate change has
also affected the strength of extreme events. Intense
rains and strong floods are now observed at the
measuring stations, as was the case in November
2014, when the upstream of Tensift watershed
received between 200 and 300 mm in a few days. The
peak flows resulting from these downpours were
pretty high for some hydrometric stations and even
exceptional for others.
Thus, in the mountainous stations, the peak flows
observed had a return period of 10 to 20 years. The
plain stations, especially Abadla and Talmest, have
collected peak flows that were observed for the first
time since the beginning of hydrometric
measurements more than forty years ago. So, whereas
climate changes affect negatively water resources,
they tend to accentuate the extreme hydro climatic
events. Improving the knowledge of the predictable
impacts of climate change on those resources should
promote the implementation of appropriate
adaptation strategies and actions to the situation of
the concerned countries. This knowledge should also
initiate and strengthen cooperation between countries
to minimize and counteract climate fluctuations
effects.
J. Bio. & Env. Sci.
2016
173 | Bennani et al.
Acknowledgments
The authors are pleased to acknowledge the Hydraulic
Agency of Tensift Basin at Marrakech for discharge
and pluviometric data and the Regional Observatory
of the Environment and Sustainable Development for
Temperature Data. Also, the reviewers are gratefully
acknowledged for their time.
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