Introduction
Water is a scarce resource in the Mediterranean region, not only in absolute
terms but also through the concentration of precipitation in the winter
months and the high interannual variability with the presence of frequent
droughts (Lionello et al., 2006). Climate change is expected to exacerbate
this situation by increasing potential evapotranspiration, decreasing
rainfall and increasing the frequency and intensity of droughts (Niang et
al., 2014; IPCC, 2014). For example Vautard et al. (2014) calculated
precipitation decreases reaching 20 % for 2 ∘C global warming and
state that southern Europe is likely to experience higher warming than the
global average, especially in summer. Additionally, minimum river flows in
southern Europe may be lowered by up to 40 % by the middle of the century
and streamflow drought conditions may continue to be intensified by human
water consumption, especially due to irrigation (Forzieri et al., 2014).
Climate change is not the only factor affecting water supply and demand;
population and economic growth in the countries of the southern Mediterranean and urbanization in the entire Mediterranean region will very
likely further increase water extractions. The urban population in northern
Africa and southern Europe is expected to increase from 51 to 63 % and from
70 to 80 %, respectively (United Nations, 2014), leading to more water
consumption, higher water demand
for energy production and changes in hygiene behaviour. Additional pressure
on water resources may arise in the southern coastal areas through increased
water use for new industries and in the northern coastal areas due to the
expansion of biofuel plantations (the EU has the objective of supplying
10 % of transport fuel through biofuels by 2020; EU, 2007). Moreover, the
expansion of tourism is expected to increase water demand, especially in the
dry period (Lanquar, 2013).
The combination of these factors will very likely intensify the debate on the
allocation of water resources between the different economic sectors and
intensify the requirements of increasing the water use efficiency in all of
them. The agricultural sector of the Mediterranean may be strongly affected
by this debate since agriculture is the sector that contributes the most to
water withdrawal. On average, around 50 % of total water withdrawal in the
Mediterranean is for agriculture, with strong subregional patterns from
around 1.3 % in Croatia and 12 % in France up to almost 90 % in Syria,
Egypt, Cyprus and Greece (FAO, 2015). These proportions are expected to
further increase in future, especially in the developing subregions
(Faurès et al., 2000). Further complexity is added to these issues by the
fact that there are environmental concerns linked to irrigated agriculture,
including groundwater over-exploitation and negative consequences of
unsustainable management, such as salinization (Souissi et al., 2013).
However, deallocating water resources from the agricultural sector would
affect food security, the economy and the environment. For example, irrigated
agriculture contributes 28 % of GDP in Syria and produces
USD ∼ 33.7 billion in Spain (Rodríguez-Díaz and Topcu, 2010;
Manero, 2008), employs 400 000 people in southern France (AIRMF, 2009) and
provides ecosystem services, such as landscape preservation and biodiversity
conservation (Nieto-Romero et al., 2014).
All this makes it certain that irrigated agriculture will be at the core of
the future discussion on the allocation of water resources. Coping with this
situation without damaging the agricultural sector and while providing the
water needed by the other sectors will require informed discussions and
decisions based on quantitative assessments. Those quantifications,
necessarily, will have to include estimations about the present and future
water requirements for irrigated agriculture as well as the water saving
potential in this sector. However, to date, only few comprehensive studies
have been made on estimations of future irrigation requirements, as reviewed
in the following paragraph.
Doell and Siebert (2002) were probably the first to quantify irrigation water
requirements at the global level. They distinguished two crop classes (rice
and non-rice), analysed with two global climate models (GCMs) and pointed out
the effect of higher climate variability on future irrigation requirement. In
a later study, Siebert et al. (2010) computed irrigation consumptive water
use by means of the Global Crop Water Model (GCWM) for the present time.
Using the FAO's agro-ecological zones model, Fischer et al. (2007) presented
estimations of future irrigation water requirements under mitigated and
unmitigated climate change for different regions, including western Europe,
the Middle East and northern Africa. They came to the conclusion that the
Middle East and northern Africa may be affected by a high water scarcity in
2080, indicating potential difficulties for meeting future irrigation water
requirements. They also indicated that mitigation of climate change would
reduce increases in irrigation requirements, the effect in Europe being
larger than in northern Africa. Konzmann et al. (2013) presented simulated
future irrigation requirements globally for around 10 crop functional types
under 19 GCMs with a previous version of the LPJmL (Lund–Potsdam–Jena
managed Land) model (without the representation of irrigated agricultural
trees) and came to the conclusion that the Mediterranean region may need more
water under climate change. Souissi et al. (2013) compiled data from various
sources, showing estimates of irrigation water use of 181 km3 per year
in 2005 for the Mediterranean region. For 2025, they show a range of 157 to
212 km3, depending on the scenario (business as usual or sustainable
development in relation to water resources policies), pointing to possible
savings in irrigation water but also to potential increases in irrigation
water requirements. Elliot et al. (2014) pointed to the risk of increasing
irrigation water requirements under climate change in some regions, including
the Mediterranean. These conclusions are complemented by a number of
local-scale studies, focused on a reduced number of crops, for example
Teyssier (2006) for the Midi-Pyrénées region in France and
Rodríguez-Díaz et al. (2007) for the Guadalquivir river basin in
Spain. The literature review shows some common elements, indicating that the
Mediterranean region may suffer in the future from a combination of increased
water scarcity and higher water demand.
The present study aims to advance substantially the present research status
by taking into account, in a comprehensive framework, several previously
unconsidered variables: climate change impacts on irrigation water
requirements in the Mediterranean region are simulated with a newly developed
version of the LPJmL model that considers 88 % of irrigated areas and
represents the special structure of Mediterranean agriculture, which is
dominated by perennial crops (Fader et al., 2015). The simulations are
performed for four warming levels and 19 GCMs. LPJmL (Sitch et al., 2003;
Bondeau et al., 2007; Gerten el al., 2004; Schapfhof et al., 2013) is a
mechanistic hydrology and agro-ecosystem model that has important features
for the quantification of irrigation requirements, such as a dynamic coupling
of water, agricultural production and plant physiology, and for the
consideration of changes in phenology through to dynamic growing periods,
sowing dates and flowering times. Additionally, we consider in this study the
link between demographic change and water demand as well as the possibility
of improving irrigation and conveyance systems in future by adopting water
saving technologies and infrastructure.
One of the largest uncertainties in climate change impact research related to
vegetation is the effect of higher CO2 concentrations in the atmosphere
on plant growth, phenology, water requirements and production. In general
higher CO2 in the atmosphere has the potential to increase
photosynthesis and water productivity of plants, especially the ones with
C3 photosynthetic pathways (Hatfield et al., 2011; Ackerman and Stanton,
2013). This is why this effect has been called the “CO2-fertilization
effect”. Nevertheless, the environmental and genotype dependencies and
consequences of co-limitations (especially nutrients and water) as well as
the order of magnitude of the CO2-fertilization
effect and changes in nutritional values are still uncertain (Porter et al., 2014; DaMatta et al., 2010). For example,
DaMatta et al. (2010) reviewed literature and came to the conclusion that the
beneficial effect of CO2 could be offset by higher temperatures and
altered precipitation patterns. FACE (free-air CO2 enrichment)
experiments, enclosure study measurements and modelling efforts have tried
and are trying to shed light on this issue but have not given consistent
results so far (Long et al., 2006; Tubiello et al., 2007; Ainsworth et al.,
2008). Modelling experiments usually deal with this uncertainty by making two
sets of simulations: one using dynamic CO2 concentrations in the
atmosphere as inputs and one with constant CO2 concentrations. The
responses of vegetation will very likely fall in the range of these two
extremes, and in order to assess this in-between space in more detail, we
additionally take one more scenario into account, which represents a
“reduced” CO2-fertilization effect.
