water Review Global Isotopic Hydrograph Separation Research History and Trends: A Text Mining and Bibliometric Analysis Study Yunlong Yu 1,2,3 , Zhao Jin 2,4, * and Junping Qiu 1,3, *   Citation: Yu, Y.; Jin, Z.; Qiu, J. Global Isotopic Hydrograph Separation Research History and Trends: A Text Mining and Bibliometric Analysis Study. Water 2021, 13, 2529. https:// doi.org/10.3390/w13182529 Academic Editor: Maurizio Barbieri Received: 11 August 2021 Accepted: 13 September 2021 Published: 15 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Academy of Data Science and Informetrics, Hangzhou Dianzi University, Hangzhou 310018, China; yuyl@hdu.edu.cn 2 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China 3 School of Management, Hangzhou Dianzi University, Hangzhou 310018, China 4 CAS Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi’an 710061, China * Correspondence: jinzhao@ieecas.cn (Z.J.); jpqiu@whu.edu.cn (J.Q.); Tel.: +86-134-6866-9232 (Z.J.); +86-199-6747-3138 (J.Q.) Abstract: Scientific research into isotope hydrograph separation (IHS) has rapidly increased in recent years. However, there is a lack of systematic and quantitative research to explore how this field has evolved over time. In this study, the methods of text mining and bibliometric analysis were combined to address this shortcoming. The results showed that there were clear periodical characteristics in IHS studies between 1986 and 2019. High-frequency words, e.g., catchment, stable isotope, runoff, groundwater, precipitation, runoff generation, and soil, were the basic topics in IHS studies. Forest and glacier/snow were the main landscapes in this research field. ‘Variation’, ‘spatial’, and ‘uncertainty’ are hot issues for future research. Today, studies involving the geographical source, flow path, and transit/residence time of streamflow components have enhanced our understanding of the hydrological processes by using hydrometeorological measurements, water chemistry, and stable isotope approaches. In the future, new methods, such as path analysis and ensemble hydrograph separation, should be verified and used in more regions, especially in remote and mountainous areas. Additionally, the understanding of the role of surface water in streamflow components remains limited and should be deeply studied in the future. Keywords: isotope hydrology; text mining; bibliometric indicator; web of science 1. Introduction In 1969, Hubert et al. [1] published their pioneering paper on hydrograph separation using stable isotope tracers. Since then, isotope hydrograph separation (IHS) has gradually become the principal method for determining the relative contributions of different sources of runoff or streamflow (i.e., event or pre-event water) [2–5]. Buttle [6] summarized the hydrological processes, including groundwater ridging, lateral throughflow, preferential flow, saturation overland flow, kinematic waves, and output from surface storage, which significantly improved the knowledge of the mechanisms of runoff generation. In 2013, Klaus and McDonnell [7] published a qualitative and comprehensive review of IHS and concluded that, despite certain limitations, water stable isotopes were still the most effective technology for understanding runoff generation processes and mechanisms. Moreover, several IHS reviews have focused on streamflow generation and associated processes and methods in special environmental settings, which has accelerated the accumulation of IHS studies [8,9]. Since 2013, there has been a rapid increase in scientific output in the field, which provides an unprecedented opportunity to explore the dynamics of IHS research based on data from a large body of published scientific work. However, to our knowledge, there has not been a quantitative assessment of the publication data in this field, and Water 2021, 13, 2529. https://doi.org/10.3390/w13182529 https://www.mdpi.com/journal/water