## 3 Results
### Sobol Results  
A total of 6200 samples of 100 groups of parameters were generated by Sobol's sequence sampling method (CN2 parameter sampling results are shown in Figure 6) [61]. Figures 7 and 8 depict the indices of first-order sensitivity (\(S\)) and total sensitivity (\(S_{T}\)), respectively. Figure 7 shows that the first-order sensitivity (\(S\)) of parameter SMTMP was significantly greater than that of other parameters, up to 0.3539, indicating that variation in this parameter significantly impacted the output results of the SWAT model. The first-order sensitivity (\(S\)) levels of CANMX, SLSUBBSN, EPCO, OV_N, CN2, and ESCO were relatively tiny, only 0.0093, 0.0032, 0, 0, \(-\)0.0003, and \(-\)0.0041, respectively, which proved that these parameters were insensitive. The first-order sensitivity (\(S\)) of parameters CN2 and ESCO  
Figure 5: Standard deviation distributions (\(\sigma\)) of each parameter of the Morris screening method.  
was negative, which was mainly due to the simplified calculation formula adopted in the calculation of first-order sensitivity (\(S\)), which was different from the real variance [61]. Except for the above parameters, the first-order sensitivity (\(S\)) of the other parameters showed a slight difference, meaning that they may have been relatively sensitive parameters.  
Figure 6: Schematic diagram of parameter sampling of Sobol sequence method, taking CN2 as an example.  
Figure 7: First-order sensitivity (\(S\)) of each parameter of Sobol analysis.  
Figure 8 shows that the total sensitivity (\(S_{T}\)) of parameter SMTMP was as high as 0.7117, indicating that the combination of SMTMP and other parameters noticeably impacted the SWAT model results. In addition to parameter SMTMP, the total sensitivity (\(S_{T}\)) levels of parameters SFTMP, SOL_Z, SOL_AWC, SOL_BD, and SOL_K were large, 0.3327, 0.2913, 0.2865, 0.2731 and 0.2679, respectively. The results show that the combination of these and other parameters influenced the results of the SWAT model. However, the total sensitivity (\(S_{T}\)) levels of parameters HRU_SLP, CANMX, ESCO, SLSUBBSN, CN2, EPCO, and OV_N were very low, all less than 0.1, suggesting that the combination of these and other parameters hardly affected the results of the SWAT model.  
According to the total sensitivity (\(S_{T}\)) and first-order sensitivity (\(S\)), the interaction effect (\(IE\)) was calculated as shown in Table 2. Among all the parameters, SMTMP and SFTMP had apparent interaction effects, suggesting that the interaction between these two and other parameters except themselves noticeably impacted the model results. The interaction effect (\(IE\)) of parameters CANMX, CN2, EPCO, OV_N, and SLSUBBSN was minimal, almost close to 0, which suggests that the combination of these and other non-self-parameters had negligible influence on the SWAT model results.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & \(IE\) & **Parameter** & \(IE\) & **Parameter** & \(IE\) \\ \hline SMTMP & 0.3578 & REVAPMN & 0.1753 & GW\_REVAP & 0.1674 \\ SFTMP & 0.2814 & ALPHA\_BF & 0.1753 & RCHRG\_DP & 0.1661 \\ SOL\_BD & 0.2226 & CH\_N2 & 0.1753 & GWQMN & 0.1636 \\ SOL\_Z & 0.2095 & SURLAG & 0.1753 & HRU\_SLP & 0.0138 \\ TIMP & 0.1977 & TLAPS & 0.1753 & ESCO & 0.0125 \\ SOL\_AWC & 0.189 & CH\_N1 & 0.1753 & CANMX & 0.001 \\ SMFMX & 0.1768 & GW\_DELAY & 0.1752 & CN2 & 0.0004 \\ SOL\_K & 0.1764 & ALPHA\_BNK & 0.1739 & EPCO & 0 \\ CH\_K1 & 0.1754 & CH\_K2 & 0.1735 & OV\_N & 0 \\ SOL\_ALB & 0.1753 & SMFMN & 0.1704 & SLSUBBSN & \(-\)0.0019 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Calculation results of the interaction effect.  
Figure 8: Total sensitivity (\(S_{T}\)) of each parameter of Sobol analysis.