Hence, this study aims to answer the following research questions:
How much irrigation water do we need today in the Mediterranean region?
What are the most water-intensive crops?
Which countries have the potential for saving water through changes in the
irrigation and conveyance systems?
How do different levels of climate change impact future irrigation
requirements? Are there subregional patterns (east, south, north)?
Are different crops affected differently by climate change?
What is the potential role of demographic change and water scarcity?
Section 2 gives an overview of the methodology of the study, including model
functioning, CO2-fertilization scenarios and scenarios of improvements
in irrigation technologies. Section 3 presents the results for the present
irrigation requirements and changes under climate, demographic and
technological change. Section 4 shows possible implications and discusses
prospects for further research.
Methods
Vegetation growth, phenology, agricultural production and irrigation water
requirements and withdrawal were simulated with the process-based
agro-ecosystem and hydrology model LPJmL (Sitch et al., 2003; Bondeau et al.,
2007; Rost et al., 2008; Gerten el al., 2004; Schapfhof et al., 2013). LPJmL
was recently developed for the inclusion of Mediterranean crops by Fader et
al. (2015); the result is a model that considers 88 % of irrigated areas
divided into 12 annual crops (temperate cereals, rice, tropical cereals,
maize, temperate roots, tropical roots, pulses, rapeseed, soybeans,
sunflower, sugar cane, potatoes), 7 perennial crops or crop classes (nut trees, date palms, citrus trees, non-citrus orchards, olive trees, grapevine, cotton) and 4 groups parameterized as herbaceous crops (fodder grass,
vegetables, managed grasslands, “other crops”).
Annual crops grow and are harvested according to the heat unit theory and
agricultural trees are implemented as evergreen or summer green trees, where
the fruits are represented by a plant-specific portion of net primary
productivity (NPP). Vegetables and fodder grasses are parameterized as C3
grass and managed grasslands as a mixture of C3 and C4 grasses (see Bondeau
et al. (2007) and Fader et al. (2015) for more details). Agricultural
management is calibrated to best match FAO yields (FAOSTAT, 2014), both for
annual and perennial crops. This routine represents differences in management
intensity (see Fader et al. (2010, 2015) for more details).
Model inputs consist of climate variables and global CO2 concentrations
(see below), soil texture as described in Schaphoff et al. (2013), and a
data set of land use patterns compiled from different sources as explained in
Fader et al. (2015) (see Fig. S1 in the Supplement for crop-specific areas in the
Mediterranean region).
Climate data for the present and past was taken from the Climate Research
Unit data set (University of East Anglia, CRU 3.10) for temperature and
cloudiness and from the Global Precipitation Climatology Centre's (GPCC;
version 5; Rudolf et al., 2010) for precipitation and cloudiness. For the
climate change simulations, the PanClim data set from Heinke et al. (2013)
was used. They performed pattern downscaling of GCM data using global mean
temperature and greenhouse gas trajectories from a reduced complexity climate
model (MAGICC6; Meinshausen et al., 2011) to derive climate scenarios
covering warming levels from 2 to 5∘ above pre-industrial levels
around the year 2100.
We take the effect of higher CO2 concentrations in the atmosphere into
account by analysing three scenarios.
Dynamic CO2 (DYN): LPJmL runs with the corresponding global CO2
concentrations (from the MAGICC6 model; see Heinke et al., 2013) in
accordance with each warming level. DYN assumes a full CO2-fertilization
effect and no limitation of this effect through lack of other resources (most
notably soil nutrients).
Reduced CO2 (RED): this scenario assumes that the CO2-fertilization
effect will occur but for higher warming levels and less strongly than in
DYN. Technically, we implemented this using the CO2 concentration values
of one lower warming level. For example, the CO2 concentrations of
4∘ warming DYN are the same as the 5∘ warming RED, but the
climate forcing is different.
Constant CO2 (CONST): this scenario assumes that plants will not
benefit from CO2 fertilization due to management deficiencies, lack of
resources, climatic stress and higher frequency of extreme events.
Technically, we implement this by keeping CO2 concentrations constant at
the level of 2009 (387.85 ppm), while varying climate forcing according to
the different warming levels.
The CO2 concentration trajectories for all of these scenarios are
plotted in Fig. S2. In LPJmL the potential, non-water-limited canopy
conductance of carbon and water depends on crop-specific net photosynthesis
and the stomata-controlled ratio between ambient and intercellular CO2
partial pressure. This ratio is dynamically simulated by LPJmL but has
maximum values slightly different for C3 and C4 plants (0.8 for C3 plants and
0.4 for C4 plants) under non-water-stressed conditions. Atmospheric demand
(i.e. “unstressed transpiration”) follows a hyperbolic function of canopy
conductance and is included, in turn, into the calculation of net irrigation
requirements (see below). Thus, with higher CO2 concentrations in the
atmosphere plants transpire less per unit of carbon fixed, i.e. they are more
water efficient, and this may reduce irrigation water requirements.
However, since CO2 is a limiting factor for most agricultural plants
(especially C3 plants), higher CO2 concentrations in the atmosphere
stimulate photosynthesis and increase net primary productivity, biomass
formation, and thus total transpiration and irrigation requirements.
Additionally, these changes in transpiration are coupled with changes in soil
evaporation (which decreases with increases in shadow effects from increased
biomass) and plants' water interception (which increases due to a higher leaf
area from the stimulation of productivity) (see more details in Sitch et al.,
2003, and Gerten et al., 2004).
Irrigation water requirements, water withdrawal and transformation of irrigation systems
Irrigation in LPJmL is triggered in irrigated areas when soil water content
is lower than 90 % of field capacity in the upper 50 cm of the soil (the
so-called “irrigated layer”). The soil water content of the irrigation
layer depends on climatic variables (notably temperature and rainfall),
vertical and horizontal water movements, soil evaporation, and also plants'
water extraction by roots. Plants' net irrigation water requirements (NIR) are modelled as the amount of water that
plants need, taking into account the relative soil moisture and the
water-holding capacity of the irrigated layer:
NIRmmday-1=min1fRilDSy-wr,1-wilWHC,
where D (mm day-1) is the atmospheric demand, which depends on
potential evapotranspiration and potential canopy conductance. When the
canopy is dry and potential evapotranspiration tends to infinity, demand
approximates the multiplication of the daily equilibrium evapotranspiration
rate (which depends mainly on net radiation and temperature) and the maximum
Priestley–Taylor coefficient (1.391) with a hyperbolic function. Sy
(mm day-1) is the soil water supply, which equals a crop's specific
maximum transpirational rate at field capacity or declines linearly with soil
moisture. fRil is the proportion of roots in the irrigated layer.
wil is the water content in the irrigated layer. wr is
the water content weighted with the root density for the soil column. WHC
(water-holding capacity; mm) is a soil-texture-dependent parameter that
represents the water content at field capacity (see Schaphoff et al., 2013).
Left four columns: efficiencies linked to irrigation and conveyance
systems. EA: field application efficiency. EC: conveyance efficiency. Right
two columns: explanation of the implementation of the improved irrigation
scenario (IMP). For example, a country with surface irrigation and open
channels, will, in the IMP scenario, move to a combination of surface and sprinkler
irrigation and to mixed-conveyance system (channels and pipelines).
Current
Current
EA
EC
Improved
Improved
predominant
conveyance
irrigation
conveyance
irrigation
system
system
system
system
Surface
Open
0.6
0.7
Mixed
Open channels and
channels
(surface
pipelines
sprinkler)
Mixed
Open
0.675
0.825
Sprinkler
Pipelines
(surface and
channels and
sprinkler)
pipelines
Sprinkler
Pipelines
0.75
0.95
Drip
Pipelines
Drip
Pipelines
0.9
0.95
Drip
Pipelines
Water withdrawal or extraction, also called gross irrigation water
requirements (GIR), is obtained by dividing NIR by the project
efficiencies (EP):
GIRmmday-1=NIREP.
