Changing Water Resources and Food Supply
in Arid Zones: Tunisia
Mustapha Besbes, Jamel Chahed, Abdelkader Hamdane,
and Ghislain De Marsily
Abstract The notion of water security in an arid country takes on another dimen-
sion when the comprehensive water balance concept is applied to water used by
rain-fed agriculture and to the water equivalent of international food exchanges.
In the case of Tunisia, this concept expands the prospects for improvements in
national food security by optimizing the food balance and the corresponding vir-
tual water ﬂux. It also prompts reconsideration of criteria and indicators classically
used to characterize water stress situations. The current situation shows that about
30% of the water used in Tunisia is imported as food (virtual water); that number
is likely to reach 40–50% in 2025 due to climate change, diet change, demographic
growth, and improved water management. Asia and North Africa will most likely
not be self-sufﬁcient in terms of food production and will need to import food from
other continents (e.g., South America). Africa, however, could be self-sufﬁcient if
its existing water resources are developed. Bioenergy production is likely to be lim-
ited to a small fraction of the global energy needs. Major food shortages in cases of
severe global droughts (e.g., during very strong El Niño events) may occur, however,
with severe consequences in terms of food availability.
Keywords Climate change ·Droughts ·Food production ·Tunisia ·Virtual water
7.1 Water Resource Planning and Management in Tunisia
Tunisia is well suited to a discussion about how water resources in an arid
area are interlinked at the national scale because it is fairly advanced in water
resource planning and management, and its scarce hydraulic resources are almost
entirely mobilized. The country is therefore obliged to apply new concepts and
paradigms, to optimize the use of different types of water resources, and to change
the behavior of some parts of the population (Chahed et al. 2005, 2007; Besbes
et al. 2007).
M. Besbes (B)
National School of Engineers of Tunis, Tunis, Tunisia
G. Schneier-Madanes, M.-F. Courel (eds.), Water and Sustainability in Arid Regions,
DOI 10.1007/978-90-481-2776-4_7, C
Springer Science+Business Media B.V. 2010
104 M. Besbes et al.
Generally, the main water resource management objective is to provide sufﬁcient
quantities to municipalities, industry, and agriculture by developing blue water—
surface water and groundwater. In arid countries where water resources are scarce
and where the economy and the population are developing, the water needs increase
and additional withdrawals are starting to pose problems. The urban and indus-
trial needs, the so-called direct water demand, depend on the standard of living but
remain moderate compared to the large quantities of agricultural water (irrigated
and rain-fed agriculture) known as green water and used in food production. When
local water resources are insufﬁcient to guarantee the food production that the pop-
ulation needs, food imports—known as virtual water—are required to ﬁll the water
deﬁcit (Allan 1998; Renault and Wallender 2000; Hoekstra 2003; Oki et al. 2002)
(see Chapters 13 and 19).
7.2 A Vulnerable System Under Intensive Surveillance
Tunisia is situated in North Africa and is bordered by Algeria, Libya, the
Mediterranean Sea, and the Sahara. The country has a surface area of 164,420 km2
and 10 million inhabitants. The average rainfall is 220 mm/yr, which translates into
a rainfall resource of 36 km3/yr. Total hydraulic resources (blue water) are estimated
at 4.85 km3/yr. The mean runoff is 2.7 km3/yr, of which 2.1 km3/yr can be exploited
through dams. Exploitable groundwater resources are estimated at 2.15 km3/yr. In
2006, groundwater abstraction was estimated to be 1.95 km3, which represents an
exploitation index of 90%. The soil water resource, which is part of the rainfall
resource inﬁltrated into the soil and available for evaporation and consumption
by plants, refers to the arable land (5 million hectares, or ha) and is estimated at
12 km3/yr, the country’s total green water potential.
The total water withdrawals reached 2.64 km3in 2006; 0.4 km3of that was allo-
cated for drinking water and 2.1 km3for irrigation. Of the irrigation water, 75%
comes from groundwater, 23% from surface water, and 2% from reuse of treated
wastewater. The area that can potentially be irrigated is estimated at 560,000 ha.
Most of the demand is concentrated in the populated coastal zones, and some of
the main irrigated areas are situated far from the wettest parts of the country. The
coastal zones therefore use more water than they receive in rainfall and inﬂows,
obliging them to import water. The whole country is now marked by major west–
east water transfers. This situation requires tremendous resource monitoring efforts:
the rainfall monitoring network is composed of more than 900 regular rain gauges;
the runoff network consists of 60 permanent stations and 60 points of regular mea-
surements; and the regular groundwater observation network is made up of 3,800
points. This constantly updated information allows the authorities to review their
resource assessments regularly, focusing mainly on groundwater, as exploitation of
that resource has increased by 250% in the past 40 years. Groundwater pumping,
mainly for agriculture, now reaches 90% of sustainable levels, creating great risks
to the quality of the resource itself. The government has deployed a number of
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 105
measures in its efforts to preserve water resources and provide farmers with incen-
tives to optimize their consumption. For example, in 2005, 75% of the total irrigated
area, which represents 400,000 ha, was equipped with water-saving devices such as
drip and sprinkler irrigation and modern surface irrigation methods because farmers
have responded to ﬁnancial incentives from the government.
