ReviewMediterranean water resources in a global change scenario
Introduction
Change is an intrinsic characteristic of the Earth's system. During the Quaternary, long cold and dry periods alternated with relatively short warm periods. Each of these was affected by climatic variability at different temporal scales (Alley et al., 1995, Bradley, 1999). Studies of fluvial, lacustrine, and oceanic sediments have demonstrated the occurrence of periodic changes in climate, runoff, flooding, and plant cover (e.g., Ruddiman, 2000, Benito et al., 2003, Valero-Garcés et al., 2006). The last significant global cold period, the Little Ice Age, finished at the beginning of the 19th century. The Earth's average temperature has subsequently increased by about 1 °C (Hansen et al., 2006), and both temperature and precipitation have exhibited varying wet and dry regional cycles that are well represented in the annual streamflow (Benito et al., 1998, Brito-Castillo et al., 1998, Fekete et al., 2002, Kargapolova, 2008). Nevertheless, it is generally agreed that the intensity and rate of recent changes have no precedent since at least the beginning of the Holocene (Mann et al., 1998, Barnett et al., 2001), and reflect natural environmental changes affected by human activities, which can counteract or enhance natural forces (Nepstad et al., 1999, Levitus et al., 2001).
Increasing efforts are being made to understand the consequences of environmental change for society, particularly in the field of water resource management. Changes in water resources are particularly relevant in areas where water availability is a limiting factor for economic development. This is the case in the Mediterranean basin, where both developed and developing countries have a common dependence on water availability to meet the needs of increasing populations and living standards, development of irrigated agriculture, and increasing industry and tourism activities (Cudennec et al., 2007). Water availability in the Mediterranean basin is scarce, and mainly dependent on runoff from mountain areas (Ives et al., 1997; Viviroli and Weingartner, 2004, De Jong et al., 2009); these supply 20–50% of the total discharge, but in semi-arid areas including the Mediterranean basin runoff can contribute 50–90% of the total supply (Viviroli and Weingartner, 2004, Viviroli et al., 2007). Challenges will be faced in maintaining the quantity and quality of mountain runoff, through preservation of mountain environments, while ensuring sustainable use of the available water resources (Messerli et al., 2004). Water scarcity is particularly intense in the Maghreb countries, including Libya and Egypt, as well as in some sectors of northern Mediterranean countries, such as southeast Spain and the Ebro Depression, where the expansion of irrigated areas and urbanization has caused increasing water supply difficulties. The disjunction between runoff-producing and water-demanding areas is contributing to increasing political tensions, for instance in Spain and the Middle East, and water supply to the large cities (e.g., Barcelona, Madrid, Athens, Jerusalem, Cairo, Casablanca and Marrakech) requires a high level of investment and complex solutions that are, in general, are not designed for a long-term perspective.
In the Mediterranean basin precipitation can be subject to high interannual and seasonal variability, with long and intense dry periods (e.g., Lloyd-Hughes and Saunders, 2002, Bonaccorso et al., 2003, Piccarreta et al., 2004, Van der Schrier et al., 2006, Vicente-Serrano, 2006, Nicault et al., 2008), or extreme rainfall and floods (e.g., Llasat and Puigcerver, 1997, White et al., 1997, García-Ruiz et al., 2000, Brunetti et al., 2002, Peñarrocha et al., 2002, Klein-Tank and Können, 2003, Martius et al., 2006, De Jong et al., 2008, Martín-Vide et al., 2008; Beguería et al., 2009). Consequently, water is relatively scarce throughout most of the year, whereas high flows threaten lives and property on a small number of days per year or decade. Under such conditions, the rising demand for water is met by increasingly expensive and complex infrastructure necessary to store seasonal or annual water surpluses in reservoirs, to transfer the water from storage to areas of demand, and to pump groundwater reserves (Croke et al., 2000; Ibáñez and Prat, 2003, López-Moreno et al., 2004, López-Moreno et al., 2008a). Unfortunately, large infrastructure developments during the 20th century were designed and constructed on the basis that water resources would be relatively stable over time, despite the occurrence of short-term oscillations in supply. However, there is mounting evidence of long-term climatic trends (Giorgi et al., 2004a, Giorgi et al., 2004b), and changes in land cover have produced marked alterations in hydrological responses at the basin scale. Indeed, the Mediterranean basin is considered to be a global “hot-spot” in terms of climate variability and change, as well as in the rate of land transformation processes (Giorgi, 2006, Rosenzweig et al., 2007).
