Impacts of climate change on olive crop evapotranspiration and irrigation requirements in the Mediterranean region

Agricultural Water Management 144 (2014) 54–68

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Impacts of climate change on olive crop evapotranspiration and irrigation requirements in the Mediterranean region Lazar Tanasijevic a , Mladen Todorovic a,∗ , Luis S. Pereira b , Claudia Pizzigalli c , Piero Lionello d a

CIHEAM, Mediterranean Agronomic Institute of Bari, Via Ceglie 9, Valenzano (BA), Italy CEER, Instututo Superior de Agronomia, Universidade de Lisboa, Portugal c SAIPEM S.p.A., Ocean Engineering Department, Fano (PU), Italy d CMCC, University of Salento, Lecce, Italy b

a r t i c l e

i n f o

Article history: Received 21 March 2014 Accepted 29 May 2014 Keywords: Climatic suitability for olive cultivation Crop water requirements Phenological dates Olive flowering Irrigation strategies Rainfed olive cultivation.

a b s t r a c t The Mediterranean basin is the largest world area having specific climatic conditions suitable for olive cultivation, which has a great socio-economic importance in the region. However, the Mediterranean might be particularly affected by climate change, which could have extensive impacts on ecosystems and agricultural production. This work focussed on the climate change impact on olive growing in the Mediterranean region considering the possible alterations of cultivable areas, phenological dates, crop evapotranspiration and irrigation requirements. Monthly climate data, with a spatial resolution of 0.25◦ × 0.25◦ (latitude by longitude), have been derived from Regional Climate Models driven by ECHAM5 for the A1B scenario of the Special Report on Emissions Scenarios (SRES). The data used in the analysis represented two time periods: (i) present, called year 2000 (average values for the period 1991–2010), and (ii) future, called year 2050 (average values for the period 2036–2065). The areas suitable for olive cultivation were determined using the temperature requirements approach known as the Agro Ecological Zoning method. Crop evapotranspiration and irrigation requirements were estimated following the standard procedure described in the FAO Irrigation and Drainage Paper 56. Results showed that the potentially cultivable areas for olive growing are expected to extend northward and at higher altitudes and to increase by 25% in 50 years. The olive flowering is likely to be anticipated by 11 ± 3 days and crop evapotranspiration is expected to increase on average by 8% (51 ± 17 mm season−1 ). Net irrigation requirements are predicted to increase by 18.5% (70 ± 28 mm season−1 ), up to 140 mm in Southern Spain and some areas of Algeria and Morocco. Differently, effective evapotranspiration of rainfed olives could decrease in most areas due to expected reduction of precipitation and increase of evapotranspirative demand, thus making it not possible to keep rainfed olives’ production as it is at present. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Olives (Olea europaea L.) are among the oldest domesticated species and one of the best adapted crops to the marginal subhumid and semi-arid lands of Mediterranean. Olives are a cash crop of great economic importance and may be considered a strategic crop in the region because highly adaptable to dry spells and drought, and able to attain acceptable yield under dry farming. Nevertheless, recently, the irrigation of olive orchards took increased

∗ Corresponding author at: CIHEAM, Mediterranean Agronomic Institute of Bari, Via Ceglie 9, 70010 Valenzano (BA), Italy. Tel.: +39 0804606235; fax: +39 0804606206. E-mail address: [email protected] (M. Todorovic). http://dx.doi.org/10.1016/j.agwat.2014.05.019 0378-3774/© 2014 Elsevier B.V. All rights reserved.

in importance because traditional olive groves are progressively abandoned giving place to high intensive orchards that produce larger yields and economic returns (Duarte et al., 2008; de Graaff et al., 2010; Freixa et al., 2011). The Mediterranean region seems to be particularly affected by climate change. The warming is projected to be greater than the global average, with also a large percent reduction of precipitation and an increase in its inter-annual variability (Giorgi, 2006). A pronounced decrease in precipitation over the Mediterranean is expected, except for the northern areas (e.g. the Alps) in winter (Giorgi and Lionello, 2008). The authors explained this drying with the increased anti-cyclonic circulation that yields increasingly stable conditions and with a northward shift of the Atlantic storm track. Giorgi and Lionello (2008) also predicted a pronounced warming, maximum in the summer season, with high inter-annual

