**4. Conclusions**

This study presented the dynamics of global research in sustainable irrigation in agriculture over the last two decades, the main agents promoting it, and the topics that have received the most attention. The main concerns stated in the Introduction section related to the improvement of irrigation water use in order to increase food production, the world overexploitation of water resources, and the effects of global climate change. Our analysis verified how these questions are addressed by countries taking into account interdisciplinary approaches, and it also proved how these questions are mirrored in the main research lines on SI.

The results of the analysis of the principal variables revealed that the study of sustainable irrigation has grown in recent years in all of the variables considered: articles, authors, journals, institutions, and countries. Despite the fact that the growth trend in this topic is higher than that of general research in irrigation, an even greater research effort using a sustainability-based approach is required to further knowledge in this area. Traditionally, studies on sustainability have focused on one of the areas of which it is composed, namely, the environmental, social or economic dimensions. In the study of irrigation, the dominant area has been the environmental dimension, far more than the social or economic perspectives. The studies that analyzed just one of these dimensions provide highly useful information, but this information is only partial. It is necessary to integrate the three aspects of sustainability in order to gain full knowledge of the feasibility of certain practices, not only in terms of their environmental impacts but also with respect to income generation for farmers and the wellbeing of the community.

The keyword study revealed the existence of diversity between studies carried out using specific approaches and in different countries. In general, the study of environmental impacts and climate change, water availability, the improvement in efficiency, sustainable development, food supply, and the conservation of water bodies, particularly aquifers that have deteriorated, are common themes. However, certain practices, such as deficit irrigation or drip irrigation and aspects related to energy consumption and certain crops, are priority issues for particular countries. The methodological approaches used and the tools applied are other points of differentiation of the research carried out by each country. The keyword analysis showed four main research lines on SI: climatic change, environmental impact, and natural resource conservation; unconventional water resources; irrigation technology and innovation; and water use efficiency. Due to the large number of analyzed documents and the scope of this work, an in-depth content analysis per topic has not been undertaken. It will be highly interesting for future studies in order to provide more detailed information of these four specific topics.

As a final conclusion, we believe that certain aspects of the research on sustainable irrigation in agriculture in each of the dimensions of sustainability should be promoted. From a technical point of view, innovation and technology have furthered the development of irrigation systems and new available water sources that can contribute to improving the efficiency of water use and the sustainability of rural areas, particularly agricultural activity in arid regions. However, effort should be made to make this technology accessible, as its cost is economically unfeasible for small-scale agriculture in many countries. New water sources, such as those derived from desalination, reuse and rainwater harvesting systems, are very expensive for farmers compared to traditional sources. The production processes for desalination and reuse should be improved, particularly with respect to energy consumption in order to bring down the final price of the water. Furthermore, although the use of these nonconventional water resources has proved to have a series of advantages for the crops and the soil, this knowledge has not been transmitted to the farmers, and therefore, they are still reluctant to use it for irrigation. Greater effort should be made to communicate the results of the research to society. Finally, greater knowledge of the environmental impacts of irrigation-related practices in different areas on plot, district, basin and regional levels is needed. Water bodies are connected to each other, so certain practices that generate a small impact on river source areas can have a multiplying effect and be experienced in the underground bodies of coastal areas.

**Author Contributions:** The four authors have equally contributed to this paper. All authors have revised and approved the final manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was partially supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund by means of the research project ECO2017-82347-P, and by the Research Plan of the University of Almería through a Postdoctoral Contract to Juan F. Velasco Muñoz.

**Conflicts of Interest:** The authors declare no conflict of interest.