Future water resource planning decisions should be based not only on water demand, but also on future scenarios of climate and stream flow. Uncertainties include how environmental change will affect the quantity and quality of water resources and the fluvial regimes, and how the new scenarios will affect water management and reservoir operating rules, which were established on the basis of constant water availability. To address these issues, special focus on mountain areas is needed, as these are the main source of water in the region (Beniston, 2003, Viviroli et al., 2003, Viviroli et al., 2007) and are particularly threatened by environmental change (Beniston, 2003, Nogués-Bravo et al., 2008). Thus, climate scenarios project a greater temperature rise in the high mountains than at lower elevations (Giorgi et al., 1994; Bradley et al., 2006; Nogués-Bravo et al., 2007). Moreover, the hydrology of snow-fed basins is noticeably more sensitive to climate variability and change, as snow and ice respond rapidly to slight variations in precipitation and temperature (Nesje and Dahl, 2000, Carrivick and Brewer, 2004, López-Moreno and García-Ruiz, 2004). Mountains are also affected by changes in water demand from human activities (new urbanizations, sky resorts, snow making devices etc.) and by intense modifications in plant cover and land uses including (1) rapid deforestation as a consequence of overexploitation and overgrazing in developing countries, and forest fires; and, (2) afforestation processes as a consequence of land abandonment and reforestation activities, especially in developed countries. Such changes in vegetation cover affect the water balance because of effects on evapotranspiration and interception rates (Joffre and Rambal, 1993, Llorens et al., 1995, Llorens et al., 1997, Cosandey et al., 2005; David et al., 2006, López-Moreno and Latron, 2008), soil moisture dynamics (Correia, 1999, Maestre and Cortina, 2004), and the recharge of aquifers (Callegari et al., 2003). For these reasons, changes in land cover, particularly in the mountains, are as important as climatic variability in explaining the observed hydrological alterations and in assessing future water availability in the region. The present work reviews the main findings on the effects of climate fluctuations and land use changes on water resources of the Mediterranean region. The aim is to provide a comprehensive picture of environmental and human changes in order to underline the main problems derived for water management and to contribute to copy and address the hydrological consequences of such environmental change. This requires a synthesis of results on (1) climate evolution and trend in the last few decades, and future perspectives according to projections from climatic models; (2) land cover and land use changes, particularly the evolution of cultivated areas and forests in mountain areas, and (3) changes in river discharge and water resources, including trends in discharge, and changes in snow processes and in river regime.
Section snippets
Climate evolution
Many studies have assessed whether consistent long-term temporal trends occur within the high natural variability of Mediterranean climatic series. For precipitation, the longest climate records do not show consistent regional patterns (Cudennec et al., 2007). Data from only a few stations cover the entire 20th century, and do not indicate significant trends for the Iberian Peninsula (Esteban-Parra et al., 1998, Rodriguez et al., 1999, Llasat and Quintas, 2004) or the Alpine region (Beniston et
Projected climatic change
Projections of future evolution of climate are always subject to uncertainty (Räisänen, 2007). However, the scenarios projected by the best available models of the climate system and the prognostics of socioeconomic development in the Mediterranean region have major implications for the hydrological cycle and water resources.
There is consensus that projections of the future evolution of temperature (error ± 1 °C) are more reliable than for precipitation (error ± 25%) (Giorgi et al., 2004b, Dequé et
Discussion: will current water management strategies be adequate to address the consequences of environmental change?
Many studies have revealed a consistent decrease in river discharge over almost the entire Mediterranean basin. The occurrence of significant and positive trends in temperature has been confirmed, as have long-term trends in precipitation during the last six decades. This climate evolution is consistent with the response of river discharges, but most studies have indicated that climate alone does not fully account for the magnitude of the observed hydrological changes. Two additional reasons
Conclusions
Water resource availability in the Mediterranean has already been affected by environmental change, and is seriously jeopardized in future environmental, economic, and demographic scenarios. Most global hydrological models are based on expected trends in precipitation and temperature. However, a number of studies have demonstrated the influence of land cover on river discharge and water resources. Climate and land cover change (artificial and natural reforestation, deforestation, and expansion
Acknowledgements
This work was supported by research projects CGL2006-11619/HID, CGL2008-01189/BTE, and CGL2008-1083/CLI, financed by the Spanish Commission of Science and Technology, and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7-ENV-2007-1-212250, financed by the European Commission, the VII Framework Programme financed by the European Commission, the project “Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica” and “La nieve en el Pirineo aragonés: Distribución espacial y su
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