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variability and a greater occurrence of heat waves and dry spell events. Hertig and Jacobeit (2008) reported a similar temperature increase, ranging mostly between 2 and 4 ◦ C when the period 2071–2100 is compared to 1990–2019, depending on region and season. Such climate changes, along with rising CO2 concentration, are expected to have extensive impacts on ecosystems and agricultural production with associated consequences on water availability and distribution, pest and disease occurrence, and overall socio-economic development. Probably, the most affected variables will be the duration of phenological stages, crop evapotranspiration, irrigation requirements and biomass growth and yield (Osborne et al., 2000; Pereira and De Melo-Abreu, 2009; Quiroga and Iglesias, 2009). Further, though the factors that prevail regionally may change over time gradually, a more rapid climate change may occur in some areas. Crop growth and development strongly depends on local climatic conditions. Each crop has different climatic and environmental requirements for normal growth (e.g., temperature, light, slope orientation, soil fertility, water availability, nutrients). These variables may be affected by climate change, especially temperature, due to its impact on the plant development (Galán et al., 2001). Temperature rise leads, in most areas, to a shift in the optimal growing period, often by a month or more into the winter season, and may sometimes even cause a change in the cropping pattern (Galán et al., 2005; Avolio et al., 2008). Thus, some crops, which currently grow mostly in Southern Europe, will become more suitable for cultivation further north or in higher altitude areas in the south (Audsley et al., 2006; Olesen et al., 2007; Moriondo et al., 2010). A study by Gutierrez et al. (2009) showed that in Italy the areas suitable for olive cultivation are expected to extend and include new zones at higher elevations in central Italy and the Po Valley in the north. Climate warming may also increase the range of olive fly northward. Similar conclusions were reported for the Mediterranean region by Ponti et al. (2013). Global warming is expected to affect the phenology dates of plants, particularly olives (Galán et al., 2005; Bonofiglio et al., 2008; García-Mozo et al., 2010; Oteros et al., 2013). There is an overall trend of earlier occurrence of key phenological events such as flowering and a consequent shortening of the crop growth phases (Osborne et al., 2000; Giannakopoulos et al., 2009; Moriondo et al., 2008, 2010; García-Mozo et al., 2010). This advance of phenological phases is more evident in arboreal than in herbaceous crops (García-Mozo et al., 2010) resulting in a shorter time for biomass accumulation and yield formation (Bindi et al., 1996; Olesen et al., 2011). Flowering is the critical phase for olive development, hence it may be a sensitive and reliable indicator of inter-annual variability of temperature in the Mediterranean, particularly for spring temperatures (Osborne et al., 2000; Orlandi et al., 2005, 2010). De Melo-Abreu et al. (2004) reported that olives require a certain period of low temperatures (chilling requirements) for normal flowering, hence with a warmer climate olive flowering could advance for almost one month, while much warmer scenarios indicate no normal flowering in some varieties. Oteros et al. (2013) demonstrated that changes in phenological dates not only vary with temperature and water availability but also with altitude and exposition. Model simulations indicated that the flowering date for olives in the western Mediterranean could occur significantly earlier by the end of this century (Osborne et al., 2000). Several studies, using different methodologies, addressed the temperature requirements before the start of flowering of olives in different sites of Spain and Italy (Galán et al., 2001, 2005; Avolio et al., 2008; Bonofiglio et al., 2008). They concluded that with the further temperature rise it could be necessary to introduce new varieties with lesser chilling requirements; otherwise, it would be required to move production into other areas with lower temperature. In fact, due to the changes in temperature and precipitation patterns, the

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area climatically suitable for olive cultivation could be enlarged northwards and to higher altitudes, thus increasing the range of areas suitable for olives into new areas of France, Italy, the Balkans and the northern Iberian Peninsula (Bindi et al., 1992; Bindi and Howden, 2004; Moriondo et al., 2008; Gutierrez et al., 2009). Olive is sensitive to longer periods of freezing and, although resistant to water shortage, produces best with high rainfall or with irrigation (Palomo et al., 2002; Moriana et al., 2003; Iniesta et al., 2009; Palese et al., 2010; Martinez-Cob and Faci, 2010). Therefore, the olive cultivation in the future could require more water input than today but water availability is likely to be reduced (García-Ruiz et al., 2011; Milano et al., 2012). The atmospheric water demand, expressed through the reference evapotranspiration (ETo ), is expected to increase directly with temperature rise and due to the changes of net radiation (Pereira, 2011): long wave radiation is predicted to increase with increased greenhouse gases, while shortwave radiation could decrease with increasing of cloudiness. Thus, ETo is expected to increase due to climate change. A greater percentage of ETo increase is foreseen for the winter season, but, in absolute value, the increases may be higher in the summer months. Various studies confirmed an increase of ETo under climate change (Döll, 2002; Rodriguez-Diaz et al., 2007; Moratiel et al., 2011). As per the review above, there is a good knowledge on olive trees processes that may be affected by climate change. However, there is large uncertainty about the temporal and spatial variation of the above mentioned impacts through the Mediterranean basin. Crop evapotranspiration (ETc ) and net irrigation requirements (NIR) are particularly uncertain, as well as the pattern of changes in the areas suitable for olive cultivation. Thus, studies that permit a spatial elaboration of data and presentation of results at both country and regional scales could be particularly relevant. This could bring additional insight regarding agricultural water management at different scales and promote active management strategies optimizing water use and yield production. Hence, this study aims at understanding the impacts of foreseen climate change on olive cultivation in the Mediterranean countries and region by comparing a baseline climate, defined for year 2000, with a future one assumed for 2050. The study focuses on crop evapotranspiration, irrigation requirements and water stress impacts on rainfed olive cultivation, while considering the expected shifting of the flowering time and future changes in the areas suitable for cultivation.

