1. Introduction

Similar to all scientific fields, the study of IHS has changed over time. Interests in certain themes have emerged and disappeared, and the breadth and depth of research content have expanded [10]. The most significant advances were made around the five well-known assumptions implicit in IHS models [9]. The five underlying assumptions are listed as follows: (1) significant isotopic differences occurred between the pre-event (old) and event (new) water; (2) the new water retains a constant isotopic signature in space and time, or any variations can be accounted for; (3) the isotopic signature of the old water is constant in space and time, or any variations can be accounted for; (4) contributions of water from the vadose zone must be negligible, or the isotopic content of soil water must be similar to that of groundwater; and (5) the contributions from surface storage to streamflow are negligible. Many IHS studies have concentrated on the validity and effects of these assumptions in recent decades [2,11,12,13,14,15,16,17]. Tracing IHS literature by quantitative approaches is interesting and vital for enhancing the understanding of runoff generation and relevant hydrological processes.

Text mining is the process of automatically extracting high-quality information from unstructured or structured texts with diverse formats and types using linguistic and statistical techniques [18,19]. Thematic analysis is an important subject of textual data mining. Studies have found that temporal changes in themes, usually represented by keywords for a given research domain, can help to uncover the evolution of a topic and its trend [20,21,22]. For example, Yao et al. [23] analyzed the frequency changes in author keywords related to nitrogen in eutrophic lakes or reservoirs at five-year intervals and successfully determined the current and future trends in nitrogen field research. Chen et al. [24] analyzed research trends in management science and engineering in China based on co-keyword analysis and concluded that the foci were game theory, supply chain management, complex networks, data mining, optimization, risk management, and data envelopment analysis. In addition, theme variations based on keyword analysis can also help new researchers to situate themselves and their topics within the field and within changing research interests. Similarly, a systemic analysis of the thematic changes in IHS research is indispensable to understanding how this field evolved over time and predicting future research trends.

Bibliometric analysis is a popular, powerful, and systematic way to analyze the performance of scientific production using mathematical and statistical methods [25,26]. The results of bibliometric analyses can provide objective views of scientific productions in a given research field and provide effective support for the subjective perceptions of researchers. Many research fields have applied this methodology to assess and predict scientific productivity, development, and future trends [27,28]. Padilla et al. [29], for example, conducted a bibliometric analysis of global nitrate leaching publications and found an overriding interest in recent decades on the theme of soil nitrogen loss in agroecosystems. Moreover, many different bibliometric indicators can be used to evaluate the literature characteristics as well as thematic influences, such as the number of documents, number of citations, and rank [30]. A quantitative analysis of bibliometric indicators can reveal the most cited papers, hot issues, and other valuable information in a given research field, which can help scientists to quickly develop an understanding of the research situation in their field [31,32]. Therefore, an analysis of the performance of themes with bibliometric indicators is of vital importance in understanding the global state of IHS.

Based on the techniques of text mining and bibliometric analysis, this study analyzed the evolving history and future trends of IHS research from a global perspective by using the Web of Science (WoS) database. The aims of this study were to (1) quantitatively summarize the characteristics of yearly publication and explore the development nature of IHS; (2) mine and assess IHS themes in different subperiods and reveal how these themes enhance our understanding of hydrological processes related to IHS, and (3) discuss the upcoming research trends and provide a certain guide for future IHS research.

2. Materials and Methods

We extracted various performance indicators from 392 downloaded IHS recodes to conduct a bibliometric analysis that included frequency calculation, co-word analysis, and science mapping. Science mapping is a spatial representation of how disciplines, domains, specialities, and individual research units relate to one another [35]. Frequency calculations are widely used in bibliometric analyses to investigate the influence of research units, such as publications, authors, and countries/regions, by counting the number of documents and total citations (TC) or global citation scores (GCS), local citation scores (LCS), and themes based on counting words of interest [28]. LCS is the number of citations of one document by other documents in the collected dataset, while TC or GCS is the number of citations of one document by the documents in the WoS core collection [23]. More information on the relevant bibliometric indicators in this study is shown in

Table 1 . In this study, the primary bibliometric index of assessing the influence of research activities was LCS, followed by the number of publications, then others.

