## 3 Bibliometric Analysis
### Evolution of Publications and Research Areas
Lastly, the highly specialized or niche themes quadrant, though well developed and specialized, exhibits high density but low centrality. This means these themes have limited relevance in the global context or for the broader scientific community but could be crucial in specific applications or future research. Keywords in this category include \"Biomass\", \"Carbon\", and \"Water supply\". These results highlight the need to continue strengthening core and emerging themes, bridging knowledge gaps, and steering research toward a more sustainable energy system [59; 60; 61; 62; 63; 64; 65]. Table 3 describes the four clusters, aligning with the parameters in Figure 4, which display the number of items in each group and the most representative keywords. In contrast, Figure 5 maps \"Index Keywords\", which offer a broader, database-driven categorization of research topics. This distinction is crucial for understanding how research themes are self-reported versus how they are classified at an indexing level. The circle size represents the frequency of keyword usage within each cluster. The connections between different groups can be observed based on the usage of each keyword across clusters. Figure 4: Thematic map of the clusters of the 57 most frequent keywords. Table 3, as mentioned, presents the keyword clusters obtained using _Bibliometrix_ and reveals several important aspects that complement the observations in Figure 5. Cluster 1, centered on Renewable energies, encompasses a wide range of keywords, such as wind energy, drought, and hydroelectric generation. This demonstrates the central relevance of renewable energies in current research. On the other hand, Cluster 2 focuses on \"Solar energy\", including key terms such as solar power generation and land use. This confirms the dominant role of solar energy in the energy transition, as highlighted in the cluster figure, where it appears as a highly interconnected topic closely related to climate change and resource optimization. \begin{table}
\begin{tabular}{c c c c} \hline _Cluster_ & _Main Keywords_ & _Number of Keywords_ & _Keywords in the Bibliometrix Network_ \\ \hline \multirow{4}{*}{1} & & & Renewable energies, wind power, drought, hydroelectric power, alternative energy, optimization, solar power, energy storage, hydroelectric power plant, electricity generation, energy planning, power generation, drinking water, offshore oil well production, optimizations, photovoltaic system, wind, artificial intelligence, California. \\  & & & Climate change, solar energy, solar power generation, land user, reservoirs (water), economic analysis, photovoltaic cells, water management, electric power generation, energy, photovoltaic systems, concentrated solar power, hybrid systems, investments, photovoltaic panels, photovoltaics, biogas. Renewable energy resources, fossil fuels, energy efficiency, water scarcity, costs, energy utilization, renewable energy sources, digital storage, electric power transmission network, electric energy storage, energy policy, greenhouse gases, irrigation, sustainable development, water resources, and cooling systems. \\
4 & Biomass & 5 & carbon, biomass, water, water supply, China. \\ \hline \end{tabular}
\end{table} 
Table 3: Cluster of keywords obtained using Bibliometrix software. Figure 5: Clusters of the most frequent index keywords assigned by the Scopus database. Node size represents the frequency of keyword occurrence, while arc thickness indicates the strength of keyword co-occurrence relationships. Larger nodes denote higher occurrence, whereas thicker links highlight stronger conceptual connections. Cluster 3, associated with \"Renewable energy resources\", emphasizes the importance of renewable energy resources, energy efficiency, and reducing dependence on fossil fuels. This area is connected in Figure 5 with terms like optimization and efficiency, reinforcing the need for continued research into improving these technologies.