EP is a dimensionless country-specific parameter calculated by Rohwer et al. (2006),
taking into account reported data on conveyance efficiency (EC), field
application efficiency (EA) and a management factor of the irrigation
system (MF):
EP[0to<1]=EC⋅EA⋅MF.
EA represents the water use efficiency in the fields and increases from
surface irrigation systems to mixed (sprinkler and surface systems) and pure
sprinkler systems to drip irrigation systems. EC represents the water use efficiency in the
distribution–conveyance systems usually belonging to farmer associations,
and it is assumed to be linked to irrigation systems. Thus, EC is smaller for
surface irrigation systems (with water assumed to be supplied by open
channels) than for sprinkler and drip irrigation systems, which are assumed
to function with water supplied by pressurized pipelines. MF varies
between 0.9 and 1 and is higher in pressurized and small-scale systems under
the assumption that large-scale systems are more difficult to manage and,
thus, prone to have slightly lower efficiency in water use, especially when
surface irrigation comes into play (see values in Table 1 and more details in
Rohwer et al., 2006).
In order to test the potential for water savings through more efficient
irrigation and conveyance systems, we assume two more scenarios with
improvements in irrigation systems and water conveyance infrastructures in
addition to the status quo regarding irrigation efficiencies as explained
above.
Improvement scenario (IMP): adoption of more water-efficient irrigation
and conveyance systems. In this scenario it is assumed that one level of
improvement in irrigation systems is achieved in every country of the region
(as shown in Table 1). Technically, we implemented this in a set of
climate change runs with changed EP parameters, assuming a higher efficiency
in irrigation and conveyance systems (see Eq. (3) and Table 1).
Most efficient scenario (DRIP): in this scenario, it is assumed that all
countries of the region implement drip irrigation systems combined with water
conveyance through pipelines.
Standard scenario (STS): this scenario represents the business-as-usual
possibility, where irrigation and conveyance systems remain as they are at
the present time.
Simulation protocol and descriptive statistics
Three simulations (STS, IMP, DRIP) were performed for the present time and
analysed as means over the years 2000 to 2009.
In total, 684 simulations were performed for the future: 19 GCMs, 4 warming levels
(from 2 to 5∘ in 1 ∘C steps), 3 CO2-fertilization
scenarios (DYN, CONST, RED), 3 irrigation scenarios
(IMP, DRIP, STS). Results are evaluated for the period 2080 to 2090 as
medians or means as explained by the following equations.
Region and subregion medians and simple means over GCMs were computed as
last steps after averaging over years and aggregation over grid cells and
crops as follows:
NIR80-90,GCMxkm3=∑P=1n∑cr=1n∑Y=1nNIRY,P,cr⋅10⋅areaY,P,cr/nY109,
where NIRY,P,cr (mm day-1) represents the net irrigation
requirement of a crop (cr), for year (Y), in a grid cell (P), according
to one GCM (GCMx). Area (ha) is the irrigated area covered in P by the cr. nY is the number of years for the
period evaluated (11 for 2080 to 2090). A factor of 10 is used to convert
values from millimetres to cubic metres per hectare, and 109 is used to
convert cubic metres to cubic kilometres.
Spatial explicit changes in variables are computed for each scenario and GCM
separately as
NIR_ChangeP[%]=NIR80-90,GCMx,PNIR00-09,P⋅100-100.
Thus, negative (positive) values represent decreases (increases).
Influence of demographic change
For all combinations of irrigation systems, CO2-fertilization effect
and warming scenarios, the influence of demographic change was taken into
account as shown in Eq. (6).
GIRPop,80-90,IRR,WAR,CO2km3=GIR80-90,IRR,WAR,CO2+POP80-90POP00-09-PROD80-09,IRR,WAR,CO2PROD00-09⋅PROD00-09⋅VWC80-90,IRR,WAR,CO2109,
where GIRPop,80-90,IRR,WAR,CO2 (km3)
is the gross irrigation requirement, as an average over the period of
2080–2090 and adjusted for population growth (Pop) for the irrigation
scenario IRR (STS, IMP, DRIP), the warming level WAR and the
CO2-fertilization scenario CO2 (DYN, CONST, RED).
GIR80-90,IRR,WAR,CO2 (km3) is the gross
irrigation requirement as computed in every combination of IRR, WAR and
CO2 without the influence of demographic change. POP (hab) is the
population numbers from the medium fertility scenario in United
Nations (2013). PROD (t) is the irrigated production of agricultural goods.
VWC (m3 t-1) is the virtual water content of irrigated
agricultural products calculated as gross irrigation requirements divided by
production. GIR, PROD and VWC are medians over the 19 GCM runs; 109 is
used to convert values from cubic metres to cubic kilometres.
This computation takes into account the production gains and losses through
different levels of climate change and CO2 fertilization as well as the
changes in the productivity of irrigation water (i.e. changes in VWC). The
output of this approach assumes that population change will linearly decrease
or increase food demand and sheds light on future irrigation requirements in
the case of (a) no future increases in import dependency by increases in
virtual water imports, (b) unchanged diets, and (c) unchanged proportions of
irrigated to rainfed areas in the case of agricultural expansion and no
changes in agricultural management besides the ones linked to modernization
of irrigation systems.
Influence of water scarcity
A quantification of water scarcity for future irrigation requirements was
carried out by comparing the simulated irrigation water requirements under
climate change (with and without demographic change) with four water
availability scenarios. These renewable water availability scenarios (RWA)
were calculated on the basis of AQUASTAT data for the current time at national
level (FAO, 2015). They differ in the external inflows considered as well as
in whether they consider environmental flow requirements or not.
For the politically assured, sustainable scenario (POL_SUS), we only
consider the external inflow secured through treaties at present and exclude
water needed by aquatic ecosystems. Thus, the calculation is
RWAPOL_SUSkm3=RWRI,S+G+InflowE,S,T-OutflowE,S,T+InflowE,G-OutflowE,G+BorderRWRLakes+Rivers-WUmunicipal-WUindustry-EFS-VS+G,
where RWA is the renewable water availability in the politically assured,
sustainable scenario. RWRI,S+G represents the
renewable water resources as a sum of the internally (I) produced surface (S)
and groundwater (G). Double counting is avoided by considering the overlap
variable of AQUASTAT. OutflowE,S,T and
InflowE,S,T are the surface water outflow and
inflow from and to other countries, respectively, secured through
treaties (T). InflowE,G and OutflowE,G
are the groundwater entering and leaving the country.
BorderRWRLakes+Rivers is each country's section of border
lakes and rivers. WUX is the water withdrawal for industry and municipal
use. EFS is equal to 30 % of internally produced
RWRI,S and represents the water needed for conservation of
aquatic ecosystems, an assumption widely used in the hydrological community.
V is equal to 30 % of internally produced
RWRI,S+G and represents the amount of water that is
unavailable due to technical difficulties, lack of infrastructure, temporal
variability, and mismatching of temporal availability and spatial needs.
In the second given, sustainable scenario (GIV_SUS) we additionally
take external inflows not submitted to treaties into account
(InflowE,S,noT):
RWAGIV_SUSkm3=RWRI,S+G+InflowE,S,T+InflowE,S,noT-OutflowE,S,T+InflowE,G-OutflowE,G+BorderRWRLakes+Rivers-WUmunicipal-WUindustry-EFS-VS+G.