Large-scale hydraulic programs make the water cycle strongly artiﬁcial, which
leads to a reduction of the water that feeds natural hydrologic systems, with conse-
quences for the behavior of continental and coastal aquatic ecosystems; a reduction
of recharge to aquifers situated downstream of large dams; and a progressive salin-
ization of soils irrigated with highly saline water. In these conditions, the protection
of the environment and resources requires a continuous assessment of the envi-
ronmental water demand. In the planning of water allowances, artiﬁcial ﬂoods for
wetlands and groundwater recharge or additional irrigation shares to prevent salin-
ization of irrigated soils should be included. Increased irrigation shares for salt
leaching in soils were applied early on, whereas the understanding of other envi-
ronmental needs requiring a direct water allowance from mobilized resources has
been progressive and now has become an essential component of water resource
planning and management. On a national level, the environmental water demand
remains small compared to the urban and agricultural requirements, but it represents
a growing concern in the planning of future hydraulic programs.
7.3 Food Security, Food Trade, and Virtual Water
Tunisia’s food security goals are to satisfy, to the greatest extent possible, the
country’s basic food needs (cereals, oil, meat, milk, etc.). However, Tunisia is not
self-sufﬁcient in some of these products. Most importantly, climate variations cause
large ﬂuctuations in the yield from rain-fed agriculture (Besbes et al. 2008). The
food trade balance of Tunisia has been negative during the last two decades, except
for the rainy years of 1991, 1999, and 2004. This balance (Figs. 7.1 and 7.2) depends
strongly on cereal imports, which represent close to 45% of the total value of food
Fig. 7.1 National cereal
production and imports in
Tunisia from 1990 to 2004, in
103tons. Source: MARH
1995 to 2004
106 M. Besbes et al.
Fig. 7.2 Food imports and exports, 106Tunisian dinars, 1990 to 2004 (1 Tunisian dinar ~0.84 US
$ in July 2008) Light gray: export. Dark gray: import
The direct water needs, which include municipalities, industry, and tourism, are
small compared to the agricultural demand. The concept of virtual water—the quan-
tity of water needed to produce a given type of product—can help in the analysis
of the relationship between agriculture and water resources. Based on production
and trade statistics of food products, one can establish the national budget of water
demand in Tunisia (Table 7.1). In an average rainfall year, half of the water required
to meet Tunisian food needs is provided by rain-fed agriculture, one-sixth by irri-
gated agriculture, and almost one-third by virtual water in the form of imported food.
Tunisia imports the water equivalent of 5.2 km3/yr, essentially in the form of cere-
als, and exports agricultural products such as citrus fruit, dates, olive oil, and early
season produce equivalent to 1.5 km3/yr for an average annual deﬁcit of 3.7 km3/yr
(Chahed et al. 2007).
Table 7.1 Comprehensive water demand of Tunisia (average values for 1990–1997)
Sector Water demand, billion m3/year
Rainfed agriculture 6.0
Deﬁcit of food balance [imported
Forests and rangelands 5.5
Water bank [storage in dams for
Environment [conservation of
Total water demand 18.5
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 107
Fig. 7.3 Evolution of per
capita water equivalent of the
Tunisian food demand during
the last 40 years.
The current water deﬁcit is likely to increase in the future because food require-
ments and the population’s tastes evolve and the pressure on the resource restricts
domestic production. Food demand can evolve quickly because of improved living
standards. The per capita water equivalent of Tunisian food demand (1,400 m3/yr)
has more than doubled over the last 40 years (Fig. 7.3) and will probably continue
to increase in future decades. The implications for agriculture and the food-trade
balance are considerable, especially as the nature of the exchanged products may
ﬂuctuate as markets evolve and agricultural policies are modiﬁed.
7.4 Factors of Change and the Comprehensive Water
Demographic, social, and economic factors will determine the future demand and
availability of water resources. After the strong demographic growth of the second
half of the twentieth century, the Tunisian population is now reducing its growth
rate and will stabilize at about 13 million inhabitants by 2050 (Institut National des
Statistiques 2005). Industrial development and urbanization will also have strong
impacts on water resources, including an increase in the per capita drinking water
requirements and water quality standards, as well as changes in the population’s diet.
