## 3 Results and Discussions
### Derivation of LDC for Each Point  
To identify the degree of water pollution in the Gul-po stream, the LDCs of BOD and T-P for each point were derived using the daily flow and water quality data over five years (from 2016 to 2020) (see Figure 4). The target water quality used as the benchmark was set based on the target water quality presented in the operation plan for the TMDL program in the Han River Basin, because the Gul-po stream was subjected to the program. Comparative analysis was conducted mainly on the target water quality of phase 1 from 2013 to 2020, and the total overage rate for the target water quality of Phase 2 from 2021 to 2030 was additionally analyzed, as it was set recently (see Table 4). Figure 4: Results of the load duration curve plot from monitoring data at each point. When the target water quality (phase 1) overage rate was analyzed for each flow section in the upper part, BOD exhibited an average rate of less than 50% for all sections, thereby satisfying the phase 1 target water quality. DC and LF, which are low-flow sections, showed higher average rates (21% and 15%, respectively) than HF and MC, which are high-flow sections. T-P exhibited a low average rate of 1.72% for the MC section and an average rate of 0% for the DC and LF sections, thereby satisfying the target water quality. In the upper part of the stream, where the overage rate is high for low-flow sections, the influence of point pollution sources is larger than that of nonpoint pollution sources when the results of the flow-pollutant loading equation are also considered. The increase in the BOD concentration in the upper part, however, can also be explained by algal growth due to the increase in stream residence time, caused by lower flow as compared to other points--especially regarding the reduction in stream flow for the DC and LF sections and the increase in the amount of native organic matter due to the growth of phytoplankton [30]. When the target water quality (phase 1) overage rate was analyzed for each flow section in the middle part, BOD showed an overage rate of 55.56% for HF and thus did not satisfy the target water quality. T-P satisfied the target water quality, as it exhibited a low overage rate of 1.79% for MC and DC. As mentioned in the analysis of water quality characteristics, it appears that BOD exceeded the target water quality for HF in the middle part due to the influence of nonpoint pollution sources caused by the discharge from the urban industrial complexes, road sections, and the inflow of the Sam-jeong stream (a tributary of the Gul-po stream). In the results of the flow-pollutant loading equation, the b value of SS was higher than that of the other variables. This can be interpreted as an increase in SS during rainfall due to nearby pollutants and increases the concentration of BOD and other organic matter [27]. When the target water quality (phase 1) overage rate was analyzed for each flow section in the lower part, BOD was 15.79% for HF and 5.26% for MC, indicating that the overage rate concentrated in high-flow sections showed an overage rate of 0% compared to low-flow sections. In the case of T-P, the overage rate was also concentrated in high-flow sections compared to low-flow sections, and this showed the significant influence of nonpoint pollution sources during rainfall on water pollution in the lower part. This appears to be closely related to land use in the lower part of the Gul-po stream when considering that the flow index values of BOD, SS, and T-P were high in the results of the flow-pollutant loading equation. According to a previous study, the concentrations of organic matter and phosphorus tend to increase as the area of farmland increases [31]. In the case of paddy fields, organic matter and soil are introduced into streams in large quantities during rainfall because of the large quantity of organic fertilizers (e.g., compost) being  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Classification** & **Watershed** &