## 3 Results
### Thematic Distribution and Evolution  
To further explore the topics of IHS, the co-word matrix and clustering methods were applied to determine the themes in different subperiods. The themes of different subperiods using strategic diagrams are presented in Figure 4a-c. Moreover, their evolution paths from \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Paper** & **Digital Object Identifier (DOI)** & **LCS** & **GCS** & **LCS Per Year** & **GCS Per Year** \\ \hline [PERSON], 1994, Prog Phys Geog (IF: 3.580) [6] & 10.1177/03091339401800102 & 118 & 297 & 4.4 & 11.0 \\ [PERSON], 2013, J Hydrod (IF: 5.722) [7] & 10.1016/j.hydrod.2013.09.006 & 92 & 221 & 11.5 & 27.6 \\ [PERSON], 1986, Water Resour (IF: 5.240) [45] & 10.1029/WR022i010p01444 & 88 & 252 & 2.5 & 7.2 \\ [PERSON], 1991, J Hydrol [46] & 10.1016/0022-1694(91)90181-G & 60 & 139 & 2.0 & 4.6 \\ [PERSON], 2002, Water Resour & 10.1029/2002 WR001510 & 51 & 80 & 2.7 & 4.2 \\ [PERSON], 1999, J Hydrod [3] & 10.1016/S0022-1694(98)00247-9 & 46 & 187 & 2.1 & 8.5 \\ [PERSON], 2001, Water Resour & 10.1029/2000 WR900341 & 44 & 129 & 2.2 & 6.5 \\ [PERSON], 1993, J Hydrol [48] & 10.1016/0022-1694(93)90005-T & 43 & 93 & 1.5 & 3.3 \\ [PERSON], 2012, J Hydrol [49] & 10.1016/j.hydrod.2012.02.029 & 41 & 78 & 4.6 & 8.7 \\ [PERSON], 1994, Water Resour & 10.1029/93 WR03246 & 39 & 110 & 1.4 & 4.1 \\ [PERSON], 1997, J Hydrol [51] & 10.1016/S0022-1694(97)00030-9 & 38 & 85 & 1.6 & 3.5 \\ [PERSON], 2001, J Hydrol [52] & 10.1016/S0022-1694(00)00391-7 & 38 & 136 & 1.9 & 6.8 \\ [PERSON], 2002, Hydrol Process & 10.1002/hyp.1232 & 38 & 69 & 2.0 & 3.6 \\ (IF: 3.565) [53] & 10.1002/hyp.312 & 35 & 94 & 1.8 & 4.9 \\ [PERSON], 2002, Hydrol Process & 10.1002/hyp.7326 & 35 & 56 & 2.9 & 4.7 \\ [PERSON], 2009, J Hydrol Process & 10.1016/j.hydrod.2008.02.017 & 33 & 62 & 2.5 & 4.8 \\ [PERSON], 1991, Water Resour Res [4] & 10.1029/91 WR02025 & 31 & 126 & 1.0 & 4.2 \\ [PERSON], 1994, J Hydrol & 10.1016/0022-1694(94)90004-3 & 31 & 137 & 1.1 & 5.1 \\ [PERSON], 2002, J Hydrol [58] & 10.1016/S0022-1694(01)00596-0 & 28 & 66 & 1.5 & 3.5 \\ [PERSON], 2003, Water Resour Res [59] & 10.1029/2003 WR002331 & 28 & 146 & 1.6 & 8.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Twenty most influential papers based on LCS. \begin{table}
\begin{tabular}{r c c c} \hline
**Name** & **Frequency** & **Top 10 Most Frequent Keywords in Each theme; the Number Following each Word is the Frequency of the Word** & **Subperiod** \\ \hline Sports; Ranoff; Groundwater & 132 & Storm 19; Ranoff 19; Groundwater 17; Flow 14; Stream 8; Chesmity 5; Transport +; Discharge 3; Environmental- Isotope 3; Environmental- Isotope 3; & 1991–1999 \\ Cathedral; Ranoff-Generation; Basin & 100 & Catchment 16; Ranoff-Generation 13; Basin & Concurrent 6; Subsurface-free 6; Waste-Chemistry 6; Hunidid-Headwater 6; & 1991–1999 \\ Computer 4; Hamilton & 8; Low-2; Zealand 3 & 1991–1999 \\ Capillary-Fring; Forest; Acidification & 52 & Capillary-Fring 6; Forest 5; Acidification 5; Snownet 6; Prevention 8; Prevention 8; Prevention 9; 1991–1999 \\ Molecular; Traover; Soil & 39 & Model 10; Tractor 6; Soil 6; Stream Streamaway 2; Metropolitan 2; Snownet 1; Three-Component 1; Two-Component 1; Two-Component 1; Two-Component 1; Two-Component 1; Two-Component 1; Two-Component 1; Two-Component 1; Double-Member 1; Multi-Analysis 1  
IHS publications in subperiod I were defined by six themes (Figure 4a). 'Capillary fringe', 'forest', and 'acidification' (frequency: 52, Table 5) were the motor themes of IHS, indicating that publications containing these keywords were more important in the research domain. Moreover, 'catchment' that linked with 'runoff-generation' and 'deuterium' showed a motor theme, which showed high centrality and high density and thus represented the processes and mechanisms of streamflow generation at the catchment scale. 'Storm'/'runoff'/'groundwater' (frequency: 132) appeared as a general theme with the highest centrality, the greatest number of publications (38), and the highest LCS (62, Table 6).