In addition to this evolution, the risks to the resources are multiple: groundwater
over-exploitation, increased water and soil salinity, urban and industrial pollution,
etc. The major problem, however, is the management, a few years hence, of a
situation in which 100% of the resources will be mobilized, and all technical, eco-
nomical, and institutional preservation measures will have been applied while the
population and the per capita needs of the Tunisians continues to rise. How can the
water security of the country be guaranteed under these conditions?
One can start with a certain number of observations. The ﬁrst one is that rain-
fed agriculture plays an essential role in food security; it represents, in an average
hydrologic year and with average monetary values, 65% of the national agricul-
tural production and 80% of the agricultural exports. The second observation is
that water security comes down to a food security problem: (a) the largest share
of the mobilized resource (blue water) is used in irrigation, (b) the blue water
exploitable resource stabilizes, (c) the direct demand (drinking water, industry) is
108 M. Besbes et al.
incompressible and increases with the population, and (d) consequently, the blue
water agricultural allowances should necessarily decrease.
The comprehensive water balance model constructed by Chahed et al. (2007)
shows that the water allocated to irrigation is the quantity of blue water available
when the direct water demand has been satisﬁed (equation 7.1). The model also
demonstrates that virtual water expresses the balance of the water equivalent of the
food needs, the water equivalent of green water, and the water equivalent of the
irrigated agricultural production (equation 7.2).
IW =EWR −(1 −RI)∗DD (7.1)
VW =FDWE −GW −k∗EWR +k∗(1 −RI)∗DD (7.2)
IW Irrigation water volume
EWR Exploitable water resource
RI Recycling index
DD Direct demand
VW Virtual water volume
FDWE Food demand water equivalent
GW Green water volume
k Irrigation factor (Converts irrigation volumes into water equivalent of the
irrigated food production; this factor integrates irrigation efﬁciency and
The veriﬁcation of this model consists of testing a certain number of values of
the parameters k (the irrigation efﬁciency factor) and RI (the recycling index for
the direct demand), while providing the data concerning EWR (exploitable water
resource), DD (direct demand), and FDWE (food demand water equivalent), and
calculating the reference variable, VW (virtual water volume). The validation of the
model on the situations of 1996 and 2004 is summarized in Table 7.2.
The calculated virtual water corresponds well to the observed import–export bal-
ance, which is a good validation. It is necessary to keep in mind, however, that
this model is strongly constrained because the total food demand has already been
estimated by the import–export balance; it is therefore not possible to do any fur-
ther calibration of the model but simply to verify the consistency of its results. The
model is very useful for running simulation scenarios. For example, from 1960 to
2000, the per capita food demand water equivalent increased from 1,200 to 1,600
m3/yr in Europe and from 600 to 1,400 m3/yr in Tunisia during the same period. The
projection to 2025 of the Tunisian per capita food demand, expressed by continuing
the trend of the last years, would be 1,700 m3/yr.
Using the comprehensive balance model, the ﬁrst simulation for 2025 maintains
the present situation, except for the population variation. All other elements are kept
at their 2004 level: food demand, direct demand, recycling index, global irrigation
efﬁciency, and the rain-fed sector production. As a result, the virtual water necessary
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 109
Table 7.2 Validation and scenarios in the comprehensive balance model
Units 1996 2004
Population 106persons 9.10 9.93 12.15 12.15 12.15
106m3/yr 2,380 2,500 2,700 2,700 2,700
Food demand water
m3/yr 1,350 1,450 1,450 1,700 1,700
Food demand water
106m3/yr 12,285 14,399 17,618 20,655 20,655
m3/yr 45 55 55 70 70
106m3/yr 410 546 668 851 851
– 0.08 0.1 0.1 0.5 0.5
106m3/yr 2,003 2,008 2,099 2,275 2,275
106m3/yr 6,500 8,000 8,000 8,000 10,000
Irrigation factor, k – 0.9 0.9 0.9 0.9 0.9
106m3/yr 3,982 4,591 7,729 10,608 8,608
Tot al u se 10 6m3/yr 12,695 14,945 18,286 21,506 21,506
– 31% 31% 42% 49% 40%
to close the budget gap increases from 4 to 7.6 km3/yr, and the water dependency
index exceeds 40% (Table 7.2). The second simulation for 2025 corresponds to the
increased living standard tendency. This scenario prolongs the tendencies of the
per capita food demand, which rises to 1,700 m3/yr; the direct per capita demand,
which reaches 70 m3/yr; and recycling. In this case, the virtual water increases to
10 km3/yr, and the water dependency index approaches 50%. In the third simulation,
the productivity of the rain-fed agricultural sector is assumed to grow very strongly,
with a gain of 25%. This would make it possible to reduce the virtual water to the
ﬁrst scenario level.