Not considering environmental flow requirements in Eqs. (7) and (8) yields
the politically assured, unsustainable scenario (POL_UNSUS) and the
given, unsustainable scenario (GIV_UNSUS), respectively.
AQUASTAT data are for the present time, and thus they do not consider
changes in water availability due to climate change and increases in water
demands through other sectors.
Annual absolute gross irrigation water requirements (GIR), as
average for the period 2000–2009, at a 30 arcmin resolution.
![]()
Panel (a): net irrigation requirements and green water
consumption aggregated for different crops in the Mediterranean region, as
average of 2000–2009. Total bar height represents the crop water needs.
Error bars show the maximum and minimum values for crop water needs published
by FAO (1986). Note that the y axis starts at 1000 m3 ha-1.
Panel (b): national gross irrigation water requirements (GIR) for
current irrigation systems (STS), improved irrigation systems (IMP) and
optimized irrigation systems (DRIP) as the average of the period 2000–2009.
![]()
Maximal and median GIR under climate change, with and without the influence
of population change and transformation of irrigation systems, are compared
to these scenarios for the period of 2080–2090.
Results
Mediterranean region could save 35 % of water at present
Figure 1 shows patterns of GIR in absolute terms for the present time.
Irrigation water withdrawals are especially high in the Nile Delta, the Po
Valley, in the eastern Mediterranean and in some Spanish regions. In total,
the agricultural sector in the Mediterranean was simulated to withdraw
≈ 223 km3 of water per year for irrigation (average
2000–2009). Only around 128 km3 of this amount represent the quantity
of water directly required by plants (NIR). Hence, around 95 km3 of
water infiltrates the ground, evaporates, or leaks
on the way to the plants before being productively used for photosynthesis.
Figure 2a shows irrigation water requirements (coloured in blue) and green
water consumption on irrigated areas (coloured in green) in the Mediterranean
region, ordered from the highest to the lowest NIR. Green water is the
precipitation water stored in the soil and directly available for plants.
Adding both values yields the crop water needs. Sugar cane, mostly
cultivated in Morocco and Egypt, is the most water-intensive crop of the region
(Fig. 2a). Also, date palms, citrus and olives have irrigation water
requirements above 7000 m3 ha-1. Nonetheless, when considering
absolute values of NIR (not shown), as opposed to values per hectare,
temperate cereals, maize, olives and cotton, with NIR above 10 km3
each, are the strongest water consumers (see crop-specific irrigated areas in
the Supplement, Fig. S1). Nevertheless, caution is imperative when
interpreting both indicators (absolute and per hectare), since they represent
averages and sums that are not independent of the location of cultivation
areas and, thus, are influenced by the patterns of potential
evapotranspiration.
Our simulations indicate that the Mediterranean region could save 35 % of
water by strongly improving the irrigation systems and the conveyance
infrastructure: GIR for the DRIP scenario amount to ∼ 143 km3. A
less dramatic improvement (IMP) yields 10 % water savings
(GIR=∼ 200 km3). Especially Egypt, Turkey, Spain and Syria
could save large amounts of water through a switch to more efficient
irrigation systems and infrastructure (Fig. 2b). On the contrary, for example
Libya and Tunisia have not only lower irrigation water requirements but also
much reduced possibilities for saving water through the optimization of
irrigation systems and conveyance (Fig. 2b).
Fader et al. (2015) showed a good agreement with other estimates for NIR at
national and subnational levels and for GIR at national level. Souissi et
al. (2013) compiled data from various sources, showing estimates of
irrigation water use of 181 km3 per year in 2005 for the Mediterranean
region. It is unclear whether they refer to net or gross irrigation
requirements, but this value is within the range defined by our NIR and GIR
values (128 and 223 km3, respectively). Blinda (2012) estimated the
water demand for irrigation use at 181.3 km3 in 2005 (Mediterranean
area excluding Portugal, Serbia and Jordan) and the water lost during
conveyance and distribution at 100 km3. It is not clear how they
computed or collected the data. These numbers are close to ours (223 and
95 km3, respectively).
The ratio of NIR to GIR in our study is 57 % and represents the current
irrigation efficiency. This is in very good agreement with Fischer et
al. (2007), who calculated an irrigation efficiency of 58 % (average of western Europe, defined by these authors as including southern Europe and Turkey, and of the Middle East and northern Africa).
There is a general lack of data on crop water needs; hence, only rather old
data from FAO (1986) could be compared with our estimates (see error bars in
Fig. 2a). FAO values are very generic, i.e. without differentiation for
period of time, region, climate and soil. The comparison yields a fair
agreement with our values for some crops but shows that spatially inexplicit
data may overestimate the water needs for sugar cane, fodder grass,
sunflower, tropical cereals and potatoes in the Mediterranean region.
Median (19 GCMs) of gross irrigation water requirements (GIR) for
four warming levels, three irrigation scenarios (STS, IMP, DRIP) and
three CO2 scenarios (column represents RED and whiskers represent DYN
and CONST). The period 2080–2090 for the whole Mediterranean region is
shown. Note that the x axis starts at 120 km3.
![]()
Climate change will increase irrigation water requirements in the future
Without improvements in irrigation technologies and irrigation water
conveyance (STS), considering no effects of higher CO2 concentrations (CONST)
and looking at the 5 ∘C warming trajectory, GIR increase by around 18 % up to ∼ 264 km3 in 2080–2090 (median of 19 GCMs; Fig. 3). The median precipitation in this trajectory by the end of
the century is around 5300 km3 for the Mediterranean region as a whole,
as opposed to 6000 km3 at present, implying a 10 % decrease. However,
the spread of GCM values is considerable, with values between a 4 and
23 % decrease in precipitation (see Figs. S3–S5 for time series). The full
CO2-fertilization effect (DYN) and the lowest warming level
(2 ∘C) yield a much lower GIR increase of about 4 % (Fig. 3). In
this trajectory changes in precipitation are less uncertain with a median
precipitation decrease of 4 % (GCM range between 2 and 10 %
decrease; see Figs. S3–S5). The influence of outlier GCMs was explored by
calculating the GIR average over GCMs, which shows slightly higher values
than the median (data not shown), demonstrating that results are very robust.
GIR in 2080–2090 will strongly depend on the irrigation technology used by
the farmers and on the efficiency of the nation's conveyance systems
(pipelines or open channels). Figure 3 shows that this factor may have a
larger influence than the strength of warming and the effect of higher
CO2 concentrations in the atmosphere (within every warming level, note
the large differences between the different irrigation scenarios). Also,
regardless of the warming level and the effect of CO2 fertilization,
strongly improving irrigation technologies and irrigation water
conveyance (DRIP) until the end of the century would have the potential of
saving around 30 % of water (compare bars from DRIP and the solid, black
line in Fig. 3).
Interestingly, the results of a limited improvement in irrigation
technologies and irrigation water conveyance (IMP) are heterogeneous. For a
2 ∘C global warming, total withdrawal for irrigation is lower than
current values. For a 3 ∘C warming, the negative effects of climate
change may counteract the gains achieved through the technological
improvements. For 4 and 5 ∘C warming, the negative
effects of climate change may exceed the savings, depending on the actual
effect of CO2 fertilization (Fig. 3).
Note that the RED scenario does not always make up the same proportion of
CONST and DYN since the
implementation of this scenario (see Sect. 2) and the non-linear trajectory
of CO2 concentrations as shown in Fig. S2 made it possible to take a
high diversity in the combination of the CO2-fertilization effect and
warming levels into account. For example, for 2 ∘C warming, the
CO2 fertilization in DYN and RED is assumed to be equal, while it
separates for higher warming levels (see Fig. S2 for more details).
