## 4 Discussion
### Advances in IHS during Subperiod III  
In subperiod III, the themes of'stable-isotope'/'precipitation'/'river' (frequency: 760, Table 5) and 'catchment'/'runoff'/'runoff-generation' (696) developed from subperiod II. These two themes were the foundation of IHS, and there was a great deal of overlapping documents in these themes. Under 'climate change' (29), increasing attention has been directed toward cryosphere meltwater (alpine and cold regions) where glaciers shrink, snowpack melts, and frozen soil degrades [83; 84]. The glaciers in the Tibetan Plateau are known as Asia's water tower, feeding the largest rivers and sustaining some 2.8 billion people [85]. Therefore, in this subperiod, most IHS documents concentrated on how changes in hydrological processes and runoff sources in the Tibetan Plateau and the surrounding mountains occur with climate warming [86; 87; 88; 89; 90]. A majority of these studies were in broad agreement on increasing cryosphere meltwater to runoff or lakes. In addition,'soil' was grouped with 'catchment', suggesting that soil water became a common topic, and the fourth assumption should be considered in IHS research. 'Groundwater'/'flow'/'recharge' (243) with 'baseflow' (13) was mainly inherited from the themes 'natural-tracer' and 'groundwater' of subperiod II. Except for the traditional IHS topic, which evaluated the contribution and flow path of groundwater to stream, the recharge of groundwater was explored using stable isotope approaches in several studies [91; 92].  
In the 'variation'/'chemistry'/'distribution' relative to subperiod II, there was increasing interest in 'variation', with 68 publications, suggesting that more research focused on the spatiotemporal heterogeneity of runoff sources [5; 16; 17; 93; 94]. Moreover, many studies evaluated the 'uncertainty' of end-member contributions to runoff [87; 95]. This result indicated that there was a strong interest in assessing the uncertainty of end-member contributions to runoff in this subperiod. Several studies have demonstrated that the uncertainty in IHS is mainly induced by the spatiotemporal variability in tracer concentrations [89; 96]. The temporal isotopic variation is mostly attributed to the amount, intensity, and fractionation of water sources, especially event water [97; 98]. Therefore, high-resolution sampling and measurement of events and streamflow water were adopted to characterize these changes in events, which could shed important light on catchment flow pathways, travel times, and short-term hydrological processes [99; 100]. Moreover, the long time series of stable isotopes in precipitation, soil/groundwater, and stream water was used to evaluate seasonal and annual hydrological patterns of catchments [101]. The spatial changes were mainly induced by the characteristics of the watershed, such as topography, soil permeability and moisture, local climate, land use/cover change, landscape characteristics, and catchment size [54; 102; 103]. To account for these variations, combined hydrometric, chemical, and isotopic approaches have been utilized to elucidate streamflow generation mechanisms [104]. Moreover, tracer-aided hydrological modeling and remote sensing are becoming increasingly popular tools as they assist with process understanding and source separation [105; 106]. In general, in subperiod III, quantitative estimates of the variations in different water sources and their flow paths became common issues in IHS studies using multiple technologies, further addressing the underlying assumptions two and three. In addition, two themes, 'nitrogen'/'three component'/'shallow' and 'transport'/'event'/'sediment', were also developed from the themes in subperiod II but had little influence on the field of IHS.