The ﬁnancial parameters that are not integrated into the model could greatly
inﬂuence the results. For example, the balances and performances of the Tunisian
food trade observed until now were in part possible because the price of wheat
has remained fairly steady and low for a relatively long period. International cereal
prices have remained low because of the state-controlled subsidies in the main pro-
ducer countries and remarkable yields due to intensive use of chemical fertilizers.
However, recent upheavals in cereal prices demonstrate that the situation can change
suddenly and can dramatically inﬂuence the world food trade balances.
110 M. Besbes et al.
The Tunisian example illustrates how blue water, green water, and virtual water
are interlinked and constitute the entirety of the water cycle at a national scale, and
how this general analysis acquires a particular and timely relevance in countries that
have limited water resources and have mobilized a large share of their resources.
This aspect of the issue should call into question certain established concepts, such
as the deﬁnition of water resources, water stress, water policies, integrated water
resource management, and demand management.
Four additional external factors can further inﬂuence the water balance of coun-
tries like Tunisia, as described above, given the more general problem of the water
resources on Earth: climate change, food availability on the global market, human
diet, and food production efﬁciency.
7.5 Climate Change
At present, it is overwhelmingly agreed that greenhouse gas (GHG) emissions
are responsible for the accelerating climate change. The review conducted by the
Académie des Sciences (2006) concluded that the effects of climate change for the
next century are fairly well predicted as far as temperature is concerned, depend-
ing, of course, on the GHG emission scenario. The hydrologic effects are much
more uncertain. Nevertheless, the current prediction is that the temperature increase
will generate a signiﬁcant acceleration of the water cycle, with more evaporation
and an increase in the amount of water vapor present in the troposphere, while the
relative humidity will remain more or less constant. The global rainfall will thus
increase, but its spatial distribution is much more uncertain. Figure 7.4 shows the
Zonal mean precipitation (DJF)
Fig. 7.4 Mean zonal precipitation (mm/day) for December, January, and February for the cur-
rent climate, observed (bold black line), and calculated with 15 models. The thick gray line is a
schematic representation of the precipitation changes for the climate towards the end of the century
(Adapted from Lambert & Boer 2001)
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 111
Fig. 7.5 Dry areas of the world today (See also Plate 9 on p. 340 in the color plate section)
zonal distribution of average rainfall (from pole to pole) for the current climate, as
measured (thick black line) and as calculated for the present time with 15 different
climate models. A large variability between models can be observed, which partly
explains why the model predictions of rainfall for future climates are so uncertain.
The expected general consequences of climate change are a shift towards the
poles of the climate zones, as shown by the schematic thick grey line in Figure 7.4.
The dry areas of the world, as shown in Figure 7.5, would move towards the north
in the Northern Hemisphere and towards the south in the Southern Hemisphere. At
the same time, the upper latitudes and the tropics would receive more rain.
Figure 7.6 shows the precipitation changes (in millimeters per day and percent)
from the second half of the twentieth century to the second half of the twenty-ﬁrst
century, for December to March and June to September, calculated by the Météo-
France model from the Centre National de Recherches Météorologiques for the
International Panel on Climate Change (IPCC) scenario B2 (Académie des Sciences
2006). The same results supplied by the latest IPCC report (2007) from an average
of different models are very consistent with the preceding ones.
Apart from the average precipitation changes, the issue of climate variability
was also considered by the Académie des Sciences (2006) and IPCC (2007). The
probability of occurrence of the annual rainfall can be described by its distribution
function. Climatologists agree that if the average annual rainfall increases, it is most
likely that the whole probability distribution will shift towards an increase. In that
case, the probability of ﬂoods will increase and that of droughts will decrease. The
opposite would be true if the average rainfall decreases. What is not known, how-
ever, is whether the shift of the mean will also affect the distribution and modify,
for instance, its variance. In that case, the probability of both ﬂoods and droughts
could increase, whatever the change in the mean. The latest IPCC report (WG1,
112 M. Besbes et al.
Fig. 7.6 Precipitation anomalies (top: mm/day, bottom: %) calculated for the IPCC B2 scenario
with the French CNRM model, comparing the averages for 1950–1999 and 2050–2099. Left:for
December–March. Right: for June–September. (Source: Académie des Sciences 2006) (See also
Plate 10 on p. 341 in the color plate section)
Chapter 3, 2007) indicates that, based on observations, an increase in the variability
of the climate seems indeed likely (i.e., both a shift in the distribution and a change
of the variance toward more variability). Unfortunately, current climate models are
unable to answer this question; only observations can be used to infer the changes,
but they obviously require long time series.
