## 4 Discussion
### Advances in IHS during Subperiod II  
A significant development of IHS occurred in subperiod II. 'Catchment'/'tracer'/'runoff-generation' was used to characterize the 'flowpath' (frequency: 31, Table 5) of runoff formation and'residence time' (19) of end-members at catchment scales. These results demonstrated that significant advances occurred in the flow path of end members of different geographic sources to runoff [68]. Moreover, the residence time could provide more information on the age of soil water/groundwater, which helped further differentiate soil water/groundwater from different soil profile layers/aquifers [69]. Inaddition, 'preferential flow' (11) and'subsurface flow' (11), in the new theme of 'hills-lope'/'streamflow/'preferential flow', focused on the flow path ('macropore') of water movement from the 'unsaturated' hillslope to streamflow [70]. 'Runoff' developed into the motor theme from'storm' and'model' in subperiod I (Figure 4b,d). In this theme, the frequency of'soil' (51) significantly increased relative to subperiod I, indicating that researchers paid more attention to the role of soil water on IHS [71]. Dissolved organic carbon (DOC) measurements were used to distinguish the dominating runoff processes (i.e., lateral subsurface flow), and DOC-rich water was considered from the upper soil horizons [72]. Moreover, 'forest' (37) was likely the main land cover in the theme [73; 74]. Generally, these studies regarding the flow path and transit/residence time of streamflow components characterized the hydrological processes of the IHS domain in more detail.  
Approximately 84% of publications in subperiod II were related to'stable-isotope'/ 'groundwater'/'precipitation', indicating that these words were general IHS terms. In addition, the frequency of 'oxygen-stable-isotope' (41) was larger than that of 'deuterium' (29), suggesting that researchers favored the oxygen stable isotope in IHS studies. Overall, compared with subperiod I, stable water isotopes played a more important role in hydrological segmentation in subperiod II. Hydrogen (H) and oxygen (O) are part of the water molecule (H\({}_{2}\)O), which can directly capture the movements or changes in water [7; 75]. Consequently, compared with solute traces, stable water isotopes are more ideal tracers for water. Moreover, as the technology upgrades and costs decrease, the percentage of hydrological separation using stable water isotopes increases. However, solute tracers were also important in IHS studies, which could provide extra information to identify the different contributing areas or flow paths in many studies [54; 76].  
'Variation' and 'end-member mixing analysis' as a novel theme emerged in this subperiod. Approximately 16% of publications belong to these themes, suggesting that the isotopic variabilities of end members attracted IHS researchers' attention [76]. Several studies found that the temporal and spatial variability in stable water isotopic components occurred widely in event, pre-event, and runoff water [56; 77; 78; 79]. In addition, there were 111 publications regarding'model' with'snowmelt' and'meltwater', indicating that snow/glacier cover was a classic landscape in IHS research. Therefore, in this subperiod, many studies focused on revealing the spatiotemporal patterns of the meltwater from snowpack, snowmelt, glacier, and frozen soil and their effect on IHS results [80; 81]. In addition, evaluating the effect of land use/cover and climate changes on IHS was accompanied by the above themes [49; 82].