3.7.2. Unconventional Water Resources

The current scenario is one in which so-called conventional water sources are being exhausted and degraded in large parts of the world. These water sources include both surface water (rivers, lakes, reservoirs) and groundwater (aquifers). The principal option for increasing the water supply for irrigation consists of using alternative sources, also called nonconventional water sources [23]. These other sources include the reuse of urban and industrial water, the desalination of seawater, and rainwater harvesting. In recent years, water from nontraditional sources has become a competitive option in the supply of quality water for irrigation, particularly in arid and semiarid regions [62] The use of these types of resources has a series of advantages—two in particular. First, the contribution of these new water sources represents an increase in the supply of the resource, which is capable of satisfying the growing demand of the different sectors (urban supply, agricultural activity, tourism sector, industrial sector, and environmental requirements) [63]. Second, the use of water from alternative sources should serve to diminish the use of traditional water sources so that the state of deterioration of the wetlands, rivers, and aquifers can be restored or at least alleviated [64]. If these two advantages are to be efficient, they have to be accompanied by demand control. In addition to these two principal functions, the use of nonconventional water resources gives rise to other advantages. They provide a greater reliability in the supply, they supply higher quality water, they can generate increases in crop yields, they contribute to ensuring the stability of agricultural incomes, and they can have positive effects on seawater intrusion processes in the aquifers [65–67].

The use of each of these alternatives also gives rise to a series of limitations and disadvantages. Evidently, the construction of seawater desalination plants is only feasible in coastal areas. A wastewater treatment plant requires a volume of a large enough size for the facility to be viable [64]. Therefore, the use of this resource is not possible in areas where the activities generating wastewater (population nuclei, industrial facilities, livestock farms, etc.) do not have sufficient water use [63]. Thus, despite water reuse being the ideal way of maintaining continuous use of the resource, this type of facility is not appropriate for many rural areas where the population is dispersed. The main problem of rainwater harvesting systems is the low volume of water that can be supplied in comparison with the demand [67]. Furthermore, the seasonality of rain in many regions means that the water must be stored for long periods of time for use when needed. To these limitations we must also add the high cost of water derived through these systems. The installation costs are usually very high, and we must also take into account the cost of production. In the case of desalination and reuse, these costs usually establish a price for water that is much higher than the price of conventional resources [68]. This means that many farmers throughout the world are not willing to pay the price of the water unless there is no alternative available. Studies have been carried out on experience with the use of desalinated seawater, reused water or harvested rainwater all over the world [69–75].