These changes are, as explained before, quite uncertain, and might perhaps occur
earlier than the second half of this century. The major expected consequences for
the water resources distribution in the world include the following:
For southern Europe, Mediterranean-latitude zones, South America, and southern
•a large decrease, on average, of soil water content (higher evapotranspiration due
to temperature increase and lower rainfall, particularly in summer); a decrease of
rain-fed agricultural production;
•an increased risk of agricultural droughts, which occur during the spring and
summer months and mostly affect vegetation;
•an increased risk of hydrologic droughts, which occur in the fall and winter and
affect the recharge of aquifers and therefore the ﬂow of rivers the rest of the time;
however, this risk is probably lower than that of agricultural droughts because
rainfall reduction occurs mostly in summer months;
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 113
•an increased risk of ﬂoods; very intense rains are likely to occur more frequently;
•an increased risk of forest ﬁres.
For northern Europe, northern Russia, northern America, and equatorial zones
•increased water resources, both in summer and winter;
•an increased risk of ﬂoods, particularly in winter;
•possible increase of droughts.
•ice melting in the Alps (and also in the Himalayas, the Andes, etc.) and on the
polar cap edges (but perhaps an increase of ice at the poles, due to increased
•warmer sea surface temperatures, likely to increase (in strength and/or frequency)
hurricanes in the tropical zones;
•an increased frequency of El Niño–La Niña events; this is still debated but would
mostly affect the monsoon zone;
•sea level rise (about 0.50 m in 2050, currently 3 mm/yr) from general warming
of the seas (thermal expansion) and ice melting;
•a possible effect on the Gulf Stream is sometimes mentioned and would reduce
the temperature in Europe; this is very uncertain and its timing is unknown, but
it would not compensate for the general temperature increase.
For Tunisia, the most likely consequences of climate change seem thus to be a
reduction of rainfall and an increase in the frequency of droughts, with rather severe
effects on green water and blue water availability. Assuming a 10% reduction in
both, and applying it to Simulation 3 for 2025 (Table 7.2), the virtual water import
(food) would shift from approximately 8.6 km3/yr (for a total of 21.5 km3/yr and
a water dependency index of 40%) to 9.9 km3/yr and a water dependency index
The situation described above for Tunisia is likely to affect most of the current
arid zones (Fig. 7.5). Figure 7.6 shows that rainfall decreases will unfortunately
occur in most arid zones, at the desert belt latitudes, as they are the most affected by
climate change, at least at their northern (for the Northern Hemisphere) or southern
(for the Southern Hemisphere) limits. According to Viviroli et al. (2007), the desert
zones cover roughly 18% of the world’s surface and are home to 8% of the world’s
population. The semiarid zone and dry tropical forest represent 18% of the surface
and 25% of the population. Keeping in mind the strong uncertainty of climate
change predictions, one may, as a ﬁrst guess, estimate that the situation likely to
occur in Tunisia may therefore affect more than 20% of the world’s population.
The scenarios developed above for Tunisia with virtual water imports are viable
solutions, provided food is available on the world market and at affordable prices.
But where can food be produced, and will it be by rain-fed agriculture or irrigation?
114 M. Besbes et al.
7.6 Food Availability on the International Market
At the World scale, food production will be a major problem, not just because of
climate change, but also because of demographic growth (Table 7.3). In 2050, it is
expected that around nine billion people will live on Earth.
To produce the necessary additional food, arable land is required. Table 7.4
presents the surface area available for agriculture, per continent. A large increase
of agricultural efﬁciency, both in rain-fed and irrigated areas, is needed but is not
sufﬁcient; fertilizers may become much more expensive, as nitrates are following
oil prices and phosphate reserves may become depleted.
