*3.3. Most Relevant Journals in the Research on SI*

Table 2 shows the main characteristics of the most prolific journals in the field of SI. The group of journals with the highest number of articles published on SI accounted for 25.7% of the total articles in the sample. This indicates that there is a high level of dispersion in terms of the journals that publish articles on this subject area. The leading journal in terms of the total number of articles published during the whole period analyzed was *Agricultural Water Management*, with a total of 52 articles on SI. This journal has the highest H index and the most citations of the journals with articles published in this area, and a Scimago journal rank (SJR) factor of 1.272. It published its first issue on this subject in 2001. Since then, it has remained among the top positions in terms of the number of articles published on SI, becoming the leader in 2014. The journal in second place was *Irrigation and Drainage*, with a total of 30 articles on SI. This journal published its first article on SI in 2001 and was the most prolific journal until 2005. It has the second highest H index, an average of 10.4 citations per article and an SJR index of 0.342. The third journal was *Sustainability*, with 17 articles on SI. This journal is among the most recently incorporated journals, as its first article on SI was published in 2013. However, in only five years, it rose to third position in terms of the number of articles for the whole of the period of study. It has an average of 3.8 citations per article, an H index of 6 and an SJR of 0.537. The journal with the highest average number of citations per article was *Science of the Total Environment* with 33.8; followed by the *Journal of Hydrology* with 31.5 and *Water Resources Management* with 29.4.


**Table 2.** Main characteristics of the most active journals related to SI research.

<sup>1</sup> Scimago Journal Rank 2017; <sup>2</sup> only sample documents; <sup>3</sup> total number of citations divided by the total number of articles.

## *3.4. Most Relevant Countries in Research on SI*

Table 3 shows the principle characteristics of the articles on SI from the most prolific countries. During the analyzed period, the United States was the leading country in research on SI in terms of the number of articles, with a total of 143. The country with the second highest number of articles was India, with a total of 74. This was followed by Australia with 67, Spain with 61, and Italy with 55. Due to the differences in terms of the size and economic development of the different countries, these data were analyzed to determine the number of articles per capita, measured as the number of articles per million inhabitants. Based on this variable, Australia was shown to be the most productive country with 2.7 articles per million inhabitants. This was followed by the Netherlands with 1.5, Spain with 1.3, Italy with 0.9, and the United Kingdom with 0.8. France was shown to be the country with the most citations per article, with 23.9, followed by the United Kingdom with 22.5, Iran with 21.9, the Netherlands with 18.6, and the United States with 18.1.


**Table 3.** Main characteristics of the most active countries related to SI research.

<sup>1</sup> Total number of articles per million inhabitants; <sup>2</sup> total number of citations divided by the total number of articles; <sup>3</sup> only sample documents; <sup>4</sup> FAO Aquastat (2019), last available data.

The percentage of cultivated area ready for irrigation per country has been included in the last column of the table, as well as the position they have in the world ranking regarding this variable. It can be stated that these countries do not occupy leading positions as far as irrigation-equipped cultivated land is concerned. From the available information about 177 countries, Japan is the country with the highest percentage of irrigation-equipped land surface—54.96%—, reaching the 25th place—followed by Iran with 51.88% (27th place), China with 51.48% (28th place), and Netherlands with 46.85% (31st place). However, some countries leading research on SI place themselves on lower positions within the irrigation-equipped cultivated surface ranking. This is the case for the USA with 16.94% (72nd place), Australia with 5.72% (110th place), or the UK with 3.41% (126th place).

Table 4 shows the principal variables related to the international collaboration of countries with the highest numbers of articles. The average percentage of articles carried out through international collaboration was 50.3%. The countries with the highest percentage of studies carried out in collaboration were Canada with 81.8%, France with 79.3%, Germany with 75.1%, the Netherlands with 65.4%, and China with 55.8%. The United States was found to have the largest collaboration network, with 33 different collaborators. In addition, similarly to Australia, this country forms part of the group of the main collaborators of 10 of the 15 countries in the table. These data reveal the global nature of research in this subject area, with very high percentages and extensive collaboration networks on a global level. The majority of the countries obtained a higher average number of citations per article when they worked in collaboration with other countries. The articles produced through collaboration obtained an average of 14.6 citations as opposed to 13.5 citations of noncollaborative articles. When comparing these results to those of related works on irrigation and water [37,39,48], it can be observed that studies on sustainability trigger a higher level of international cooperation.