2. Materials and methods 2.1. Climatic data This work used climate data that were derived within the WASSERMed project (EC-FP7-ENV) from the Regional Climate Models (RCMs) outputs that have been produced by the ENSEMBLES project (EC-FP6-ENV). Two sets of RCMs forced by two different Global Circulation Models (GCMs) were considered: RACMO2, REGCM3, RCA and REMO were forced by ECHAM5, and HIRHAM5, PROMES, CLM and HadRM3Q0 by HadCM3Q0. Model time series were divided into three time slices: (i) Past: 1961–1990, (ii) Present: 1991–2020, and (iii) Future: 2036–2065. The time slice “Past” has been used to “validate” models through the comparison between their outputs and the gridded observational CRU (Climate Research Unit) dataset of East Anglia University (Mitchell and Jones, 2005). This validation suggested that two RCM datasets have comparable quality, though those driven by ECHAM5 have lower overall bias for precipitation in spring and temperature in summer. Therefore, RCM simulations driven by ECHAM5 have been used for producing a multi-model ensemble, which, in general, has been shown to provide robust and reliable results for all climate

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Table 1 Temperature profile. Temperature interval [◦ C] Number of days

<−5 L9

−5 to 0 L8

0–5 L7

5–10 L6

10–15 L5

15–20 L4

20–25 L3

25–30 L2

>30 L1

Source: adapted from Fischer et al. (2002).

variables and statistics (Christensen and Christensen, 2007; Jacob et al., 2007). RCM results have been interpolated on a 0.25◦ × 0.25◦ (latitude by longitude) grid and they refer to the A1B scenario of the Special Report on Emissions Scenarios (SRES). Further explanations on data derivation and statistics, as well as the creation of the various data time slices are given by Lionello et al. (2013). Monthly values of the following climatic variables have been considered: average, maximum and minimum temperature at 2 m height (◦ C), total precipitation (mm month−1 ), average, maximum and minimum relative humidity (%), mean solar (incoming) radiation (W m−2 ) and wind speed at 10 metre level (m s−1 ). A data generation procedure was used to convert monthly data into daily for appropriate ETo and phenological calculations (Saadi et al., 2014). Results covering the time slices “Present” and “Future” have been averaged over the periods 1991–2010 and 2036–2065 and labelled “year 2000” and “year 2050”, respectively. The same base data were used for a study on climate change impacts on winter wheat and tomato cultivation (Saadi et al., 2014). Fig. 1 shows the spatial pattern of the average annual near surface air temperature over the Mediterranean region for the present, year 2000, and its predicted changes over the next 50 years, by 2050. The increase of annual mean temperature might vary from 0.8 to 2.3 ◦ C, being the largest in some areas of Northern Africa and Middle East, and in Southern Turkey (Fig. 1b). Seasonal patterns (Saadi et al., 2014) indicated that, in winter, the continental areas of South-Eastern Europe and Eastern Mediterranean could warm more rapidly than other areas. In summer, on the contrary, the western Mediterranean would likely warm more than the other parts of the region. Considering the whole Mediterranean region, the total annual precipitation is foreseen to decrease by approximately 5.7% (Fig. 2). This change is the result of two contrasting trends: an increase over France and the Alps, and a decrease in almost all other areas. There is a marked contrast between the winter and summer precipitation change (Saadi et al., 2014). In the future most of Europe is expected to become wetter in the winter season, except Greece, Southern Italy and Turkey, which could become drier. In summer, precipitation over Europe could decrease, while a small increase is predicted for some areas of Northern Africa and Middle East. Important changes in spring and autumn are also expected for the Iberian Peninsula and Morocco, where precipitation could be significantly reduced. 2.2. Methodology