The current rate of increase of irrigated surfaces is 1.34 million hectares per year
(Mha/yr); with 234 Mha of irrigated land in 2000, the irrigated surfaces would thus
cover 331 Mha in 2050. This is insufﬁcient to produce the amount of food necessary
for nine billion people. Unless the present expansion rate of irrigated surface areas
is multiplied by approximately 10, irrigation will not be able to provide the food
needed by 2050; food production will depend on rain-fed agriculture in areas where
Table 7.3 Food needs in 2000 and estimated for 2050, in million tons per year (Mt/yr) equivalent
cereals, taking into account diet changes and population growth
Regions Asia Latin America
Wes t As ia
2000 1,800 272 154 262 –
2050 4,150 520 390 1,350 Same as 2000
2.34 1.92 2.5 5.14 ~1
Source: Griffon (2006): Collomb (1999)
1. OECD: Organisation for Economic Cooperation and Development
2. CIS: Commonwealth of Independent States (former Soviet Republics)
Table 7.4 Cultivated area in 2000 in million hectares (Mha) and area suitable for agriculture
Area World Asia
West Asia and
and CIS OECD
(2000) (a) 1,600 439 203 86 228 387 265
Area suitable for
agriculture (b) 4,152 585 1,066 99 1,031 874 497
a/b 39% 75% 19% 87% 22% 44% 53%
Source: FAO 2006, in Griffon 2006
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 115
Table 7.5 One possible scenario for food production (in Mt/yr) in 2050
Region Asia South America
West Asia and
needed 4,150 520 390 1,350
grown 3,190 ±100 1,704 ±100 166 ±10 1,350
Shortage/Surplus –960 ±100 +1,184 ±100 –224 ±10 0
Source: Griffon 2006
land is still available—mainly South America and Africa. It is also clear from Tables
7.3 and 7.4 that some regions, in particular Asia and West Asia–North Africa—
where the food multiplying factor is very large (around 2.5) and the population
constitutes more than half of the World’s total—do not have sufﬁcient land to grow
their own food: they already use 75% and 87% of the area suitable for agriculture,
Table 7.5 presents one possible scenario of food production that would meet the
demand in 2050, after Griffon (2006). This scenario assumes signiﬁcant technolog-
ical changes to improve agricultural efﬁciency (+50% and +33% in rain-fed and
irrigated agriculture, respectively) in Asia, Latin America, and sub-Saharan Africa;
nominal investment in irrigation; and a major areal increase of rain-fed agriculture
in sub-Saharan Africa and South America to compensate for the deﬁcits in Asia and
West Asia–North Africa that cannot be self-sufﬁcient.
The distribution of cultivated land in 2050, assuming some land is used for
energy production with the same food production scenario as above, is given in
Table 7.6. According to this scenario, the cultivated area is projected to increase
from 1.574 billion ha in 2000 (1.34 rain-fed + 0.234 irrigated) to 3.152 billion ha
in 2050, with 2.587 billion ha for food production (2.174 rain-fed + 0.413 irrigated)
and 0.565 billion ha for bioenergy production. Even if energy production is not
Table 7.6 Cultivated surface areas per continent in 2050 for food and energy production and
remaining protected areas, in Mha
Wes t As ia
and CIS Total
Area suitable for
cropping 585 1,066 99 1,031 874 497 4,152
Protected areas 100 300 0 200 300 100 1,000
Area for food 460 646 99 711 424 247 2,587
Irrigated 250 26 49 17 24 47 413
Rain-fed 210 620 50 694 400 200 2,174
Area for energy 25 120 0 120 150 150 565
Source: Griffon 2006, with some numbers adapted from other sources
116 M. Besbes et al.
included, feeding the planet will require increasing the cultivated area by 1 billion
ha. The natural ecosystems will have decreased from 2.578 billion ha in 2000 to
1 billion ha in 2050, or 1.565 billion ha if there is no bioenergy production. In
conclusion, water and climate change are not likely to be the limiting factors in
controlling the current demographic growth of the planet. There will be enough
land and water to produce the required food in normal years, but with enormous
virtual water trade between continents1and a dramatic reduction of the biodiversity
and natural ecosystems all over the world.
7.7 Virtual Water: Political and Economic Feasibility
As long as food production was in excess (and heavily subsidized) in the developed
world, buying food on the World market was a feasible option and strongly encour-
aged by the United Nations, the World Bank, and other international organizations
and agencies. Local food production was even discouraged and export crops encour-
aged. The recent food price crisis, with the wheat price rising from about $165 (US)
per metric ton ($/t) to more than 400 $/t in a couple of years, has shown that cheap
and easy procurement of food on the World market is no longer a reality. It is likely
that many countries will now turn to food security by increasing their own food pro-
duction. The increased food prices will make it possible for lower-efﬁciency crops
(in terms of man-hour per ton) in the developing world to become economically
feasible and to allow the farmers to survive, as most of the current poverty and
malnutrition occur in rural communities. But in the longer term, food will remain
in strong demand on the international market and food prices are likely to remain
high. Emerging countries like Tunisia that have a strong economy, some mineral
resources for export (mostly phosphates), and limited oil and gas resources will be
able to pay the price of food. If food prices continue to increase and if energy costs
increase less, it may even become economically feasible to use desalinized water for
some high-yield crops, or rather to partly supplement local water resources through
methods such as diluting slightly brackish water. But what about poor countries
with no economic development or natural resources? The only option left is migra-
tion to less deprived areas, as has always been the case in the past when climatic or
demographic conditions have changed. But where to go?
