## 3 Results and Discussions
### Derivation of the Flow-Pollutant Loading Equation  
The flow-pollutant loading equation was derived using the flow and water quality data for each point in the Gul-po stream from 2016 to 2020, as shown in Table 3. Regarding the degree of dispersion of the pollutant load according to flow, only the results of BOD and T-N with the distinct characteristics of the flow coefficient are shown in Figure 3. R-squared (\(R^{2}\)) is a statistical measure that represents the proportion of the variance for a dependent variable and independent variable.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Watershed**} & \multirow{2}{*}{**Factor**} & \multicolumn{6}{c}{**Water Quality Factor**} \\ \cline{3-8}  & & **BOD** & **COD** & **TOC** & **SS** & **T-P** & **T-N** \\ \hline \multirow{3}{*}{Upper} & b & 0.7222 & 0.9177 & 0.8494 & 0.8806 & 0.8965 & 1.4379 \\  & \(R^{2}\) & 0.3977 & 0.7676 & 0.6896 & 0.4807 & 0.4078 & 0.7400 \\ \cline{1-1} \cline{3-8}  & b & 1.0919 & 1.0265 & 1.0703 & 1.1268 & 1.0702 & 0.8779 \\ \cline{1-1} \cline{3-8}  & \(R^{2}\) & 0.8390 & 0.9050 & 0.8811 & 0.7798 & 0.8195 & 0.9011 \\ \cline{1-1} \cline{3-8}  & b & 1.6988 & 1.1285 & 1.0922 & 1.3851 & 1.2413 & 0.8569 \\ \cline{1-1} \cline{3-8}  & \(R^{2}\) & 0.5087 & 0.7718 & 0.7516 & 0.5947 & 0.5472 & 0.7606 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Results of the flow–pollutant loading equation of the Gul-po stream.  
Figure 2: Comparison of T-N concentrations in Gul-po stream water and ozone-treated reclaimed water: (**a**) T-N of ozone-treated discharge water (Cheong-cheon stream); (**b**) T-N of the Gul-po stream (Gulpo A1 measure point).  
As shown in Table 3, the b values of BOD, COD, SS, TOC, and T-P in the upper part were lower than 1--they were 0.72, 0.92, 0.85, 0.88, and 0.89, respectively, showing that their concentrations decreased as the flow increased. This indicates that the stream's flow increase rate was higher than the pollutant inflow increase rate during rainfall. In addition, when compared to the flow distribution data, flow discharge significantly increased after April 2019. This indicates that the flow fluctuation characteristics in the upper part, between 2016 and 2020, were directly affected by ozone-treated reclaimed water discharge, which is responsible for the improvement of water quality (b value is lower than 1).  
On the contrary, the b value of T-N (1.44 in Table 3) was greater than 1, showing that its concentration increased as the flow discharge increased. This is because the T-N concentration of ozone-treated reclaimed water is higher than that of the Gul-po stream water, as shown in Figure 2.  
In the middle part, the b values of BOD, COD, TOC, SS, and T-P were greater than 1--they were 1.09, 1.03, 1.07, 1.13, and 1.07, respectively, showing that their concentrations increased as the flow increased. This can be interpreted as the significant influence of nonpoint pollution sources during rainfall. The b value of T-N (0.8779), however, was lower than 1, showing that its concentration decreased as the flow increased. This appears to be because the dependence of the discharge water concentration on the flow decreases, since nitrogen has high fluidity in the atmosphere or soil pores and is thus easily discharged, even during small-scale rainfall [29]. In the lower part, the b values of BOD, COD, TOC, SS, and T-P were greater than 1, showing that their concentrations increased as the flow increased. The b value of T-N (0.8569) was lower than 1, showing that its concentration  
Figure 3: Scatter graph of the flow–pollutant loading equation for the Gul-po stream.  
decreased as the flow increased. In particular, the b values of BOD, SS, and T-P in the lower part significantly increased to 1.6998, 1.3851, and 1.241, respectively, compared to the middle part. This shows that the influence of nonpoint pollution sources was larger as the concentration of the water quality variables rapidly increased alongside the flow discharge.