**Table 4.** International collaboration of the most active countries related to SI research.

<sup>1</sup> Number of articles made through international collaboration divided by the total number of articles; <sup>2</sup> number of citations obtained by articles made through international collaboration divided by the number of articles; <sup>3</sup> number of citations obtained for articles not made through international collaboration divided by the number of articles.

Figure 4 shows a network map of the collaborations carried out between countries, where the size of the circle represents the number of documents per country and the color corresponds to the cluster formed by the different groups of countries. Three clusters can be distinguished, led by the United States, Australia, and Spain in terms of the number of articles. The first (shown in blue) includes some of the most prolific countries, such as India, Italy, China, France, Japan, and the Netherlands, and others, such as Mexico, Egypt, and Bangladesh. Together with Australia, the second cluster (shown in red) includes Canada, Iran, South Africa, Sweden, Switzerland, Pakistan, Sri Lanka, and Uzbekistan. The group led by Spain (shown in green) includes some European countries, such as the United Kingdom, Germany, Belgium, Portugal, and Greece, as well as countries in the Mediterranean basin, such as Israel, Jordan, Morocco, and Turkey, and others, such as Brazil, Thailand, and New Zealand.

**Figure 4.** Main relationships between countries in SI research.

A keyword analysis was used to detect the preferences in the research conducted by the countries included in Table 4 (Table 5). We established that there is a group of terms that make up a general line from which the different specific topics are derived.

In the studies conducted in the United States, the central themes included food supply and food security (food-supply, food-security), the conservation of natural resources and environmental impacts (conservation-of-natural-resources, environmental-impact), and land use (land-use). Agronomic issues, such as crop evapotranspiration and productivity, were prominent. Both surface water and groundwater were studied with special emphasis on the availability of the resource, the water budget, the water table, and water stress (surface-water, water-budget, water-availability, water-table, water-stress). The term most used in relation to methodology was numerical model (numerical-model), and the term most used with regard to crops was Zea mays (Zea-mays). No noteworthy geographical terms other than the United States were identified.


**Table 5.** Main keywords of the most active countries related to SI research.

The articles from India were found to place more emphasis on new alternative agriculture systems (alternative-agriculture) and the agricultural activity in arid areas (arid-regions) on an ecosystem level (ecosystems). Agronomic aspects such as the nutrition and fertilization of crops (fertilizers, nutrient) and their productivity (crop-yield, productivity) were also prominent. Both surface and groundwater were studied (surface-water), as was the planning and development of irrigation projects for optimizing the resource (irrigation-planning, irrigation-project, optimization), with particular emphasis on irrigation processes (drainage, waterlogging, water-table) and alternative water sources (wastewater). The term most used in relation to methodology was numerical model (numerical-model), and with regard to crops, the most used terms were rice and vegetables (rice, vegetables). Prominent geographical terms, such as the State of Maharashtra or the region of South Asia, were identified.

The studies conducted in Australia covered food supply (food-supply), runoff (runoff), and environmental protection (environmental-protection). Agronomic issues, such as crop evapotranspiration (evapotranspiration) and irrigation efficiency (irrigation-efficiency), were prominent. With respect to water resources, particular emphasis was placed on the availability of the resources, the water budget (water-availability, water-budget), and the use of alternative sources through recycling and wastewater treatment (wastewater, water-treatment, recycling). From a methodological point of view, the hydrological and geological approaches were prominent (hydrology, hydrogeology), as were controlled studies (controlled-study), and in terms of crops, the prominent term was rice (rice). Noteworthy geographical terms, such as the basin of the river Murray–Darling (Murray-Darling-basin), the region of Australasia and the country of Canada, were identified.

