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Top1. Introduction
Climate change is one of the largest threat the world ever faced as it widely affects different aspects of social activity and the earth’s natural resources from tropical to arctic and from sea to land and atmosphere (IPCC, 2013). Scientists are more than 90% certain that warming of the climate is primarily caused by increasing concentrations of greenhouse gases (CO2, CH4, O3, CFCs and Nitrous oxide) produced by human activities such as the burning of fossil fuels and deforestation (IPCC, 2007). According to the Intergovernmental Panel on Climate Change (IPCC) (2013), warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished and the sea level has risen. The IPCC Fourth Assessment Report (AR4) (IPCC, 2007) projects that the global mean temperature will rise by the end of the 21st century (2090-2099) relative to 1980-1999 by 1.8°C to 4°C for SRES scenarios. The most recent IPCC Fifth Assessment Report (AR5) projections (IPCC, 2013) give an increase of global mean surface temperatures by 1°C to 3.7°C by the late 21st century (2081-2100) relative to 1986-2005, according to RCPs scenarios.
Climate warming observed over the past several decades affects water through a number of mechanisms and is associated with changes in a number of components of the hydrological cycle and hydrological systems. An issue of global concern is the possible change in water resources availability in response to different scenarios of climate change. Water resources play a crucial role in crop production regionally and worldwide. On the one hand, in the regions, that agricultural land is rainfed, crop productivity depends solely on sufficient precipitation to meet evaporative demand and associated soil moisture distribution (FAO, 2003). Crop production will be very vulnerable to climate change especially in regions such as the Mediterranean Region where precipitation is limited (FAO, 2003). On the other hand, crop production depends on the available water resources for irrigation. In most countries, water use has increased over the recent decades, due to population and economic growth, changes in lifestyle and expanded water supply systems, with irrigation water use being by far the most important cause. Irrigation accounts for about 70% of total water withdrawals worldwide and for more than 90% of the consumptive water use and generates about 40% of total agricultural output (Fischer et al., 2006). The above indicate that changes in water resources in relation to shifts in climate will affect crop production and can become critical for the sector of agriculture; as a result, climate change impacts on water resources require careful assessment.
At a global scale, there is evidence that climate change will have a range of impacts on water resources, with some regions experiencing an increase in annual runoff and others experiencing a decrease (Milly et al., 2005). Areas under high water stress in central and southern Europe will expand by the end of the century. In Northern Europe (Andréasson et al., 2004) annual runoff is projected to increase while the opposite is expected in the rest of Europe included Mediterranean Region (Etchevers et al., 2002; Menzel & Bürger, 2002). Annual average runoff will increase in Northern Europe by approximately 5-15% up to the 2020 and by 9-22% up to the 2070, under the SRES scenarios (IPCC, 2007). Meanwhile, runoff in Southern Europe is projected to decrease by 0-23% up to the 2020 and by 6-36% up to the 2070 according to the SRES scenarios. Summer flow is projected to decrease by up to 50% in central Europe and by up to 80% in some rivers in southern Europe (IPCC, 2007). The reductions in annual runoff under climate change can be attributed to a combination of evapotranspiration increases and precipitation decreases, with the overall reduction in river flow exacerbated by human water demands on the basin’s supply (Sanchez-Gomez et al., 2009). The Mediterranean region is considered to be one of the regions that is the most vulnerable to climate change and to cope with water resource vulnerability through water management practices is challenging.