criteria for olives cultivation and marked as optimal, limited or non-suitable zone. Temperature profiles, providing quantification of temperature seasonality considering the year-round temperature regimes, were calculated for each grid-cell. They are expressed as the number of days falling into pre-defined temperature intervals (Table 1). These intervals consist of 5 ◦ C steps. A complete account of time periods of individual temperature intervals provides a year-round temperature profile. Temperature profile requirements and accumulated temperature requirements of olives (Table 2) were compared with the actual temperature profile in each grid-cell. The cell was considered suitable for cultivation when the temperature characteristics matched, respectively, the temperature profile requirement and the accumulated temperature requirements. Otherwise, the cell was not suitable for cultivation. In addition, cultivation was considered only in areas where minimum monthly air temperature exceeds 0 ◦ C. Two climatic suitability conditions for growing olives were finally considered: suitable (with full cultivation potential) and acceptable (with limited cultivation potential). Olive flowering dates were determined considering the base temperature of 8.5 ◦ C for olive development (De Melo-Abreu et al., 2004) and the 1st of February as the date of the beginning of season. It was assumed that the flowering of olives occurs when the temperature sum reaches 514 growing degree-days. The reference evapotranspiration (ETo ) was estimated from full weather data sets by the FAO Penman–Monteith equation (Allen et al., 1998) as: ETo =

0.408 ·  · (Rn − G) +  · (900/(T + 273)) · U2 · (es − ea )  +  · (1 + 0.34 · U2 )

where Rn is the net radiation available at the crop surface (MJ m−2 d−1 ), G is the soil heat flux density (MJ m−2 d−1 ), T is mean air temperature at 2 m height (◦ C), U2 is wind speed at 2 m height (m s−1 ), (es − ea ) is vapour pressure deficit at 2 m height (kPa),  is the slope of the vapour pressure curve (kPa ◦ C−1 ) and  is the psychometric constant (kPa ◦ C−1 ). Computations of considered parameters were performed following the recommendations by Allen et al. (1998). The wind speed data obtained for 10 m height (Uz , m s−1 ) were adjusted to the standard height of 2 m, U2 , using the logarithmic wind speed profile equation (Allen et al., 1998). The ETo maps for present and future, computed with the climatic data as referred to above, are presented by Saadi et al. (2014). Olive evapotranspiration, which seasonal sum corresponds to the crop water requirements (CWR), was calculated using the single crop coefficient Kc approach: ETc = Kc ETo

The overall methodology adopted in this study is schematically illustrated in Fig. 3. The elaborations have been done for both the baseline (year 2000) and future (year 2050) scenarios. The main methodological steps include: (a) determination of the areas suitable for crop cultivation; (b) estimation of the dates of phenological phases, particularly flowering; (c) computation of the reference evapotranspiration (ETo ); (d) estimation of crop evapotranspiration (ETc ) and net irrigation requirements (NIR) for various irrigation scenarios; and (e) estimation of ETc and stress coefficients for rainfed olives. The assessment of the areas suitable for olives cultivation was done using the temperature requirements approach developed by Fischer et al. (2002) and known as the Agro Ecological Zoning (AEZ) method. Each grid-cell was evaluated following the specific

(1)

(2)

Kc values suggested by Pastor and Orgaz (1994) and Er-Raki et al. (2008) were adopted. It resulted in the following monthly values for January through December: 0.50, 0.50, 0.65, 0.60, 0.55, 0.50, Table 2 Crop temperature requirements for olives cultivation (L = 12 months) (Fischer et al., 2002). Acceptable

Suitable

L8 = L9 = 0 L7 + L6 + L5 + L4 > 0.400 × L L4 + L3+ L2+ L1 > 0.333 × L TSgc > 4000

L8 = L9 = 0 L7 + L6 + L5 + L4 > 0.400 × L L4 + L3 + L2 + L1 > 0.333 × L TSgc > 5000

TSgc – temperature sum of the growing crop period.

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Fig. 1. Spatial pattern of average annual near surface air temperature over the Mediterranean for 1991–2010 (a) and predicted change over the period 2000–2050 (b).

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Fig. 2. Spatial pattern of average annual precipitation over the Mediterranean for 1991–2010 (a) and its predicted change over the period 2000–2050 (b).

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Fig. 3. Scheme of methodology used for estimating crop evapotranspiration and irrigation requirements (AEZ – agro-ecological zoning, ETo (PM) – reference evapotranspiration estimated by the Penman–Monteith equation, ETc – crop evapotranspiration, Kc – crop coefficient, NIR – net irrigation requirements).

