## 3 Results  
### Publication and Keyword Performance  
The primary information and statistics regarding the analyzed IHS collection are reported in Table 2. The 392 IHS documents published in 87 sources were written by 1138 authors from 453 institutions in 53 countries. The numbers of TC and average citations per document were 12,413 and 31.67, respectively. The annual growth rate was, on average, 11.3%. Figure 2 shows the yearly output of IHS documents published from 1986 to 2019. The output presented obvious periodical characteristics in terms of the mean value of yearly publications. The whole period could be divided into three subperiod based on the significant changes in average publication output. Before 2000 (subperiod I), the literature output grew slowly, with only 50 documents in 13 years and an average annual output of only 3.6. From 2000 to 2014 (subperiod II), the annual volume of documents increased, with an average production of 11.7 documents per year. The most influential literatures were published in subperiods I and II (Table 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 \(y=157,\!000-158x+0.04x^{2}\) (adjusted \(R^{2}=0.988\), \(p<0.001\)), indicating that there is a generally increasing trend in IHS publications.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Description** & **Results** \\ \hline Total documents & 392 \\ Timespan & 1986:2019 \\ Annual growth rate & 11.28\% \\ Total citations (TC) & 12,413 \\ Average citations per document & 31.67 \\ Sources & 87 \\ Authors & 1138 \\ Institutions & 453 \\ Countries & 53 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Principal information about the 1986–2019 IHS collection.  
Figure 2: The publication output of IHS between 1986 and 2019. The transverse dashed lines present the mean values of the number of documents per year in each subperiod.  
Two papers published before 1991 were excluded from the textual analysis because there were no keywords in these documents. Table 4 presents the number of publications and keywords per period. The number of keywords used per year was 17.6, 42.1, and 125.8 in subperiods I, II, and III, respectively, showing a noticeable expansion in research topics over time. There were 158 keywords in all of subperiod I, including 'runoff' (19),'storm' (19), 'groundwater' (17), 'catchment' (16), and 'flow' (14), located in the center of the word cloud of'subperiod I (Figure 3a). There was a significant increase to 632 keywords in subperiod II. 'Catchment' (99) rose in rank from the fourth to first position (in the center of the word cloud of subperiod II), and'soil' rose from the thirteenth (6) to third position (51), which was close to the 'catchment' (Figure 3b). There were 629 keywords in subperiod III, including'stable-isotope' (99), 'catchment' (90), 'groundwater' (61), 'runoff' (60), 'precipitation' (59), 'river' (55), and 'basin' (51) (in the center of the word cloud of subperiod III (Figure 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.