**4. Approaching the problem from an environmental perspective**

The rational use of water should imply its moderate and efficient consumption, and the conservation of its quality after use and release back into the water environment. This idea is a fundamental concern in the Water Framework Directive (Directive 2000/60/EC of October 23rd, 2000). Several authors point out that improving water management at the fields level is the most effective tool in reducing the impacts of irrigation on the environment [48, 49], is the achievement of this objective, inseparable from convenient management of water resources at the scale of irrigation district [16, 50]. Agricultural and livestock activity is one of the primary sources of non-point source pollution, occurring in extensive areas and highly dependent on its hydrological behavior, which leads to the water bodies substances previously deposited in the soil (fertilizers, phytosanitary products, organic matter) [51]. For this reason, the focus on controlling this type of pollution has been more on indirect instruments such as codes of good agricultural practice embodied in agri-environmental measures complemented with monetary incentives to farmers [52]. In the climatic scenario of water scarcity, a considerable increase in irrigation costs is expected due to the pressure of water demand, so its rational and efficient use, combined with environmental concerns, is an unavoidable issue in modern irrigated agriculture.

*Resilience of Irrigated Agriculture to Face the Challenges in Mediterranean Climatic Conditions… DOI: http://dx.doi.org/10.5772/intechopen.107882*

#### **4.1 Rational application of fertilizers**

The amount of nitrogen fertilizers incorporated into the soil should be in proportion to its removal by the crops, safeguarding unnecessary spending with excess fertilizer and situations of contamination of soil, water, and atmosphere [53], since, in more oxidized forms, nitrogen is very soluble and mobile, presenting itself as one of the most problematic substances in water pollution [54]. However, this desideratum has not been achieved in recent years in Portugal, and there has even been an increase in the excess of nitrogen fertilizers between 2000 and 2017 from 144.7 to 153.1 thousand tons, corresponding to an increase of 5.8% (**Figure 2**). It should also be noted the importance of gaseous nitrogen emissions from the application of fertilizers in agricultural activity, which in 2017 amounted to 36.3 thousand tons, with a considerable impact on air quality and the water cycle. Nitrogen losses to the atmosphere are volatilized in the form of elemental nitrogen, ammonia ammonium ion, and nitrous oxide, the latter having a very harmful influence on the greenhouse effect in the atmosphere [56].

Concerning the nutrient phosphorus, the amounts applied in fertilizations are much lower, and its mobility in the soil is lower than that of nitrogen and is preferentially loaded outside of agricultural fields together with sediments [57, 58]. Even considering the improvements that have been seen in the characteristics of fertilizers, particularly in the efficiency of absorption of this nutrient by plants, the excess phosphorus applied still has a very high value, contributing to the progressive saturation of the soil with this element. We can verify by reading the graph in **Figure 2** that the phosphorus balance had a significant decrease between 2000 and 2017 (35%), passing the surplus amount of this element from 36.6 to 238 thousand tonnes. In the same period, the situation regarding the nitrogen balance presents an opposite trend.

#### **4.2 Irrigation water quality and the problem of salinization**

The high concentration of salts in the various compartments of its cycle, depending on its nature, may cause inconveniences to economic, environmental, and social order. This problem usually occurs under particular climatic conditions, being typical in irrigated areas where water with high salinity is used [59], frequently by downstream reuse of water that has already been used once or several times in irrigation [60, 61]. In a given agricultural system, the balance of salts is considered adequate when the level of soil salinization is compatible with the expected crop yield [62]. In a scenario of climate

#### **Figure 2.**

*Gross balance (incorporation – Removal by plants) of nitrogen and phosphorus from fertilizers applied in agricultural activity in Portugal (Adapted of [55]).*

change for Portugal and Spain, where an increase in temperature is predicted, with a greater incidence of drought phenomena, and a decrease in total annual precipitation, especially in the south of the Iberian Peninsula, the problem of salinization may become an agro-environmental problem with some acuity, due to the decrease in the washing of salts from the soil and its evapoconcentration in the plant root zone [41]. Of particular concern are the areas of more intensive agriculture, with massive applications of fertilizers, namely the areas of irrigation perimeters in southern Portugal and Spain, and, for similitude of climatic conditions, the southern European countries [63]. In Portugal the estuarine zones of some rivers and other coastal areas, and in Spain, especially in the Ebro River valley and large areas between Almeria and Valencia, the soils are potentially affected by salinization. A significant problem is the particular case of salinization called sodization, currently affecting more than 50% of areas near the coast of Cadiz in Spain. Areas near the coast may be affected by the advancement of the seawater interface due to the intensification of groundwater abstractions [64].