It is necessary to distinguish between local and global droughts. In case of a
severe drought occurring in a country like Tunisia, but not over an entire conti-
nent, it is likely that the World food production will not be affected, or at least not
signiﬁcantly. The food deﬁcit in one region will be compensated by food availabil-
ity elsewhere and by existing stocks. In Tunisia, the government has established a
set of drought management rules (Louati et al. 1999) with the objective of guaran-
teeing national food security in case of a large reduction in seed stocks and fodder
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 117
reserves. In case of crisis, it is recommended that the authorities buy large quantities
of grain on the international market—mainly wheat for human consumption and bar-
ley as fodder—to prevent farmers from selling or consuming the seed for the next
crop; this would eliminate local varieties that are well adapted to the climate and
unavailable on the international seed market. Assuming that the World food market
or stocks are not severely affected by the local drought, this plan seems acceptable.
But severe droughts have occurred in the past, simultaneously affecting sev-
eral continents and very large areas of the planet. In 1998, following a strong El
Niño event, large deﬁcits in grain production were seen simultaneously in China
and Indonesia. These two countries were able to import the required amount of
grain from the World stocks, and no major adverse consequences were felt. The
current global food stocks of cereals, in the order of 400 million tons, which rep-
resents about two months of the current global consumption, fell to a very low
level but were sufﬁcient. These stocks have been decreasing regularly for the last
few years. But a brief look at history may be of interest here. It is well known,
for instance, that the Krakatoa volcanic eruption in 1883 had a worldwide effect
on temperature and rainfall (a global 5% rainfall reduction is often mentioned);
eruptions can thus have a large simultaneous effect on several continents. In 2001,
M. Davis published a historical analysis of the nineteenth century famines and
described two major drought episodes in 1876–1878 and 1896–1900 that simultane-
ously affected at least Australia, Brazil, China, India, and Ethiopia. Contrary to the
general belief that droughts occur locally and are compensated by surplus elsewhere,
severe droughts in this case occurred at the same time on different continents; Davis
(2001) relates these droughts to very strong El Niño events affecting the monsoon
The consequences of the 1876–1878 and 1896–1900 famines were very severe;
about 30 million people died in China and India alone in each of the two droughts
(Davis 2001). Amartya Sen, the 1998 winner of the Nobel Prize in Economic
Sciences, also analyzed these events and determined that in most cases of drought,
which he called “Food Availability Decline,” the major cause of death and famine
was not really the lack of food (Sen and Drèze 1999). Rather, it was the lack of eco-
nomic resources of the poor farmers whose crops had been lost and who therefore
were no longer able to afford the high cost of food. Sen showed, for instance, that
drought and agricultural disaster in one part of Ethiopia caused a large famine and
many deaths in 1975, even as food and the means of transporting it along a major
highway to the famine zone were available in other parts of the country.
In this context, it is of interest to look at the observed historical frequencies
of very strong El Niño events. Ortlieb (2000) tried to reconstruct, from historical
archives in South America, the years of strong and very strong El Niño events from
1525 to 1950. It can be seen from his list that 1876, 1877, and 1899 were indeed
very strong El Niño years, but also that, on average, such very strong El Niño events
occur about twice every century.
In conclusion, it seems that once or twice per century, or perhaps more often if
climate changes affect El Niño variability, a major drought period, possibly last-
ing several years, may affect several continents simultaneously, impacting food
production at the global scale. Stocks will not be sufﬁcient to satisfy demand, as
118 M. Besbes et al.
the current level of stocks will soon be used up, and transporting food to remote
places will still be a problem. The international market prices of food will suddenly
become very high, and “Food Availability Decline” will occur, generating famines
of unknown magnitude. The poor countries or the poor rural communities affected
by the droughts will be the ﬁrst to suffer, but they may not be the only ones. There
is no reason to assume that this cannot occur. What is unknown, however, is when it
will occur. The only feasible measure to prevent such a catastrophe would be to very
signiﬁcantly increase the World food stocks. A study conducted in India has shown,
however, that building up such food stocks is difﬁcult and expensive due to the costs
of constructing the storage facility, storing and preserving the food, and preventing
losses. It would be preferable if the storage facility costs could be shared between
several nations, which also would allow the facilities to be sited where conditions
are the most favorable in terms of expense, climatic conditions, transportation net-
works, means of minimizing losses, etc. Such stocks would also contribute to the
stability of food prices, which have become much too “volatile” since 2007, with
very severe social consequences. It is also true that during severe food shortages,
one could return to a low-energy diet and consume less, but hunger would then be
the fate of the less-favored citizens.
