It is clearly known that the increasing of carbon dioxide and other greenhouse gases will raise global temperatures, resulting in global warming. This, in turn, will result in climate change which is expected to impact the world by affecting winter snow-
Department of Civil Engineering, Izmir Institute of Technology, Gulbahce Kampus,
Urla, Izmir, Turkey e-mail: [email protected]
fall and snowmelt, minimum water temperature, summer average temperature, and growing season rainfall amounts and intensities . Temperature changes are expected to alter precipitation and evapotranspiration which are the prime drivers of water availability and agricultural production. Agriculture is an important economic activity in the world and the global warming is expected to have a great impact on water resources and agriculture .
Elgaalin and Garcia  investigated the impact of climate change on water supplies in Arkansas River Basin of Colorado under two transient climate change scenarios, employing artificial neural network method. Since monthly runoff is the primary factor in determining the amount of water available for irrigation, they linked the available potential water for agriculture to climate change on a monthly scale. They employed the two general circulation models (GCM) - HAD (Hadley Center for Climate Prediction and Research), and CCC (Canadian Climate Center) -to generate future climate projections assuming a progressively 1% annual increase in carbon dioxide concentrations . Minville et al.  investigated the impact and uncertainty of climate change on water resources management in the Perobonka River System, Canada. They evaluated the impact of the change on medium-term reservoir operations for the Perobonka water resources system (Quebec, Canada) with annual and seasonal hydropower production indicators and flood control criteria.
Agricultural systems are more sensitive to the climate change due to the common lack of buffering capability in agricultural response to climate events. For example, a single month of extremely low rainfall may affect a reservoir by decreasing storage over the course of a few months, but the reservoir system might be able to recover quickly with single large rainfall. On the other hand, extremely low rainfall period of a month will cause death of a region's crops with no hope of growing new crops until next growing season. Hence, agricultural water resources planning must consider the variability in agricultural systems over time and the primary cause for temporal variation in climate .
Irrigation is a principal adaptation mechanism to climatic variability and economic studies have shown that climatic variability can be a factor in determining private investment in irrigation infrastructure more important than any others including credit availability, governmental price policies, and local violence . Already irrigated agriculture takes place under water scarcity. This situation definitely will worsen in future. To cope with scarce supplies, deficit irrigation, i.e. application of water below full crop-water requirements, is an important tool to achieve the goal of reducing irrigation water use .
One of the major adverse effects of deficit irrigation, on the other hand, is the salini-sation of the soil. Salinisation, which is also known as alkalisation or sodification, is the process that leads to an excessive increase of water-soluble salts in the soil. The accumulated salts include sodium, potassium, magnesium, calcium, chloride, sulphate, carbonate and bicarbonate that lead to severe deduction of soil fertility. Primary salinisation involves salt accumulation through natural processes due to a high salt content of the parent material or in groundwater. Secondary salinisation is caused by human interventions such as inappropriate irrigation practices, e.g. with salt-rich irrigation water and/or insufficient drainage. Salinisation is often associated with irrigated areas where low rainfall, high evapotranspiration rates or soil textural characteristics impede the washing out of the salts which subsequently build-up in the soil surface layers. Irrigation with high salt content waters dramatically worsens the problem.
Salinity is one of the most widespread soil degradation processes on the Earth. According to some estimates, the total area of salt affected soil is about one billion hectares. They occur mainly in the arid-semiarid regions of Asia, Australia and South America. In Europe, salt affected soil occurs in the Caspian Basin, the Ukraine, the Carpathian Basin and on the Iberian Peninsula. Soil salinity affects an estimated one million hectares in the European Union, mainly in the Mediterranean countries, and is a major cause of desertification. In Spain 3% of the 3.5 million hectares of irrigated land is severely affected, reducing markedly its agricultural potential while another 15% is under serious risk. The Euphrates, Tigris and Van basins are presenting an alarming situation with over 75,000 ha facing salinity-alkalinity problems . Accordingly Kendirli et al. , 1.5 million ha of land in Turkey is salt effected and about 74% of barren land is saline soils.
The accumulation of salts, particularly sodium salts, is one the main physiological threats to ecosystems. Salt prevents, limits or disturbs the normal metabolism, water quality and nutrient uptake of plants and soil biota. When water containing a large amount of dissolved salt is brought into contact with a plant cell, the protoplasmic lining will shrink. This action, which is known as plasmo-lysis, increases with the concentration of the salt solution. The cell then collapses. In addition, sodium salts can be both corrosive and toxic to organic tissue. The nature of the salt, the plant species and even the individuality of the plant (e.g. structure and depth of the root system) determine the concentration of soil-salt levels at which a crop or plants will succumb. Examples of plants and crops with a high tolerance to salt include bermuda grass, cotton, date palm, peas, rape and sugar beet while apples, lemons, oranges, potatoes and most clovers have a very low tolerance.
Salinization processes are near to irreversible in the case of heavy-textured soils with high levels of swelling clay. Although a combination of efficient drainage and flushing of the soil by water is often used, the leaching of salts from the profile is rarely effective. Because the reclamation, improvement and management of salt affected soils necessitate complex and expensive technologies, all efforts must be taken for the efficient prevention of these harmful processes. Permanent care and proper control actions are required. Adequate soil and water conservation practices, based on a comprehensive soil or land degradation assessment, can provide an "early warning system" that provides possibilities for efficient salinity control, the prevention of these environmental stresses and their undesirable ecological, economical and social consequences.
This study presents a mathematical model for simulating salt transport in satu-rated/unsaturated soil. The effect of deficit irrigation on the salt accumulation in the soil column is quantitatively investigated and tested against measured data.
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