In the case of Spain, relevant topics were the management of the use of energy resources (energy-efficiency, energy-resources), the productivity of water (water-productivity), efficiency (efficiency), semiarid regions (semiarid-regions), and environmental protection (environmental-protection). Agronomic issues such as soil moisture (soil-moisture) were prominent, as were innovations in agricultural and irrigation systems (alternative-agriculture, irrigation-networks, drip-irrigation, deficit-irrigation) and irrigation efficiency (irrigation-efficiency). In terms of methodology, the economic approach was prominent (water-economics, economic-analysis, profitability), as was the social approach (stakeholders, decision-support-systems). With respect to crops, fruit was the most relevant term (fruit). Noteworthy geographical terms, such as the regions of Southern Europe and Eurasia (Southern-Europe, Europe, Eurasia), were identified.

The studies conducted in Italy were similar to those conducted in Spain. The main differences were found in certain agronomic terms related to crops (crop-yield, dicotyledon, stomatal-conductance, stem-water-potential) or irrigation (drainage, soil-moisture). The studies were carried out on a farm level (farms, orchard) and focused on the Mediterranean environment. In terms of methodology, the numerical models focused on decision making (decision-making) were noteworthy. With respect to crops, as well as fruit (fruit), Zea mays was also prominent. The central themes included water stress (water-stress), the environmental impact (environmental-impact), and forestry (forestry).

The articles conducted in China placed greater emphasis on environmental protection (environmental-protection), agricultural activity in arid regions (arid-regions), decision-making processes (decision-making), and issues at the district level (irrigation-district). Agronomic issues such as crop evapotranspiration (evapotranspiration) and crop productivity (crop-yield) were also prominent. With respect to water, the central theme was the level of water resources (water-level), while in the methodological area, hydrogeological models were prominent (hydrogeological-modelling). With respect to crops, rice, corn maize and wheat were found to be noteworthy (rice, Zea-mays, Triticum-aestivum, maize). The most prominent geographical terms were the regions of the North China Plain and Xinjiang Uygur.

The central themes in the studies conducted in the United Kingdom were crop yield (crop-yield); the environment and the assessment of the environmental impact (environment, environmental-impact), particularly in arid regions (arid-regions); and hydrocarbon (hydrocarbon). Processes related to water and irrigation, such as drainage (drainage), runoff (runoff), and the recycling of water in wetlands

and wastewater treatment (water-treatment, wastewater, wetlands-recycling), stood out. The most prominent term related to methodology was cost benefit analysis (cost-benefit-analysis), and in terms of crops, the study of rice was predominant (rice). This may be due to the country's connection with Asian countries (Asia, Eurasia, and South Asia).

The articles conducted in Germany, as in the case of other countries, were conditioned by its collaborative ties with other nations. The main themes included food supply and food security (food-supply, food security); the new alternative agricultural systems and the intensification of agricultural activity (alternative-agriculture, agricultural-intensification); agricultural activity in arid regions (arid-regions, oasis); and the effects of agricultural activity on economic and social levels and environmental pollution (economic-and-social-effects, greenhouse-gas, particle-size). The prominent agronomic aspects were fertilizers (fertilizers), leaching (leaching), and drip irrigation (drip-irrigation). The most commonly used term with respect to methodology was related to the study of the effects of climate change: climate models (climate-models). Further fields of interest are common pool resources; and, with respect to crops, studies on wheat (Triticum-aestivum). One of Germany's principal collaborators was China, which is why this country appeared prominently among the keywords of German studies. Similarly, Uzbekistan is one of Germany's main trading partners and also appears among the keywords.

The studies carried out in France particularly focused on the policy and institutional dimension (environmental-policy, decision-making, governance-approach, public-private-partnership, pricing-policy). French studies analyzed the level of exploitation (farming-system) and contemplated the different agents involved (stakeholders). The priority issues included the overexploitation of groundwater and surface water (groundwater-overexploitation, surface-water); the chemical composition (chemical-composition); crop evapotranspiration (evapotranspiration) and the response to possible alterations (ecophysiological-responses); and deficit irrigation (deficit-irrigation). In terms of methodology, dynamic models and linear programming were prominent (dynamic-model, linear-programming), together with economic issues related to water (water-economics).