7.9 Food Production Efﬁciency
In agriculture, reducing the water consumption by reducing plant transpiration is
not likely to be effective. As shown by Tardieu (2005), genetically reducing transpi-
ration by the leaves is feasible, but this would also reduce carbon dioxide input into
the leaves through the same stomata and thus reduce biomass production. There
is no known method to reduce one without affecting the other, even with geneti-
cally modiﬁed organisms. Plants can be made more tolerant to periods of drought:
they will survive but not produce. Water can only be saved by reducing the losses
between what the plant actually uses and the water brought to the ﬁeld. These
“losses” include evaporation into the atmosphere (water sprinklers and irrigation
of bare soils) and inﬁltration into the ground. But inﬁltration into the ground is not
really a loss; the underlying aquifer is recharged and the water can be used again or
will ﬂow into rivers. Furthermore, it is essential, particularly in arid zones, to bring
more water than strictly required to leach and drain the soil to eliminate the salts that
accumulate in the upper layer due to evapotranspiration. If soils are not drained, soil
salinization becomes a major threat to productivity.
The major water savings in agriculture will come about by changes in crops and
dietary habits (Table 7.7). The current trend is a strong increase in meat consump-
tion. It is clear from Table 7.7 that a meat diet uses much more water and soil than a
vegetarian one. To obtain water savings in agriculture, one must address the human
diet issue, together with that of crop efﬁciency. Human diet also touches on one of
the growing major health problems of mankind—obesity—which is due to exces-
sive consumption of high-energy food. Initially limited to a few developed countries,
7 Changing Water Resources and Food Supply in Arid Zones: Tunisia 119
Table 7.7 Water needed for food production. Average values of water used in cubic meters per
ton (m3t–1) to produce raw food (consumed fraction, not dry matter).
Plant product Water needed (m3t–1) Animal product Water needed (m3t–1)
Vegetable oil 5,000 Beef 13,000
Rice 1,500 −2,000 Poultry 4,100
Wheat 1,000 Eggs 2,700
Corn 700 Milk 800
Citrus fruits 400
Vegetables 200 −400
Source: Académie des Sciences 2006
this problem is now observed in many areas around the World, including developing
countries, and requires urgent attention.
Food savings can also be achieved by better care and management. It is reported
that 30% of the food bought by consumers in developed countries is actually thrown
out, while up to 50% of the crops can be lost to humidity, rats, and other poor storage
conditions in many developing countries.
7.10 Water Resources Management
in the Twenty-First Century
In normal conditions, the water and soil resources in arid and semiarid zones will
not be sufﬁcient to produce the food needed by their populations because of popu-
lation growth, diet changes, and climate change. Large percentages of virtual water
imports—food produced in humid or temperate zones—will be required. Given the
growing global food demand, increasing agricultural production should be a major
world priority. Although some water savings in agriculture can be expected together
with important increases in crop efﬁciency, it is most likely that food production will
require the development of additional rain-fed agriculture. This is particularly true
in Africa and South America, where arable land is still available for virtual water
trade. Domestic and industrial water supply is, in comparison, a minor problem and
can be resolved by water savings or technology. Food saving and diet constraints
should be implemented.
Although as yet uncertain, the most likely scenario is that climate change will
increase drought frequency, particularly in arid zones. Drought plans should there-
fore be developed and measures implemented from the beginning of a drought
period to organize savings and allocate water to priority users. Food stocks should
be increased, particularly in arid zones; in the future, these zones may have to import
up to 50% of their food to prepare for periods of worldwide drought.
Societies are much more vulnerable to hydrological changes than to changes that
affect only temperature. Therefore, strong research efforts are still needed in cli-
mate modeling, extreme events analysis, and paleoclimatology to better constrain
120 M. Besbes et al.
the predicted effects of climate change on the water cycle. There is a lack of opera-
tional scenarios balancing demand and resources over the next 10–50 years, region
by region, as presented for Tunisia, taking into account water withdrawal, water
consumption, recycling, and water quality.
Soil conservation should also be a priority, as soil availability will be a major fac-
tor for food production. Preventing increases in soil salinity, soil erosion, and loss of
organic matter that increases erosion and prevents inﬁltration is necessary. Creating
contour ridges to prevent erosion and increase inﬁltration should be reconsidered.
But the ﬁnal goal of twenty-ﬁrst century water resources management should be
to protect and maintain biodiversity. This is probably the most fragile and endan-
gered resource on the planet—far more than water and soil. Research efforts are
urgently needed to develop tools to predict the future status and health of ecosystems
for any new agricultural, urban, or industrial development. Which compensating
measures are required to maintain the ecosystems? Should minimum protected
areas be established to preserve nature? When alarming signs are observed, can
an endangered ecosystem be restored before it disappears? These are today’s unre-
solved issues, and their solutions require strong cooperation between hydrologists
1. As Asia and North Africa will not be self-sufﬁcient and will have to import food essentially
from South America.
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