*3.2. Evolution of Research in SI by Subject Area*

Figure 3 shows the evolution of the main subject areas into which the articles on SI included in the Scopus database were classified. It should be noted that an article may belong to more than one category. From the beginning of the period, the category in which the highest number of studies were classified was Environmental Sciences, which accounted for almost 65% of the total sample. The second largest block of studies was classified in the Agricultural and Biological Sciences category, with 44.3% of the total sample. In third place was the Social Sciences category with 21.1% of the articles. These three categories have dominated research on SI since the beginning of the studied period. However, in contrast to some previous works [37,39,48], our results revealed that over the last five years, the Earth and Planetary Sciences, Engineering, Energy, and Economics categories have begun to gain relevance, although none of them include more than 15% of the total articles in the sample. The Scopus classification distinguishes between the following categories: Business, Management, and Accounting; and Economics, Econometrics, and Finance, which also differ from Social Sciences. For the purpose of simplification, we grouped these two categories into only one and termed it "Economics".

**Figure 3.** Comparative trends of subject categories related to SI research.

The keyword analysis revealed that there is a series of commonly used terms in research on SI, irrespective of the approach of the study. These terms include, among others, agriculture, alternative agriculture, climate change, crops, groundwater, irrigation system, salinity, sustainable development, water conservation, water management, water resources, water supply, water use, water use efficiency, and water quality. When we took the analysis beyond these terms, we identified a group of keywords used specifically by each discipline.

In the studies classified in the category of Environmental Sciences, there was an emphasis on the state of the soil (soils, soil moisture), aquifers, and surface water. With respect to processes, from the environmental approach, recycling and wastewater reclamation were prominent. In terms of methodology, the keywords that stood out were numerical model and decision making, particularly related to the management of the available water resources (water budget, water availability). With regard to the geographical dimension, China, the United States, and India were particularly prominent, as were the regions of Eurasia and Asia.

In the studies classified within the category of Agricultural and Biological Sciences, technical terms were predominant. Studies in this category mostly focused on soils and groundwater. There was particular emphasis on different types of crops (Triticum aestivum, Zea mays, fruit, Gossypium hirsutum, and rice) and irrigation processes (deficit irrigation, drainage, drip irrigation, leaching, waterlogging, agricultural irrigation). Furthermore, from the agronomic perspective, the environmental dimension was also considered (Environmental Impact). In these studies, China and the United States stood out, together with the regions of Africa and Asia.

The studies carried out from a Social Sciences approach had a more multidisciplinary perspective. They focused primarily on the stakeholders, water demands, and food security. However, technical concepts were also prominent, particularly those related to irrigation and water management (drip irrigation, sustainable water management), crops (Triticum aestivum), and economics (water economics). Unlike the over categories, land use was found to be one of the prominent subjects of this group of studies. A focus on management and decision making at different levels was also characteristic of these studies (governance approach, water planning, policy making, resource management, decision support system). With respect to geographical distribution, the United States, China, India, Spain, and Australia were among the most cited countries, and Asia, Europe and Africa stood out on a regional level.

Finally, the studies carried out based on an economic approach (Economic Sciences) were the most multidisciplinary, including technical, social, and environmental aspects. Among the main themes analyzed in these studies, we found food supply, food security, the development and innovation of irrigation systems (agricultural technology, irrigation performance) and management processes (integrated resource management, managed change, project management, strategic change, strategic management, strategic planning, decision making), and issues related to economic and social management (efficiency, investment, performance, economic and social effects) and the environment (environmental impact, environmental sustainability).