Souissi et al. (2013) compiled data from various sources using the database
of the Blue Plan. They showed estimates of irrigation water use for 2025 in
the range of 157 to 212 km3 (compared to 181 km3 in 2005),
depending on the scenario (sustainable development in relation to water
resource policies and business as usual). The values are naturally lower
than ours (Fig. 3), very possibly due to the shorter time frame, but they
show that both increases in irrigation water requirements and potential
savings in irrigation water are possible depending on the scenario chosen.
This is in good agreement with our results.
The results shown in Fig. 3 represent medians and do not show the spread of
results from different GCMs. Figure 4 shows for which areas there is a high
agreement in NIR results, even using different GCMs as inputs. Under low
warming and DYN, most of GCMs compute that the Mediterranean agricultural
plants would need slightly less water than today. However, 60 to 80 %
of the GCMs also agree on increases in NIR of 15 to 45 % for some French
regions (Fig. 4, upper left panel). With increasing warming, and even
taking into account some realization of the CO2-fertilization effect (RED), increases in NIR between 15 and 45 % spread to the rest of the Mediterranean
region. The GCM agreement under 4 ∘C (RED) is generally lower than
for 2, 3 and 5 ∘C warming but still
robust for large areas in Spain and Algeria (Fig. 4). High warming and
excluding the CO2-fertilization effect yields very high GCM agreement
on important NIR increases, especially strong (> 80 %) in
central France (Fig. 4, lower right panel).
Change in area net irrigation water requirements (NIR) from
2000–2009 to 2080–2090 and GCM agreement (saturation) for 2 to 5 ∘C
warming combined with different CO2 scenarios. See the Supplement
(Fig. S6) for all scenarios arranged according to warming level.
![]()
Regional gross irrigation water requirements (GIR) for the CONST (a),
DYN (b) and RED (c) CO2-fertilization scenarios, as
median of 19 GCMs for the period 2080–2090, for different combinations of
warming levels (x axis), different irrigation scenarios (bars represent the
IMP scenarios; whiskers represent the STS and DRIP scenario), and with or
without consideration of demographic change (light grey bars: without
population change; dark grey bars: with population change). Current regional
gross irrigation water requirement is represented by the red lines. Note that
the x axis starts at 20 km3.
![]()
Figure 2c in Konzmann et al. (2013), calculated for 19 GCMs, the SRES
(Special Report Emissions Scenario) A2 (warming between 2 and
5.4 ∘C globally) and constant CO2 concentrations by the 2080s,
show similar patterns to our Fig. 4 (lowest panel), confirming robust,
generalized increases in the Mediterranean region, except for Egypt.
Moreover, Haddeland et al. (2013) found comparable results, with increases in
potential irrigation water consumption with increasing global mean
temperature for Spain, Portugal and France. Tables 5 and 6 in the study of
Fischer et al. (2007) presented net irrigation water requirements aggregated
for regions for two GCMs (with SRES A2 and B1) by the end of the century and
taking into account the CO2-fertilization effect. They agree with our
results in two points: first, regarding stronger increases in net irrigation
requirements with unmitigated climate change and, second, regarding stronger
effect for western and southern Europe than for the Middle East and northern
Africa.
All the changes shown in Fig. 4 are the result of complex interactions
between a region's management intensity, the chosen mix of crops, the
climate forcing as well as changes in physiological plant responses (e.g.
in the transpiration) and agronomic changes, such as length and beginning
of growing period and yields. Figure S7 gives an overview of the
influence of decreasing precipitation in this signal. With some exceptions
in Libya, Italy and the Balkans, the ensemble median shows that precipitation
decreases with increasing warming, especially in the northern and eastern
Mediterranean for > 2 ∘C global warming. Despite this, the physiological changes (especially yield reduction
due to shorter growing periods) seem to counteract precipitation decreases
in Turkey, the eastern Mediterranean and Egypt, yielding reductions in NIR
for global warming levels < 4 ∘C (Fig. 4). Section 3.4 gives some
insights into this.
Population change may aggravate the water situation
Since some decisions and adaptation measures are taken in supranational
institutions, for example at the levels of the European Union or the Arab
Maghreb Union, we here show results aggregated according to regions (northern,
eastern, and southern Mediterranean; the eastern Mediterranean includes the
area from Turkey to Israel and Jordan). Figure 5 shows an overview of future water
withdrawal per region, taking climate change,
CO2 fertilization, technology improvements in irrigation systems and
population change into account. The eastern Mediterranean is today the highest water
extractor, followed by the northern Mediterranean, and with a smaller
difference, by the southern Mediterranean (red lines Fig. 5).
Climate change alone (without population change, CO2 fertilization and
transformation of irrigation systems) may increase gross irrigation water
requirements 28, 16 and 11 % in the northern, eastern and southern
Mediterranean respectively (Fig. 5a). Full realization of the
CO2-fertilization effect may decrease these numbers to 17, 7 and 3 % (Fig. 5b).
Population growth in combination with stagnation in irrigation technologies,
strong climate change and the impossibility of realizing the
CO2-fertilization effect may drive GIR up to 185 and 118 km3 in
the in the eastern and southern Mediterranean, respectively (Fig. 5a). This
would mean almost a doubling of current GIR in both regions
(∼ 95 % increase in both regions). In the northern Mediterranean, taking population change into account eases the situation slightly since the population is
expected to decrease in this region. However, GIR still increase by around
25 % because the climate change effect offsets the reduction in GIR due to
population decrease.
Improving irrigation technology and the efficiency of irrigation systems has
a large water saving potential, especially in the eastern Mediterranean
(Fig. 5). However, it can compensate for population growth and climate change only when
combined with some degree of CO2-fertilization effect. Looking at the
most optimistic CO2-fertilization scenario (DYN) in the eastern and
southern Mediterranean, only the DRIP scenario delivers lower GIR than
today, when increases due to population and climate change are taken into
account (IMP is always above the red lines in Fig. 5b).
Comparing the red lines with the bars or lower whiskers in Fig. 5a shows
situations where the effects of climate and population change may
compensate for the water savings achieved through improvement and optimization of
irrigation and conveyance systems: climate change would compensate for gains
through IMP (DRIP) in the northern (eastern) Mediterranean at 3 ∘C
global warming if CO2 fertilization does not take place (Fig. 5a). Even
with some degree of CO2 fertilization, the eastern and southern
Mediterranean would need more water than today already at 2 ∘C
global warming if irrigation technology follows the IMP scenario.
Adding up the different values for the subregion yields the magnitude of the
influence of population change on total GIR: without changes in irrigation
systems and conveyance, the Mediterranean region may face increases in NIR of
between 22 and 74 % (2 ∘C global warming combined with DYN and
5 ∘C combined with CONST, respectively).
To summarize, assuming that (a) population change will take place, (b) all
Mediterranean regions can afford some degree of modernization of irrigation
and conveyance systems (IMP), and (c) CO2 fertilization will happen to
some degree but nutrient limitations and other co-dependencies with other
production factors will limit its positive effects (RED), the northern
Mediterranean will need less water than today and the eastern and southern
Mediterranean will need around 35 % more water than today, with the highest
values under 3 and 5 ∘C global warming, respectively.
There is a general lack of analyses from other studies that can be compared
with this section, but our results are in line with some global studies that
detected a strong influence of population growth on other water-related
issues, for example water scarcity indicators (e.g. Vörösmarty et
al., 2000; Schewe et al., 2013).
The results presented so far give an overview when considering all
agricultural products together. However, different crops present contrasting
patterns of change, as shown in the next section.
