## 4 Discussion  
### Advances in IHS during Subperiod I  
Although the development of IHS during 1991-1999 was slow, with an average of 3.6 publications per year, there were nine papers in the top 20 most influential documents (Figure 2 and Table 3). These papers focused on issues regarding the processes and methods of runoff generation and the role of soil and glacial water on IHS [6; 45; 51; 57]. These results indicated that some advances occurred during subperiod I compared with the previous studies before 1986. Our results showed that'storm'/'runoff'/'groundwater' and 'catchment'/'runoff-generation'/'basin', whose publication, LCS and LCS per paper were located at the top two positions, represented classified IHS research (Table 6). Moreover, 'catchment' (frequency: 16, Table 5) and 'basin' (6) indicated that IHS research was primarily conducted in small watersheds. This phenomenon was likely induced by two causes: (1) the larger the watershed size was, the larger the isotopic spatial-temporal variation in new and old water likely was. Consequently, these potential changes in events and pre-events in large-scale basins were difficult to characterize quantitatively, which would violate the IHS underlying assumptions 2 and 3; (2) If these variations were to be accounted for, a highly involved watershed sampling strategy should be adopted. However, the funds and labor forces were limited. Therefore, to ensure the accuracy of streamflow separation and save costs, many IHS studies have been implemented in small watersheds where changes in isotopic compositions of runoff sources were as small as possible [3; 4; 60]. Moreover, in this subperiod, a very important literature review was published that systematically summarized the processes and mechanisms of the event and pre-event water transfer to streamflow [6]. Several studies were carried out to test these processes in the forest catchment represented by 'capillary fringe'/'forest'/'acidification' [61; 62]. These studies improved the understanding of how the end member water reached a stream in small watersheds. Enhanced recognition, in turn, inspired hydrologists to assess the effect of underlying assumptions on IHS, especially assumptions 2 and 4. [PERSON] et al. (1990) [11] developed the incremental mean and incremental intensity mean techniques to capture the changes in the contributions of new water throughout events and considered that incremental weighting was better than the standard mean weighting technique. Therefore, the effect of time isotopic variation in event water on IHS began to be noticed by researchers using the within-storm incremental weighting approach [63; 64]. 'Model' combined with 'three components' (1) and 'end-member mixing analysis' (1) suggests that research in subperiod I started to use multivariable methods to separate runoff into three or more components [3; 4]. Among these components,'soil' (6) has attracted more attention [57]. [PERSON] et al. [65] considered that runoff sources came from three reservoirs (direct precipitation on saturated areas, a near-stream saturated zone, and subsurface water in upslope areas), and streamflow was composed of precipitation on saturated areas and soil water exifftracted from the near-stream saturated zone. Oddly, the themes regarding'stable-isotope' and 'oxygen-stable-isotope' were the marginal topics, which is likely because several IHS studies combined with 'chemistry' (5), 'water-chemistry' (6), or'streamwater-chemistry' (6) weakened the role of stable water isotopes in the theme analysis. In addition, the conditions for stable water isotope use were not in all events, and the analysis of samples is expensive [45]. However, streamflow separation in many studies utilized deuterium (6) and oxygen stable isotopes as environmental tracers in this subperiod because of their unquestionably conservative nature [64; 66; 67].