0.45, 0.45, 0.55, 0.60, 0.65, and 0.50. These values were selected as average values for semi-intensive orchards as they are smaller than those reported by Allen and Pereira (2009) for dense or intensive olive groves, and by Pac¸o et al. (2014) for super-intensive olive orchards. It has been observed that Kc values, particularly in rainy months, vary with factors influencing soil evaporation (Villalobos et al., 2000; Orgaz et al., 2006; Ramos and Santos, 2009); differently, the variation of the basal crop coefficients representing transpiration is smaller but depends on crop density and height (Allen and Pereira, 2009; Pac¸o et al., 2014). Net irrigation water requirements (NIR), i.e., the quantity of water necessary for crop evapotranspiration in excess of effective precipitation (Peff ), were computed through a simplified balance between ETc and Peff as: NIR = Kc ETo − Peff = ETc − Peff

(3)

Peff was calculated as 80% of total precipitation. The actual evapotranspiration (ETa ) of olive groves under rainfed conditions was estimated as: ETa = Ks Kc ETo

and they were due to the fact that these authors used the mean temperature of 2 ◦ C in January (the coldest month) as threshold for olives cultivation whereas, in the present study, this threshold was reduced to 0 ◦ C. Results of the present study showed that the area climatically suitable for growing olives, about 39% of the total area of the region in 2000, could increase to almost 50% in 2050. It is expected that the suitable areas will shift to the north and towards inland and higher altitude than at present. Italy could have the highest relative increase in the suitable area (24%) and Spain would get an additional 19% of territory. The optimal suitable area of France would increase from 3 to 6% of the territory. The most evident change is expected to occur in the Balkans, where some inland areas in Serbia (Vojvodina) and Croatia (Eastern Slavonija) could pass from unsuitable to acceptable for olive cultivation (Fig. 4b). These results are in agreement with the studies of Bindi et al. (1992), Gutierrez et al. (2009) and Ponti et al. (2013).

3.2. Olive flowering dates

(4)

where Ks is the water stress coefficient [0,1.0], taking the value of 1.0 when there is no stress. Ks was computed from the simplified water balance and it was used to characterize water stress for the present and future scenarios. The estimations of ETc and NIR per country were performed with GIS on a cell basis considering the climatic suitability areas for olives cultivation for both 2000 and 2050. Hence, the maps showing the difference in results between 2050 and 2000 were elaborated in GIS. The statistics considering minimum, maximum and average values of ETc and NIR and standard deviations were computed at country and regional (Mediterranean) scale. The overall results referred to the spatial domain of a climatic grid database over the Mediterranean region which extended from South-West (−9◦ W, 30◦ N) to North-East (37.3◦ E, 57◦ N). 3. Results 3.1. Areas suitable for olives cultivation The areas suitable for olive cultivation in the Mediterranean region are presented in Fig. 4 for years 2000 and 2050. Results for present scenario were compared with the present suitability map elaborated by Moriondo et al. (2008). Differences were small

The indicative dates (i.e. days of year, DOY) when olives’ flowering starts for the present and the expected difference for the period 2000–2050 are given in Fig. 5. In the Mediterranean region, flowering of olives occurs between April and June. The dates of olive flowering estimated in this work for the present fitted well with the data reported in previous studies for different locations in the Mediterranean (Table 3). More accurate assessments would be expected using local crop growth models but their extension to the entire Mediterranean area could be difficult. Nevertheless, the potentially suitable areas and the duration of the growing season generally appear to reflect the reality and the results for the present scenario have shown to compare well with the literature data (Table 3). An advance in the future flowering dates is foreseen (Fig. 5b) over the whole area suitable for olive cultivation due to the predicted temperature rise. The anticipation of flowering over the whole Mediterranean could be 11 ± 3 days on average, varying from 8 days in Egypt and 9 days in France and Portugal to 15 days in Lebanon. A larger flowering anticipation was foreseen for the Middle East and the Balkan Peninsula (Fig. 5b), that might be due to the expected higher temperature rise during spring over the eastern part of the Mediterranean region. The greatest anticipation of flowering (up to 18 days) could be expected in the coastal areas while in few inland areas of North-West Spain and Northern Portugal the

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Fig. 4. Climatic suitability for growing olives in the Mediterranean region for present, year 2000 (a) and future, year 2050 (b).

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Fig. 5. Starting date of olive flowering (DOY, days of year) for present conditions, year 2000 (a), and difference 2050 vs. 2000 in days (b).

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Table 3 Comparison of the olive flowering dates (in DOY, days of year) estimated in this work for present climate with the start of flowering reported in the literature.

Table 4 Comparison of olive net irrigation requirements (NIR) obtained in this work with the results from other studies (mm season−1 ).