Agricultural trees affected the most
Figure 6 presents a summary of crop-disaggregated results for NIR change
under different warming levels in the DYN and CONST CO2-fertilization
scenario (see the RED scenario in Fig. S8). Most of the crops will need more
water per area under climate change, even if the full CO2-fertilization
effect is taken into account (Fig. 6a). Grapes are the crop affected most
strongly with increases of up to 30 % in the 5 ∘C warming
trajectory (DYN). Also, all other agricultural trees, especially olives, nut
trees, cotton and fruit trees, show high increases; this is of
particular concern since they are already major water consumers today
(compare Fig. 2a). For 2 ∘C warming, increases are mainly limited to
below ∼ 8 %, but already at 3 ∘C warming, cotton, orchards
and grapes are pushed above ∼ 10 % increases. The second most
strongly affected group of crops is the C4 crops (maize and sugar cane).
Since these crops already have a high water use efficiency, gains through
CO2 fertilization are very limited and, thus, NIR increase between
7 and 9 % in the 5 ∘C warming trajectory. Groundnuts and rice are
less strongly affected, but the areas of these two are very small in the
Mediterranean region. The increases are much stronger when considering that
the CO2-fertilization effect may not be realized: already at
2 ∘C warming NIR increases are as high as 13 % (compare Fig. 6b).
Change in per unit of net area irrigation water requirements (NIR)
from 2000–2009 to 2080–2090 for different crop classes and the
DYN (a) and CONST (b) CO2-fertilization scenario.
Negative (positive) values indicate a decrease (increase) in NIR (see Fig. S8
for the RED scenario).
![]()
The NIR of some C3 annual crops (mainly food and oil crops) could decrease with
increasing warming along with the full CO2-fertilization effect. The NIR
reduction tends to saturate with increasing warming (Fig. 6a). Observing
these crops in Fig. 6b leads to the conclusion that the NIR decreases are
due to the CO2-fertilization effect, i.e. in the case of the realization of this effect not being possible, most C3 crops would face NIR increases.
The patterns observed in Fig. 6 are the results of complex, interlinked
effect chains. Climatic variables and CO2 concentrations affect irrigation requirements directly, for example through the modification of soil
evaporation and interception via the modification of the atmospheric demand
(potential evaporation). Climate change also affects irrigation requirements
via indirect impacts on growing period length, sowing dates and, most
importantly, agricultural yields (Fader et al., 2010). Figure 7 sheds light on
this topic leading to two main conclusions.
First, the reduction in NIR of most C3 crops under the full
CO2-fertilization effect seems to be produced by a predominance of
higher water productivity (lower transpiration) over a shortening of the growing
period and lower yield. The opposite seems to be true for perennial crops and
C4 annual crops, where the stimulation of photosynthesis (and higher biomass
production), the lengthening of growing periods and the positive changes in
potential evapotranspiration seems to offset the reduction in transpiration
due to shorter opening times of stomata (compare Figs. 7a and 6a). These
crops have both yield and NIR increases, with yield increases being stronger.
Second, yield increases peak for many C3 food crops at a 3 ∘C
increase, while yield increases in fruit trees peak at a 4 ∘C or even
5 ∘C increase (Fig. 7a). Most importantly, however, yield increases
and the location of the yield peak depend on the realization of the
CO2-fertilization effect (Fig. 7b). The yield of many important Mediterranean crops and crop classes like olives, non-citrus orchards and vegetables will decrease
already at low warming if CO2-fertilization effect does not take place.
Yield decreases and increases can be minimized and maximized, respectively,
by limiting warming to 2 ∘C in the case of other limiting factors
threatening the realization of the CO2-fertilization effect (Fig. 7b).
Change in yields from 2000–2009 to 2080–2090 for different crop
classes and the DYN (a) and CONST (b) CO2-fertilization scenario.
Negative (positive) values indicate a decrease (increase) in yields (see
Fig. S8 for the RED scenario).
![]()
These results are in good agreement with some detailed studies on specific
crops. For example, Voloudakis et al. (2015) projected yield increases for
cotton in Greece with warming between 2 and 4 ∘C
using the AQUACROP model and taking the CO2-fertilization
effect into account. Tanasijevic et al. (2014) projected a 18.5 % increase in
irrigation water requirements of olive trees in the Mediterranean region for 2050.
Saadi et al. (2015) projected for 2050 a decrease in irrigation water
requirements of wheat by 11 % in the Mediterranean region.
Water scarcity may constrain future irrigation
This section closes the results section by relating the calculated water
requirements to the national water availability scenarios. In the most
restricted scenario, i.e. taking into account population growth, 5 ∘C
global warming, no realization of CO2 fertilization, no improvements in
irrigation technology and assuming that every country reserves 30 % of
internal surface water for aquatic ecosystems (scenarios GIV_SUS and
POL_SUS; see Sect. 2.4), Algeria, Libya, Israel,
Jordan, Lebanon, Syria, Serbia, Morocco, Tunisia and Spain would not have
enough water for satisfying irrigation requirements in 2080–2090. This is
10 out of 22 Mediterranean countries. The rest of the Mediterranean countries,
mostly situated in the northern Mediterranean, seem to have enough renewable
water resources for meeting irrigation requirements in all scenarios.
Six of the countries that cannot meet irrigation water requirements
(Algeria, Libya, Israel, Jordan, Syria and Serbia) would not be able to meet
them even in the most optimistic scenario of climate change and irrigation
technologies (DRIP, 2 ∘C warming, DYN CO2-fertilization scenario).
The other four could potentially meet their irrigation requirements under
some scenario combinations. Tunisia and Lebanon could do so by strongly
improving irrigation and conveyance systems (DRIP) and ensuring the
beneficial effects of CO2 fertilization (DYN or RED) if global warming
were limited to < 5 ∘C in the case of Tunisia and to < 3 ∘C
in the case of Lebanon.
Morocco and Spain could meet their requirements already with a medium
improvement of irrigation and conveyance systems (IMP) regardless of the
warming level or while ensuring that global warming stays below 3 ∘C.
The comparison between the POL_SUS and GIV_SUS
scenarios indicates that Tunisia and Spain have more possibilities of
meeting irrigation requirements if they ensure the given external inflow
from other countries through international treaties.
The comparison of all these scenarios with the ones without reserving water
for environmental flow requirements (POL_UNSUS and
GIV_UNSUS; see Sect. 2.4) gives an indication of countries
that can be at risk of having trade-offs between food production and the protection of aquatic ecosystems. This is the case of Algeria, Syria,
Serbia, Tunisia, Morocco, Lebanon and Spain. In some scenarios these
countries meet irrigation requirements only when not taking environmental flow requirements into account.
Caution is imperative when interpreting these results since they represent
an optimistic scenario by comparing irrigation water needs with current
water availabilities, i.e. they do not take into account increases in
industrial and domestic water use or direct impacts of climate change on the
water resources. A global multi-model assessment of climate change impacts
on water resources yielded strong and robust reductions in surface runoff for most of the Mediterranean region (Schewe et al., 2013). A second study
that takes into account current dams, practices and land use patterns also
showed that reduction in surface runoff is likely in this region (Haddeland
et al., 2013), but the uncertainty regarding the magnitude remains high. It is worth
highlighting that the change in river discharge is especially uncertain
among global hydrological models for the eastern and southern Mediterranean
(Schewe et al., 2013). In order to test the sensitivity of results to a
drastic decrease in water availability, we compared the irrigation
requirements and water availability of the most pessimistic and optimistic
combinations of scenarios. Assuming 5 ∘C warming, CONST
CO2-fertilization effect, STS irrigation technologies and 30 %
reduction in the POL_SUS water availability scenario
(politically secured with reserves for environmental flow requirements),
results in terms of the capacity of meeting requirements only change for
Egypt and Cyprus. Both countries would be able to meet requirements with
POL_SUS water availabilities but not under a reduction of
30 % in POL_SUS water availability. Assuming 2 ∘C
warming, DYN CO2-fertilization effect, DRIP irrigation technologies and
a 30 % reduction in the GIV_UNSUS water availability scenario
(given availability, no reserves for environmental flow requirements), the results only change for Algeria and Syria, which would be able to meet
requirements with current water availabilities according to
GIV_UNSUS but not with a 30 % decrease in GIV_UNSUS.