Studies conducted in Japan considered issues such as the development of irrigation, particularly through participative processes on both institutional and irrigation water user levels (irrigation-development, water-users'-organization, participatory-irrigation-management, institutional-development, water-policy). The Japanese studies analyzed aspects related to the salinity of irrigation water and soil (saline-water-irrigation, soil-water-salinity) and the electrical conductivity of water (electrical-conductivity). Water shortages due to drought, particularly in semiarid regions, were also found to be relevant issues (drought, semi-arid-region). The studies from this country analyzed the use of rainwater as a source for irrigation (rainfall). It is the country with the highest number of crops, and the study of the sustainable production of rice was found to be particularly relevant (sustainable-rice-production, Triticum-aestivum, Zea-mays, sorghum, sorghum-bicolour).

In the Netherlands, the primary topics identified were the management and development of agriculture, particularly towards the use of new alternative systems (agricultural-development, agricultural-management, alternative-agriculture), and there was special emphasis on the extension of agricultural practices (agricultural-extension). Other subject areas of many of this country's studies were the security and production of food (food-supply, food-production). The crop that is most studied was wheat (Triticum-aestivum). The articles focused on groundwater and rainwater with respect to water resources (well, groundwater-abstraction, rain), recirculation processes and network development (drainage, recirculations, networking-system) and the studies on the level of the basins were predominant (catchments). On a social level, the point of view of the small farmers (smallholders) was given special attention, particularly in relation to the decision-making processes (decision-making).

The studies conducted in Brazil contemplated the environmental impacts (environmental-impact), water availability (water-availability), and the expansion of the agricultural activity (agricultural-expansion). With respect to crop processes, evapotranspiration, metabolism, sustainable production, and carbon dioxide were prominent (carbon-dioxide, sustainable-production, evapotranspiration, metabolism). This country has published a large number of studies on the use of rainwater for irrigation (rainwater). Particularly noteworthy is the research on crops related to the use of biomass for different purposes (biofuel, bioenergy, biomass-power, sugar-cane, glycine-max, saccharum-officinarum). On a geographic level, studies in the region of Cerrado were predominant. Additionally, the most prominent methodological approach was found to be chemistry (chemistry).

In Canada, the most relevant themes were the new forms of agriculture (alternative-agriculture), environmental protection (environmental-protection), food security (food-security, food-production), and decision making (decision-making, water-policies). The global perspective of this line of research (global-perspective) was found to be noteworthy. From a methodological point of view, the sensitivity models, stochastic programming, and uncertainty analysis were prominent (sensitivity-analysis, stochastic-programming, uncertainty-analysis). The predominant terms with respect to water were water availability, water stress, and water sharing. With regard to crops, wheat and cotton stood out (Gossypium-hirsutum, Triticum-aestivum).

In the case of Iran, concern for the food supply was found to be prominent, despite its relationship with the FAO—Food and Agriculture Organization of the United Nations (food-supply, FAO, crop-yield). On a methodological level, genetic algorithms stood out (genetic-algorithm), together with processes for optimizing decisions (optimum-decision), and Bayesian networks (Bayesian-networks). The economic and social levels were represented through the motivation of farmers and the assessment of the economic and social effects (economic-and-social-effects, farmers-motivation). In the agronomic field, cropping patterns and the use of untreated wastewater were priority areas (cropping-pattern, untreated-wastewater-irrigation). In addition to wastewater, the combined integral use of surface water, recycled water, and rainwater for irrigation was prominent (rainfall, surface-water-resources, recycling).

Finally, the articles from South Africa were based on controlled studies, geographical information systems, and computer simulations (computer-simulation, simulation, controlled-study, GIS). The environmental impacts (environmental-impact), particularly those related to the use of mine water and wastewater (mine-water, wastewater); food supply (food-supply); project management (project-management); and the economic and social effects related to irrigation (economic-and-social-effects) were priority themes. With respect to the agronomic dimension, the evapotranspiration processes, drought, electric conductivity, and water balance stood out (evapotranspiration, drought, electric-conductivity, water-balance).