In addition to the excessive application of fertilizers to crops, especially irrigated crops, the quality of the water derived for irrigated areas is also a determining factor in inducing salinity in soils [65]. This aspect is crucial when the water abstracted for an irrigation district already has return flows enriched with nutrients (salts) from several irrigation zones and portends a deteriorated quality [66]. If the irrigation water has of good quality, it is usually not degraded after being used and returned to the natural drainage network. To exemplify this idea, the results of two irrigation seasons observed in an experimental basin located at the Irrigation District of Campina da Idanha, in the center-eastern region of Portugal, are presented below. Indeed, it is possible to verify the low mineralization of the irrigation water derived from a distribution channel, whose quality refers to a category of excellent, and slight variation throughout the irrigation season, rarely exceeding the limit of 100 S/cm (**Figure 3**). On the other hand, the return fluxes from this basin also present a good quality regarding this parameter, not compromising its use downstream [68].

Once the problem of soil salinization is installed, some corrective measures can be adopted to improve crop yields, such as (i) adoption of rotations that include crops more tolerant to salinity and that ensure acceptable yields for farmers; (ii) paying particular attention to the crops tolerance to salinity in the emergence phase, practicing

#### **Figure 3.**

*Evolution of water quality (electrical conductivity) in a distribution channel of Idanha Irrigation District (Portugal), and the return fluxes of an irrigated small basin, in irrigation seasons 2004 (a) and 2005 (b), and the maximum recommended value (MRV) of irrigation water quality in Portugal (Adapted of [67]).*

*Resilience of Irrigated Agriculture to Face the Challenges in Mediterranean Climatic Conditions… DOI: http://dx.doi.org/10.5772/intechopen.107882*

one or more irrigations to decrease the osmotic potential in the soil; (iii) more frequent irrigations than contemplate an adequate leaching fraction for the maintenance of an acceptable salinity level; (iv) adoption of localized irrigation systems, allowing to have high moisture contents in the humidified soil volume mitigating the salinity effect [67].

#### **5. Conclusions**

The edaphoclimatic particularities of the Mediterranean basin, namely in the Iberian Peninsula, determine that irrigated agriculture is unavoidable to guarantee the quality of agricultural products and levels of production to the necessary economic viability. However, this goal implies strong challenges, like the compatibility between agricultural activity and the conservation of natural resources.

The great increase in the efficiency of water use in agriculture was accompanied by a great increase in energy consumption, due the modernization of irrigation systems. In the last 60 years in Portugal, the variation was 15,000 to 6000 m3 /ha.year, and 200 to 1500 kWh/ha, respectively. The use of modern/smart technologies in irrigated agriculture, increasingly widespread in the last time, like Information and Communication Technologies, allow the rapid share of information between all the system components, according to the actual or forecasted situation, and using models and artificial intelligence, can promote optimized answers at different scales (irrigation systems in field, distribution water network in the irrigation project, water storage).

The alternative/unconventional sources of water to supply the necessity in agriculture, already quite contribute to the resilience of this activity in Spain. For example, in 2010, the volume of unconventional water resources in Spain rose to 4.540 hm3 /year. Of the total used in agriculture, 450 hm3 of water comes from the reuse of treated water, and 690 hm3 comes from desalination.

Nevertheless, the main difficulty lies in the price, too high to be supported by farmers, and, many times, in the characteristics of the water obtained. Special mention must be referred about the water transfer between regions under rules of equity, and periodically reviewed. An example in Spain is the water transfer from the Tagus River in a volume of 421 hm3 /year to the Segura River.

The rational application of fertilizers, in line with the crops needs, is a priority, to prevent the contamination of superficial and subterraneous waters, and to prevent the gradual process of soil salinization in semi-arid regions. Equally important is the evaluation of the water quality to irrigation and accounting the amount of nutrients that already contain.

#### **Acknowledgements**

This work is supported with Portuguese national funds by FCT - Foundation for Science and Technology, I.P., within the following projects: GEOBIOTEC—Project UIDB/04035/2020; CERNAS-IPCB—Project UIDB/00681/2020.

#### **Conflict of interest**

The authors declare no conflict of interest.

*Irrigation and Drainage - Recent Advances*