The figures shown in this section are mainly in line with the multi-model
effort of Elliot et al. (2014) and confirm that future irrigation
requirements will face water availability constraints, especially in the
southern Mediterranean. Also, the study by Fischer et al. (2007), in spite of
methodological differences, indicates that the Middle East and northern
Africa may be affected by a high water scarcity index (agricultural water
withdrawal to internal renewable water resources up to 96 %) in 2080,
designating potential difficulties for meeting future irrigation water
requirements. For Europe and Turkey they have a much lower value (up to
10 %), which is also in good agreement with our results.
Conclusions and discussion
This study systematically assesses how climate change and increases in
atmospheric CO2 concentrations may affect irrigation requirements in
the Mediterranean region in the context of demographic and technological
change. The comparisons with other estimates presented in the results section
reveal a strong robustness of our results that allows drawing some
conclusions.
At present the Mediterranean region could save 35 % of water by
implementing more efficient irrigation and conveyance systems. Some countries
like Syria, Egypt and Turkey have a higher saving potential than others
(e.g. Tunisia, Libya and France).
Without the positive effects of higher CO2 concentrations in the
atmosphere, a large proportion of climate models gives a robust signal of increasing
net irrigation requirements in the Mediterranean region at 3 ∘C global
warming and above. This is the result of a spatially explicit, complex interplay
of modifications in growing periods, potential evapotranspiration, precipitation
patterns and physiological responses.
Currently some crops, especially sugar cane and agricultural trees, consume
on average more irrigation water per hectare than annual crops. Different
crops show different magnitudes of changes in net irrigation requirements,
the increases being most pronounced in agricultural trees. The
CO2-fertilization effect can lower or counteract the increases in NIR of
some C3 annual crops.
Gross irrigation water requirements may increase or decrease depending
on the future efficiency of irrigation and conveyance systems, the effect of
population growth on food (and water) demand and the climate change impacts,
while the first two seem to have the strongest influence. The Mediterranean
area as a whole may face an increase in gross irrigation requirements
between 4 and 18 % from climate change alone if irrigation systems and
conveyance are not improved (2 ∘C global warming combined with the full
CO2-fertilization effect and 5 ∘C global warming combined with
no CO2-fertilization effect, respectively). Population growth increases
these numbers to 22 and 74 %, respectively.
Subregional patterns of GIR change are complex and vary depending on the
combination of climate change, irrigation technologies and CO2 fertilization.
The northern Mediterranean will need less water than today, and the eastern and
southern Mediterranean will need around 35 % more water than today, assuming
that population growth may increase food demand, that all subregions can afford
some degree of modernization of irrigation and conveyance systems (IMP), and
that CO2 fertilization will happen to some degree (RED).
In some scenarios water scarcity may constrain the supply of the irrigation
water needed in future in Algeria, Libya, Israel, Jordan, Lebanon, Syria,
Serbia, Morocco, Tunisia and Spain.
Similar forcing, heterogeneous implications
As explained in the introduction, the amount of water needed for
agricultural production in the Mediterranean region is a topic of economic
and social relevance with political implications. The results of this study
show that political incentives for water saving technologies as well as the development of efficient public water conveyance systems may help to reduce
water extractions already today but also under future climate change. This
is especially true for the eastern Mediterranean (Figs. 2b, 3 and 5).
Taking into account that irrigation water availability may be increasingly
limited in the future by competing uses, land use change, and climate change,
the Mediterranean region may be very interested in supporting the limitation
of climate change to 2 ∘C global warming in order to potentially
reduce irrigation requirements and require lower investments in irrigation
technology and infrastructure. Already at 3 ∘C global warming, the
investment and incentives needed to compensate for climate change may be much
more important than under 2 ∘C (Fig. 3).
Climate models deliver a consistent picture in the Mediterranean region:
France seems to have the highest risk of suffering from higher irrigation
requirements, even at low warming levels but especially pronounced at high
warming levels. The agreement of climate models for Spain, Turkey and Greece
are shown to be especially strong in the case of the
CO2-fertilization effect not being realized (Fig. 4). For these
countries, sustainable management of soil nutrients and soil water
conservation techniques may help to benefit from the CO2-fertilization effect.
The importance of drivers of change in irrigation requirements differs from
region to region. Climate change may be the most threatening factor for the
northern Mediterranean, while population change combined with strong water
scarcity seem to be the most important detrimental factors in the eastern
and southern Mediterranean. Strong technological improvement may compensate
for the increases in irrigation requirements due to climate change in the
northern Mediterranean. In the eastern and southern Mediterranean a medium
improvement of technologies even combined with low warming and a full
CO2-fertilization effect would not be enough to avoid increases in
gross irrigation water requirements (Fig. 5). And most importantly, these
increased irrigation requirements have a high risk of not being met due to
water scarcity (Sect. 3.5). In this context, the governments of the southern
and eastern Mediterranean may be interested in supporting climate change
mitigation along with economic development in order to produce the financial
means for virtual water imports, increasing sustainable water supply
infrastructure and decreasing the water demand of all sectors.
Improving irrigation technology is not the only way of coping with water
scarcity. For example soil water conservation techniques, such as mulching
and zero tillage, may help to reduce irrigation requirements, especially in
the regions where irrigation is meant to complement rainfall. The influence
of these factors is being analysed by a group at the Mediterranean Institute
of Marine and Terrestrial Biodiversity and Ecology in order to explore
adaptation options under climate change. Switching the type of crops within
the agricultural areas may offer another adaptation option to cope with
increases in irrigation requirements. Annual crops seem to be less prone to
increases in irrigation requirements and decreases in yields than
agricultural trees, the relationships between both being complex (Figs. 6
and 7). Given that agricultural trees are an essential part of Mediterranean
culture and agriculture, two implications follow: first, governments may be
interested in developing plans for protecting and supporting farmers linked
to agricultural trees and perennial shrubs; second, research agencies and
researchers may be interested in focusing efforts for a better understanding
of this kind of
trade-offs and assessing the potential for adaptation in more detail.
Another option for several countries is avoiding a direct relationship
between food demand and population growth by increasing virtual water imports
and improving agricultural management (additionally to improvements in
irrigation technology) (see, e.g., Fader et al., 2013). These are much discussed topics with political, environmental and
economic implications, and both the topics and implications need careful consideration in order to avoid self-induced food security risks. Other countries with a high risk of
depleting the water needed by aquatic ecosystems, as is the case in Algeria,
Serbia, Tunisia, Morocco, Lebanon and Spain (Sect. 3.5), may want to combine
different strategies: first, supporting climate change mitigation; second,
changing their land use strategies, including changing crops; and third,
developing monitoring systems that allow keeping control of water extractions
that are to the detriment of aquatic ecosystems.
Finally, collaboration, know-how transfer and cooperation in mitigation and
adaptation, including a coordinated Mediterranean negotiation at the
Conference of the Parties linked to the United Nations Framework
Convention on Climate Change, may help the region to tackle the challenges described. As described in the next section, this has to go hand in hand with
future research efforts aimed at both reducing the uncertainty of results
and improving the understanding of the functioning of the Earth system as a
managed space.
Strengths, weaknesses and perspectives for future research
This section offers an overview on the strengths and weaknesses of the
present study and ideas for improving research on this topic in future.
