Optimization of a Filter Medium Suitable for Direct Irrigation with Seawater through a Water Table

Soil salinization is the process of accumulation in the soil of water-soluble salts. It can occur naturally, when it comes to low and flat soils, which are periodically flooded by rivers or streams; or if the groundwater level is shallow and the water rising by capillary contains dissolved salts (Chhabra, 1996).

When this process has an anthropogenic origin, it is usually associated with irrigation systems. It is called saline soil to a soil with excess soluble salts. Typically, the dominant salt is sodium chloride (NaCl). One consequence of soil salinization is the loss of fertility, which harms or makes agricultural cultivation impossible. Usually, the process is slowed or reversed by expensive washing of the soil to leach the salts or, alternatively, to grow plants that better tolerate salinity (Hoorn & Alphen, 2006).

On the other hand, it is known that the Inca culture developed a technology based on modifying the surface of the soil to facilitate the movement and storage of water. The main feature of this system is the construction of a network of embankments and channels, currently in use, as shown in Figure 1.

The water channels serve two very important functions, the first is to provide the water necessary for the growth of plants, since the proximity to water allows the area to remain moist and the plants can absorb the nutrients they need. The second function is to achieve a temperature more suitable for plants than the environment. The body of water allows to absorb excess cold in the nights and winter periods, preventing frosts from affecting the plants. Likewise the water absorbs the midday heat and radiates it at night where it is necessary to balance the cold of the night (Carolina Sparavigna, 2010). FIGURE 1: Set of embankments known as waru in the highlands of Peru.

Terraced agriculture is a phenomenon developed since time immemorial and distributed throughout the globe. In Asia: Sumatra, Philippines, Yemen, Nepal, China, Turkey, Pakistan and elsewhere. In Africa: Ethiopia, Sudan, Uganda, Tunisia, Algiers and others. In Europe: Spain, Portugal, Italy, Romania, France, Switzerland and other places. In America: from the southern United States to northern Argentina (Donkin, 1979).

Almost two thirds of the water used by mangoes to agriculture. In Asia, the proportion increases to four fifths. Agriculture also impacts the basis of its own future through land degradation, salinization, excess water extraction and a reduction in agricultural genetic diversity. To date, issues related to water resources have not been adequately addressed in climate change analyzes or in climate policy-making (Bates, et al., 2008).

Globally, a water volume of more than 1,000 m3 per inhabitant per year is considered more than necessary for domestic, industrial and agricultural uses. As a result, a basin is estimated to suffer from water stress when its water availability per capita is less than 1,000 m3 /year (based on the historical average runoff) or when the ratio between water extraction and the annual historical average of runoff is greater than 0.4. There are such basins in North Africa, the Mediterranean region, the Middle East and the Middle East, South Asia, Northern China, the United States of America, Mexico, northeastern Brazil, and the western coast of South America. The population living in these basins amounts to an estimated total of between 1.4 billion and 2.1 billion people (V.R.Smarty et al., 2000; Alcamo et al., 2003a, b; Oki et al., 2003; Arnell, 2004). Currently, agriculture is one of the main contributors of greenhouse gases, with 13.5% of the world'semissions. At the same time, climate change increases the risks and uncertainty of farmers, by warming and consequent aridity, by changes in rainfall regimes and by the increasing incidence of extreme weather events (FAO & Earthscan, 2011).

Irrigation accounts for about 70% of the water extracted worldwide and accounts for approximately 40% of agricultural production (Fischer et al., 2006). In fact, irrigated lands, which account for only 18% of the world'sagricultural land, produce 1 billion tonnes of cereals per year, accounting for about half of the total world supply; this is because irrigated crops produce, on average, between 2 and 3 times more than rain-dependent crops (Alexandratos, N., 2005).

Overall, global warming appears likely to benefit agriculture in developed countries intemperate areas and have adverse effects on the production of many developing countries in tropical and subtropical areas. Climate change could therefore increase developing countries' dependence on imports and accentuate the differences between north and south in food security (Canadell et al.,2007). Water management aims to improve the quantity and quality of available water. The ways to achieve this are: regulate the use of surface and groundwater, develop alternative sources of water, rationalize their consumption, control the supply of pollutants and recover initial conditions through purification processes. The objective of good water conditions should be pursued in each watershed, so that measures relating to surface water and groundwater belonging to the same ecological, hydrological and hydrogeological system are coordinated (Directive 2000/60/EC). From this perspective, the reuse of purified waters is an essential element of the natural water cycle and is, in fact, seen as a measure to solve the problems of water scarcity.

Given the high pollution rates of rivers, reservoirs and groundwater, an important option is presented: the desalination of seawater to obtain consumable water (Lechuga et al., 2007). Their demand has increased considerably in recent years. This is mainly due to the serious water resource shortage suffered in various parts of the planet.

In recent years, the idea that water management should be understood as an instrument in the service of an explicit territorial policy has been reinforced and that it will also be supported by the growing demand for integration between water management and sectoral policies (Moral 2009). From this perspective, this study proposes the direct use of seawater, without going through a process of prior desalination, as a fluid to be used in the irrigation of various types of cultivation.