Reference

Location

Reported

This study

Reference

Location

Reported

This study

De Melo-Abreu et al. (2004) De Melo-Abreu et al. (2004) Osborne et al. (2000) Osborne et al. (2000) Osborne et al. (2000) Osborne et al. (2000) Osborne et al. (2000) Fornaciari et al. (2000) Orlandi et al. (2009) Orlandi et al. (2009) Orlandi et al. (2010) Orlandi et al. (2010) Orlandi et al. (2010) Orlandi et al. (2010) Orlandi et al. (2010) Orlandi et al. (2010) Bonofiglio et al. (2008) Bonofiglio et al. (2008) Bonofiglio et al. (2008) Bonofiglio et al. (2008) Bonofiglio et al. (2008)

Santarem, Portugal Cordoba, Spain Cordoba, Spain Lisbon, Portugal Malaga, Spain Thessaloniki, Greece Barcelona, Spain Cordoba, Spain Perugia, Italy Southern Italy Taous, Tunisia Zarzis, Tunisia Jammel, Tunisia Brindisi, Italy Lecce, Italy Agrigento, Italy Salerno, Italy Benevento, Italy Avelino, Italy Apulia, Italy Sicily, Italy

130–140 121–141 120–160 120–146 120–130 140–150 138–156 115–145 150–175 125–170 100–120 90–118 100–110 120–140 120–140 110–140 134–164 137–166 138–169 123–158 115–158

140–150 140–150 130–140 130–140 120–130 150–160 150–160 140–150 170–180 130–170 110–120 110–120 110–120 130–140 130–140 120–130 130–140 140–150 140–150 130–170 120–170

Rodriguez-Diaz et al. (2007) Moriana et al. (2003) Iniesta et al. (2009) Palomo et al. (2002) Tognetti et al. (2007) Palese et al. (2010) Ezzahar et al. (2007)

South Spain Cordoba, Spain Cordoba, Spain Seville, Spain Benevento, Italy South Italy Marrakech, Morocco

407 454 460 403 212 293 800

380-390 420 420 397 237 280-290 780

flowering date is predicted not to change or could advance by a few days only (Fig. 5b).

for Benevento, Italy, in 2004 (530 mm) similar to those estimated herein (548 mm). For Southern Italy, Palese et al. (2010) reported ET of 660–700 mm, while our estimation was 640 mm. Olive ET for Seville, Spain, was reported at 660 mm by Palomo et al. (2002) while this study estimated 700 mm. Ezzahar et al. (2007) reported 920 mm for Marrakech, Morocco, whereas the present study indicated an overestimation of 60 mm. Results were, therefore, quite similar to those reported in the literature, with the differences smaller than 7%. These results permitted to consider the ET estimation procedures adopted in this study sufficiently adequate for the present climate.

3.3. Crop evapotranspiration 3.4. Net irrigation requirements Reference ET estimates by Eq. (1) are reported in Saadi et al. (2014) and followed the spatial pattern of ETo recently studied for the Mediterranean area by Todorovic et al. (2013). The elaboration of results for ETc and NIR referred to the potentially cultivable area at present and in the future. However, the difference in ETc and NIR between 2050 and 2000 was estimated considering only the areas actually suitable for cultivation. The spatial pattern of ETc in 2000 and 2050, together with the difference between both climatic scenarios, is illustrated in Fig. 6. Up to the middle of this century, olive ET could increase on average by 8% or 51 ± 17 mm season−1 over the whole Mediterranean. However, it was observed that olive ET could increase up to 100 mm season−1 in some areas of the central Iberian Peninsula, Eastern Turkey and Northern Africa, while a smaller increase, up to 40 mm, is expected around the Adriatic coast, the Apennine Peninsula and Southern France. At a country scale, the lowest increase of ETc was for Slovenia, Croatia, Bosnia and Herzegovina, France and Italy (20–30 mm season−1 ), where the present ETc is also small. The largest increase of olive ET was predicted for Morocco and Algeria 71 ± 10 and 65 ± 10 mm season−1 , respectively. Analysing at a country level, the highest relative increase is expected for Spain and Portugal, at about 9%, followed by Algeria and Morocco, with an ETc increase of about 8%. A relatively small variation in ETc agrees with the assumption that rising CO2 could decrease stomatal conductance which, in turn, could lower water transfer into the atmosphere and, thus, crop evapotranspiration (Kruijt et al., 2008). However, the effects of rising CO2 are associated with some uncertainty since the impacts of temperature in increasing evapotranspirative demand of the atmosphere could be larger than possible effects of rising CO2 (Lovelli et al., 2010; Pereira, 2011). The evaluation of results relative to computed ETc through the comparison with literature data was difficult because the scale and purpose of studies were different. In particular, the previous studies referred to the local experimentation results while this study used simulations on a grid of 0.25◦ × 0.25◦ , which permitted only a rough approximation of the experimental sites. However, some comparisons were possible. Tognetti et al. (2007) reported olive ET