One of the strengths of this study is the dynamic simulation of growing
periods, sowing dates, crop varieties, and phenology in agricultural trees.
This allows for more realistic estimates of future irrigation requirements
since some adaptation options and changes in the length of growing periods
are already included in future simulations. However, the possibility of
adapting through deficit irrigation (fulfilling only a part of vegetation
water needs and thus lowering future irrigation water needs) was not taken
into account. Challenges regarding the implementation of deficit irrigation
are not only the complexity and non-linearity of the physiological response
to low levels of water deficit but also that these responses vary largely
depending on the growth stage at which the water deficit is induced (FAO,
2002). Jägermeyr et al. (2015) recently made advances related to this
topic and, in a sensitivity analysis, found that C4 plants can tolerate more
water deficit in the soil than C3 plants. In order to give an indication of
the sensitivity of our results to the implementation of deficit irrigation,
we performed an additional run for the present time lowering the threshold of
soil water deficit at which irrigation happens in LPJmL (from < 90 to
< 70 %). Irrigation water withdrawal was around 8 % lower in this run
than in the standard run, i.e. water savings of 20 km3 for the entire
region were achieved. However, yields did not stay unchanged: while annual
crops were comparatively insensitive (yield decreases < 1 %), the yield
of vegetables and fodder grasses decreased by around 4 % and the yield of
agricultural trees by around 8 %, with the highest values in non-citrus
orchards and for citrus trees (∼ 11 %). This experiment highlights
the importance of including agricultural trees in studies focused on water
savings through deficit irrigation.
The present study is focused on the water needs for keeping production and
cultivation mainly on current irrigated areas. Land use change and irrigation
expansion were considered in a simplified way through a linear relationship
between food demand and demographic change. Future diet changes with
associated land use shifts towards more meat and water-intensive products
were not taken into account. There is a general lack of information on these
issues, especially on crop-specific land use patterns in the future, and this
is the reason why this could not be taken into account. However, a recent
global estimate yields a potential for compensating for between 12 and 57 %
of productivity loss caused by climate change in around 2090 (RCP8.5) by the expansion of irrigation (Elliot et al.,
2014). Nevertheless, further research that includes various land use
scenarios according to different drivers, taking into account groundwater
dynamics, specifically including regional crops, and coupling water resources
and vegetation growth is urgently needed for the Mediterranean area and will
be part of future research efforts.
In the modelling framework used for simulating irrigation requirements plant
growth is influenced by parameters representing different components of
current agricultural management intensity (see also Fader et al., 2010).
Assuming that nutrient deficits, soil erosion and salinization may limit the
realization of the CO2-fertilization effect, this study deals with the
linked uncertainties by analysing different scenarios of the CO2-fertilization effect, and, for the first time, including a scenario
of a reduced CO2-fertilization effect. This is one of the strengths of
this study but indicates the necessity of further model development towards
a process-based representation of the phosphorus, potassium and nitrogen
cycles coupled with the photosynthesis and respiration routines (see,
e.g., Soussana et al., 2010). A research group in the Potsdam Institute for
Climate Impact Research is working on tackling the implementation of the
nitrogen cycle, which will also open up the possibility of better representing
alternative farming practices. Also, one has to keep in mind that crops
grown under increased CO2 concentrations may have a lower nutritional
value and the realization of the CO2-fertilization effect may require
large efforts for managing efficiently all production inputs, which may
represent an important challenge (Porter et al., 2014; DaMatta et al., 2010).
Our research shows large adaptive potential through the implementation of
drip irrigation and efficient conveyance systems (pressurized pipelines).
High-tech irrigation systems may offer advantages such as the conservation of
fertilizers, reduction of water logging and higher yields due to high
uniformity. However, less efficient irrigation and conveyance systems with
high percolation and infiltration rates may have benefits, for example
groundwater recharge, salt leaching, crop cooling, frost protection and high
return flows in downstream areas, supporting food production and food
security (e.g. Bastiaanssen et al., 2007). Thus, in order to avoid conflicts
between up- and downstream water users, efforts must be put into local
solutions based on the integral management of water resources at the
watershed level. In addition to this point, drip and high-tech irrigation
systems require high investment and high maintenance from qualified
technicians for avoiding problems related to clogging and salinization
(Belhouchette et al., 2012). Also, farmers may use the saved water for
planting higher-value crops or base their crop choices on water productivity
rather than on total water consumption. More research on the socioeconomic
and cultural constraints of the implementation of efficient irrigation
systems and on complementary measures like climate-smart agriculture,
planting drought-resistant crops, rainwater harvesting and the
diversification of production systems is needed to bring these conclusions to
the fields (Pedrick, 2012; Blinda, 2012).
When analysing the water saving potential of more efficient irrigation
systems and water conveyance infrastructure, we disregard the fact that more
efficient irrigation systems usually require more energy (and have higher
investment costs). If this additional energy was to be provided by burning
fossil fuels, a positive feedback would be created: climate change increases
irrigation requirements, which lead to technological transformation, which in
turn leads to higher energy demand and finally to more fossil fuel burning
and more climate change. Even if this omission was intentional in order to
assess non-energy-limited potential for adaptation, the non-fossil-fuel possibilities for supplying the energy needed for more efficient irrigation
systems should be at the core of future research efforts.
The analysis of water scarcity carried out in the present study is a way of
pointing out which countries could potentially face water shortage with
regard to future irrigation. Table S1 in the Supplement summarizes the characteristics, advantages and
disadvantages of this approach. On the one hand these results are based on a
rather optimistic scenario by comparing potential (not limited) irrigation
water needs with current, renewable water availabilities at national level,
i.e. they may mask subnational, seasonal patterns of water stress and they
do not take into account future changes in industrial and domestic water use
as well as direct impacts of climate change on the water resources. On the
other hand, they may represent a pessimistic scenario by not considering
fossil groundwater availability, desalination potential and the potential for
water recycling and reuse. The first point is justified by the continuously
dropping groundwater levels (e.g. Wada et al., 2010), which may lead to
water depletion in the near future. Regarding desalinization, water
recycling and reuse, these processes are very energy and cost
intensive at the moment (Elimelech and Phillip, 2011; Blinda, 2012), making their future
development very uncertain. As important all these factors are, there are
unknown variables and a general lack of data that constrain large
improvements in the approach applied in the present study. Interdisciplinary
efforts aimed at the development of socioeconomic, technological and
political scenarios that can be integrated with studies on the impact of climate
change on water resources are urgently needed to fill these gaps.
The model used for this study (LPJmL) includes a dynamic coupling of
photosynthesis, water stress and CO2 uptake (Gerten et al., 2004) and
was recently further developed and successfully tested for including the
most important crops in the Mediterranean region (Fader et al., 2015). Thus,
LPJmL is probably the most complete, mechanistic agro-ecosystem model for
the Mediterranean region at present. However, this study is focussed on
potential net and gross irrigation requirements, i.e. we assumed that
irrigation needs are always met and, as a posterior step, we compare these
needs with water availabilities. This implicitly constrains the assessment
of production increases in water-scarce regions through the supply of water
saved in other regions as well as the assessment of “more crop per drop”
potential. The study of Jägermeyr et al. (2015) looks into these issues
for the present time and argues that transpiration and non-beneficial water
consumption are not as closely related as previously assumed, i.e. it states
that there is a large potential for producing more food with less water.
Further research on the dynamics of this relationship under climate change
is greatly needed to complement our findings.
In summary, the present study offers new, detailed evidence about potential
increases in water needs and possible water shortages for irrigation due to
future climate and demographic change. These results are complemented by a
comprehensive analysis on how Mediterranean societies could adapt to this
situation by improving irrigation and conveyance systems.