3.7.3. Irrigation Technology and Innovation

Irrigation technology has evolved continuously over the last few decades. Flood irrigation, sprinkler irrigation, furrow irrigation, and drip irrigation are some of the methods that have emerged, and their advantages and disadvantages have been studied with respect to different types of crops, soils, and climatic conditions. New technologies have given rise to the development of comprehensive automated systems that combine the use of tensiometers, lysimeters, software applications, and even geographical information systems. However, drip irrigation and deficit irrigation are the terms that appear among the most used keywords.

De Wrachienb et al. [76] date the beginning of drip irrigation systems to the 1940s in Australia. The development of this system came about after the emergence of polypropylene tubes. It was not until two decades later that this system was improved in Israel, from where it was exported all over the world. Currently, thanks to automation and the use of microcontrollers, sensors, and integrated systems, this method has been perfected, and the drip irrigation system is now considerably more advantageous than traditional systems such as flood irrigation or sprinklers [77–79]. The main

contribution of this system is that it enables a substantial saving in the use of water for irrigation, which enables the development or expansion of agricultural activity in arid and semiarid regions, where it would not be possible otherwise [80,81]. Another advantage is that it can prevent evaporation, as it supplies water directly to the roots of plants [82]. Different studies show that the use of drip irrigation increases the marketable yield and quality of crops and stabilizes production when deficit irrigation is used and that fertigation through drip irrigation helps to reduce the use of fertilizers and, therefore, the risk of pollution due to leachate [80,83]. Salvador and Aragüés [84] analyzed the advantages and disadvantages of the use of underground drip irrigation systems. They demonstrated their usefulness, profitability, and sustainability and indicated that the design, handling, and maintenance of this system, together with the quality of the irrigation water and type of soil, are fundamental aspects that determine their sustainability. On the other hand, Puy et al. [85] indicated that this type of system can have harmful consequences in terms of the degradation of the soil or the production of greenhouse gas emissions.

Deficit irrigation was introduced as a measure to limit the vegetative growth of crops [86]. This irrigation technique has been fully developed, and it is used extensively [87]. This method has been used with both drip irrigation and microsprinkling on different crops and can be combined with remote sensing technology or infrared techniques to produce significant water savings while crop yields remain unaffected. Du et al. [88] analyzed the use of deficit irrigation as a sustainable strategy for managing water resources in agriculture for food security in China. These authors concluded that the current understanding of physiological processes enables the deficit irrigation methods to be adjusted to different crops and environments in order to increase water use efficiency and the yield and quality of crops. Many studies have been carried out on this subject area [89–93].

Though many authors support drip irrigation as a sustainability measure, some recent studies question it. Perry et al. [94] confirm the "zero-sum game" hypothesis which argues that the impact of high-technology watering in a farm increases the demand of local water and land production at the expense of water availability and production in other places. Furthermore, due to the advantageous effects of drip irrigation, it makes water more affordable and, at the time, it allows irrigating larger areas, obtaining greater profits, and shift to more valuable crops. The most foreseeable impact of water efficiency improvement will be the increase of current water demands. In this sense, water scarcity would remain difficult to manage. Paul et al. [95], in their review of the rebound effects on the management of land and cultivation soils, found evidence for the presence of rebound effects and the Jevons paradox, together with productivity increases and efficiency of irrigation water due to technological innovations. Further studies agree with these results [96,97].

## 3.7.4. Water Use Efficiency

All of these innovations have the objective of improving water use efficiency for irrigation. In the year 2000, Kofi Annan, the Secretary General of the United Nations, proposed a "Blue Revolution in Agriculture" that was proposed to be capable of increasing productivity per unit of water. This strategy became known by the slogan "more crop per drop" [24]. According to Yang [98], obtaining the ideal water efficiency for irrigating crops involves the reduction of losses caused by evaporation, runoff, and underground draining while increasing production. Zhang et al. [26] indicated that the use of technology to save irrigation water not only saves water and increases production but also improves the nutritional value of agricultural products and guarantees food safety by improving the environmental conditions. Water use efficiency in agriculture generally implies a reduction in water use to meet a specific production objective or to increase the production of a specific water supply [99]. The aim of improving water use efficiency is to increase food production, boost financial gains, and guarantee the supply of ecosystem services at lower social and environmental costs per unit of water used [100,101]. The practices used to achieve this objective include rainwater harvesting, complementary irrigation, deficit irrigation, and the use of precision irrigation techniques and practices to conserve groundwater [24,102]. The priority areas where it is possible to significantly increase the

productivity of water include areas with a high level of poverty and a low level of water productivity; areas with physical water shortage, where competition for water is high; areas with limited development of water resources, where the high yields of additional water have a considerable impact; and areas with degraded ecosystems driven by water, such as depleting water tables and dried-up rivers [103,104].

Among the different improvements developed over the last few decades, the use of drip irrigation has been fundamental in the improvement of water use efficiency and saving. Different studies have shown that drip irrigation has a water-saving potential of between 18% and 75%. According to Narayanamoorthy [82], drip irrigation saves an average of 25 to 75% of water compared to flood irrigation. Similar results were found, although with different percentages, in studies by Ibragimov et al. [105], Maisiri et al. [106], Yazar et al. [107], or Peterson and Ding [108], Abdulai et al. [109], Cremades et al. [110], and Jalota et al. [111].