The spatial distribution of olive NIR over the Mediterranean is given in Fig. 7 for the present and future climatic scenarios and the difference between both. The larger NIR were observed in the southern and eastern Mediterranean areas which are strongly characterized by aridity. Olive groves are relatively marginal in these areas since they require a very large amount of water, which is not likely to be available. NIR are expected to increase for the 2050 horizon everywhere in the Mediterranean. An overall increase over the whole region is estimated at 70 ± 28 mm season−1 or about 18.5%. The northern central part of the Mediterranean (Italy, Adriatic and French coast) showed the smallest increase of NIR (up to 50 mm) while in southern Spain NIR could increase up to 130–140 mm season−1 . Similar increase of NIR is expected for some areas of Algeria and Morocco due to foreseen precipitation reduction in the future. In the new potentially cultivable areas, NIR are expected to be quite large by 2050 (up to 870 mm); therefore, the future cultivation of olives could strongly depend on the availability of water resources for irrigation. A few studies have been published on this topic, e.g., the detailed analysis on irrigation requirements relative to various future scenarios (Schaldach et al., 2012). Rodriguez-Diaz et al. (2007) reported a NIR increase of 9 and 16% in southern Spain for SRES scenarios B2 and A2 respectively, while in this study a NIR increase of 29.6% (81 ± 25 mm season−1 ) is foreseen for Spain. However, as shown in Fig. 7, southern Spain may have an increase in NIR above average due to the expected significant decrease of precipitation. Published data were used to assess the accuracy of the NIR estimations for present climate. However, any comparison was affected by high uncertainty because published data used different approaches and scales as referred to ETc . Nevertheless, there is a satisfactory agreement between the results of this study and those carried out in different areas of the Mediterranean region (Table 4), with differences not exceeding 10%. As for ETc , the results allowed to assume that the estimation procedures are adequate and likely to not produce large errors when forecasting the 2050 scenario.

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Fig. 6. Olive crop evapotranspiration (ETc ) assuming optimal water supply for present conditions, year 2000 (a), future scenario, year 2050 (b) and expected difference between 2050 and 2000 (c) in mm season−1 .

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Fig. 7. Net irrigation requirements (NIR) of olive groves for achieving optimal yield under present conditions, year 2000 (a), future scenario, year 2050 (b) and difference between 2050 and 2000 (c) in mm season−1 .

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Fig. 8. Stress coefficient (Ks ) relative to olives cultivated under rainfed conditions: present (a) and future (b).

3.5. Olives cultivation under rainfed conditions Contrarily to the results obtained for irrigated olives, a quite large decrease in actual ET is expected for rainfed olives cultivation. The water stress coefficient Ks , which is a multiplier of Kc to

reduce it from optimal to actual (limited) ET conditions (see Eq. (4)), could decrease very much from the present to the horizon of 2050 (Fig. 8). At present, most cultivated areas in the Northern Mediterranean have Ks of 0.7 or higher, and those in the Southern and Eastern Mediterranean have a much smaller Ks of 0.3–0.8

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(Fig. 8). This agreed with the fact that olives have been traditionally cultivated under rainfed conditions in the northern Mediterranean areas and in various parts of southern and eastern countries. Nevertheless, olives need irrigation in the regions with arid climate and in part of semi-arid Mediterranean areas. Results in Fig. 8 indicated a large increase of water stress over most Mediterranean areas as shown by the decrease of the Ks factor in 2050. This coefficient could decrease to values smaller than 0.4, often much lower, in the southern and eastern areas of the Mediterranean, thus indicating that rainfed olive cultivation could not be feasible in the future. These results were in agreement with the foreseen decrease in precipitation (Fig. 2) that could be greater than 200 mm season−1 in some areas actually characterized by sub-humid or semiarid climate. This reduction of water availability could be intensified due to longer and more frequent dry spells. Olives rainfed cultivation could also not be feasible in the southern regions of the Iberian countries, Italy and Greece since southern Europe is foreseen to be highly affected by dry spells and droughts (Lehner et al., 2006; Beniston et al., 2007). In other words, in most Mediterranean areas, cultivation of olives in the future would require appropriate irrigation. This trend is already visible with the progressive abandonment of traditional olive groves and the plantation of intensive olive orchards. However, it is not guaranteed that water supply would be sufficient where climate is characterized by aridity and water resources are highly scarce and under large competition, mainly for non-agricultural uses.