The main concepts of a specific field could be discovered using co-word analysis. Moreover, co-word analysis is a powerful method to discover and describe the interactions between different research topics [38]. The methodological foundation of co-word analysis is the idea that the co-occurrence of keywords describes the contents of the documents in a file [42]. An equivalence index and clustering were applied to measure the keyword co-occurrence matrix [42,43] ( Tabl2

). In this study, co-word analysis, including the co-occurrence and clustering of keywords, was used to detect themes through each subperiod. It was possible that there was more than one theme in one paper.

Then, two indicators, Callon’s centrality and Callon’s density, can be used to measure the performance of the themes in each subperiod [42] (

Table 1 ). Callon’s centrality is an indicator of the importance of a theme across a full set of publications, while Callon’s density is an indicator of the theme’s development. Then, a strategic diagram was applied to show the themes with different Callon’s centrality ( x -axis) and density ( y -axis) [37] (

3). During 2015 and 2019 (subperiod III), there was a significant upward trend in the number of cumulative publications, increasing from 225 in 2014 to 392 in 2019. The mean output of 33.4 in this subperiod was much higher than that in the other two subperiods. No important literature was published in subperiod III, but this may be due to delayed citing. The number of cumulative documents grew from 1986, following a quadratic function in the form of y = 157,000 − 158x + 0.04x² (adjusted R² = 0.988, p < 0.001), indicating that there is a generally increasing trend in IHS publications.

Figure 1 ). The themes with high density and high centrality were named motor themes (important and developed topics that show strong links with other themes in other quadrants), which were located in quadrant I; those with low density and high centrality were developed and isolated themes, which were located in quadrant II; those with low density and low centrality were emerging or declining themes, which were located in quadrant III; and those with low density and high centrality were basic and transversal themes (focusing on general issues that were transversal to the different research areas of a domain), which were located in quadrant IV. Then, a strategic diagram was constructed to assess the role of each theme in each subperiod qualitatively. Severe bibliometric indicators were used to quantify the impact of themes in different subperiods. Finally, a Sankey diagram was applied to present how different themes were connected and developed over previous decades. Thematic analysis and visualization, supplemented by the interpretation of important documents, help us clearly capture the details of IHS development and enhance our understanding of hydrological processes in streamflow generation.

All analyses were performed using R 4.0.2 software (R Core Team, 2020). The bibliometric and thematic analysis was mainly carried out using an open-source R package named the bibliometrix package built by Aria and Cuccurullo [44].

3. Results

To further explore the topics of IHS, the co-word matrix and clustering methods were applied to determine the themes in different subperiods. The themes of different subperiods using strategic diagrams are presented in Figur2.

a–c. Moreover, their evolution paths from subperiod I to subperiod III are clearly shown using a Sankey diagram in Figure 4 d.

Table 5 shows the top 10-most frequent keywords of each theme. The bibliometric indicators in

Table 6 were used to measure the influence of each theme in different subperiods.

IHS publications in subperiod I were defined by six themes (

Figure 4 a). ‘ Capillary fringe’, ‘forest’, and ‘acidification’ (frequency: 52, Tabl3

) were the motor themes of IHS, indicating that publications containing these keywords were more important in the research domain. Moreover, ‘catchment’ that linked with ‘runoff-generation’ and ‘deuterium’ showed a motor theme, which showed high centrality and high density and thus represented the processes and mechanisms of streamflow generation at the catchment scale. ‘Storm’/‘runoff’/‘groundwater’ (frequency: 132) appeared as a general theme with the highest centrality, the greatest number of publications (38), and the highest LCS (62, Table 6 ). ‘Stable-isotope’/‘recharge’/‘tropical’ (publications: 5; LCS: 0) in quadrant II were isolated themes regarding tropical climates. One specialized theme, ‘oxygen-stable-isotope’/‘nitrate’/‘temperate’ (publications: 8; LCS: 1), had low density and the lowest centrality. These themes had little influence on IHS research, with few publications and low LCS. However, the final theme for subperiod I linked with ‘model’/‘tracer’/‘soil’ attained certain attention, with 15 articles and 17 LCSs.