4. Discussion Climate change would significantly affect olive cultivation, which may have relevant consequences since olives are a main cash crop in the Mediterranean area, thus with great economic importance particularly in sub-humid and semi-arid areas. It is expected that the foreseen increase in temperature could extend the potentially cultivable areas for growing olives. Main increases in the suitable area are expected for Italy and Spain, but the most drastic change is foreseen to occur in the Balkans, where the areas suitable or acceptable for olive cultivation would greatly increase. Suitability was defined upon what is known about the currently available varieties. However, one of the main issues for adaptation is plant breeding (Fabbri et al., 2009; León et al., 2011) aiming to adopt new varieties that respond better to the foreseen changes in cultivation conditions and, hopefully, to expected variations in air temperature, water availability and CO2 concentration. Therefore, it is likely that by 2050 the definition of climatic suitability would be different to what is foreseen today. Global warming will lead to the faster accumulation of thermal units in the future and to the anticipation of olive flowering dates (Fornaciari et al., 2000; Galán et al., 2005; Moriondo and Bindi, 2007; Avolio et al., 2008; Orlandi et al., 2009). An advance of the flowering dates in the future could go from one week in France and Portugal to two weeks in the Middle East and the Balkan Peninsula. The larger flowering anticipation in the latter areas is due to expected higher temperature rise in the spring over the eastern Mediterranean region. The greatest anticipation of flowering could be expected in the coastal areas, where temperature rise is predicted to be the highest. Nevertheless, further investigations are needed to consider the impact of temperature variation during the winter and spring season over the Mediterranean because olive flowering is mainly affected by late spring temperature. Precipitation and high wind could decline pollen concentration in the atmosphere and affect flowering of olives (Bonofiglio et al., 2008). However, the impact of these two factors could be studied in the context of increased frequency of extreme events in the future, which is not the subject of this work.

The spatial pattern of ETc in 2000 and 2050 showed the variation of changes for different regions following the expected temperature trend (Fig. 1b). Nevertheless, the present analysis did not consider the impact of rising CO2 on stomatal closure, hence reducing the flux of water vapour to the atmosphere (Kruijt et al., 2008). Net irrigation requirements would increase more than ETc due to the expected decrease in precipitation and larger impact of droughts and dry spells, associated with reduced soil infiltration. However, this will likely reduce the soil water availability. The increase of NIR could be particularly relevant in Southern Spain and in semiarid and arid areas of southern Mediterranean countries. Rainfed olive cultivation is expected to be more stressed in the future than at present and to become not feasible in southern and eastern Mediterranean regions. This fact has large consequences when land is abandoned (Duarte et al., 2008) and landscape deteriorates with loss of soil and water conservation measures, thus with increased erosion (de Graaff et al., 2010). In addition, the social consequences of abandoning the traditional olive groves may be enormous. Olive evapotranspiration and irrigation requirements strongly depend on the type of cultivation (traditional, intensive or super-intensive), i.e., trees density and fraction of ground cover (Fernandez et al., 2006; Orgaz et al., 2006; Martinez-Cob and Faci, 2010; Pac¸o et al., 2014). Accordingly, crop water requirements could increase in respect to the values reported in this work if super-intensive olive groves are introduced (Orgaz et al., 2006; Pac¸o et al., 2014). Hence, due to potentially high water requirements and limited water availability, regulated deficit irrigation will likely become a common practice in most areas as it is developing at present; however, achieving water savings and high yields require a careful irrigation scheduling as noted by several authors (e.g., Lavee et al., 2007; Iniesta et al., 2009; Palese et al., 2010; Mezghani et al., 2012).

5. Conclusion In this work, assessments were performed with a simplified model adapted to the regional scale. Nevertheless, the presented results, considering the potentially suitable areas for cultivation, occurrence of flowering, olive crop evapotranspiration and irrigation requirements, appeared to reflect reality and the results for the present scenario have shown to fit well with the literature data. Certainly, a more accurate assessment could be possible using local, site-specific crop phenological parameters and water balance models. Though, the extension of this more detailed approach to the entire Mediterranean area would be very difficult. Further research efforts should be devoted to estimate water use and productivity in a more accurate way using the dual Kc approach, orchards density factor and with support of remote sensing. Certainly, the possible adaptation measures should be investigated focussing on the local socio-economic and environmental factors. Due to the combined effects of temperature increase and precipitation decrease, the available soil water in the root zone could decrease and water stress conditions are likely to occur for rainfed olive cultivation in the future. Therefore, rainfed olive groves are expected to become unviable in the future and, if water is available, to be replaced by irrigated intensive olive orchards, whose economic viability is already known. This fact may have no negligible socio-economic consequences and could also lead to changes in landscape, water redistribution among the economic sectors and within the agricultural uses. The irrigation of olives will be more easily feasible in the new growing areas northward, where water availability is larger than in areas where olives are traditionally cultivated. Therefore, the future of olives cultivation in the Mediterranean would face a great challenge: how to keep sustainable development of two opposite growing systems: (i) the

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