In subperiod II (

Figure 4 b), eight principal topics emerged. The theme ‘runoff’, ‘soil’, and ‘storm’ (frequency: 298, Table 5 ) developed mainly from ‘model’/‘tracer’/‘soil’ and ‘storm’/‘runoff’/‘groundwater’ in subperiod I and shifted to quadrant I as a motor theme with a significant increase in the number of publications (117) and LCS (264) ( Table 6 ). ‘Stable-isotope’, which was inherited from the previous ‘stable-isotope’ and ‘oxygen-stable-isotope’, moved to quadrant IV as a basic theme, with the largest centrality and lowest density and was especially linked with ‘groundwater’ and ‘precipitation’. This theme had the second-largest LCS (401), indicating that it was cited widely by other themes. ‘Catchment’ and ‘runoff-generation’ had changed to a basic theme in quadrant IV with the largest publication (156) and LCS (425), which came from five themes in subperiod I. A transversal theme in this subperiod includes ‘chemistry’, ‘mountain’, and ‘nitrogen’ (publication: 78; LCS: 123). A new theme named ‘hillslope’/‘streamflow’/‘preferential flow’ was located in quadrant III as an emerging theme with a certain number of publications (53) and LCS (41), which consisted of few terms in subperiod I. Some studies focused on ‘variation’/‘end-member mixing analysis’/‘China’ (publications: 22) with high density scattered in quadrant II as a marginal and new theme together with ‘natural-tracer’ (publications: 5). ‘Model’ shifted to quadrant II with relatively high centrality and approached quadrant I, and its focus changed from ‘soil’ to ‘snowmelt’ in IHS. In general, many new contents sprang up in this subperiod.

Subperiod III was characterized by seven clusters, all of which developed from the themes of subperiod II (

3c)). Some keywords, such as ‘variation’ and ‘spatial’, merely appeared in subperiods II and III, and the sum frequency of the two terms increased from subperiod II to subperiod III.

Figure 4 c,d). Three themes, ‘stable-isotope’/‘precipitation’/‘river’, ‘catchment’/‘runoff’/‘runoff-generation’, and ‘groundwater’/‘flow’/‘recharge’, consolidated their roles as basic themes with high centrality and low density. The number of publications and LCS of these themes were much greater than those of other themes in this subperiod (

Table 6 ). More publications (68) focused on ‘variation’ in regions and/or countries other than China, with increasing centrality compared with subperiod II, and linked to ‘chemistry’ and ‘distribution’. In addition, a new, similar theme was ‘source’/‘uncertainty’/‘mass balance’ with publications (45) and LCS (45), which combined the partial information of six themes from subperiod II. ‘Transport’/‘event’/‘sediment’ in quadrant II, almost from ‘catchment’/‘tracer’/‘runoff-generation’ in the previous subperiod, appeared as peripheral themes with 25 publications and 4 LCSs. The final theme was ‘nitrogen’/‘three component’/‘shallow’ (publications: 18; LCS: 1), which could be considered a marginal theme mainly from the theme ‘chemistry’/‘mountain’/‘nitrogen’ in subperiod II.

4. Conclusions

IHS is a novel research field that has shown a great shift in separating runoff into pre-event and event water throughout its production lifetime. Although IHS has developed over several decades, many questions and challenges remain in the generation of runoff. This study used text mining and bibliometric analysis to present a quantitative and systematic review of IHS documents published from 1986 to 2019, thus tracing the evolution of the field. The number of IHS documents has increased rapidly in recent years, but the total sum of documents remains relatively small. The most influential documents in the field were published in subperiod I and subperiod II. Significant improvements can still be made. The topic analysis indicated that the contributions and mechanisms of precipitation, groundwater, and soil water to runoff were the core and base topics in the IHS field. Most IHS studies were carried out in watersheds covered with forest, glaciers, and snow. How to clearly characterize the process of runoff generation, the effects of spatial-temporal heterogeneity of precipitation and groundwater, the assessment of the uncertainty of the IHS result, the contribution of soil water in arid and semi-arid areas, and the verification and application of the new IHS methods are future trends in IHS research.

The study had some limitations caused by the method. First, documents published before 1986 were not included in our dataset. Second, it is possible that some IHS documents are not included in the WoS core database or that we did not retrieve some papers. A possible solution to overcome these limitations is the use of more sources and the retrieval of a larger number of IHS search terms. In general, however, this study has provided novel insights into the research characteristics and evolution of topics in the field of IHS. Moreover, the methods described in this study have great value for all research fields and thus have broad applicability.