## 3 Results and Discussion
### Optimization Results for the Jinggualiao Watershed (Step by Step)
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Year**} & **TP Concentration (\(\upmu\)g/L)** & **Achievement Rate** \\  & **(\(\mathbf{n}\) = 12 per Year)** & **(\(\mathbf{n}\) = 12 per Year)** & **(\(\mathbf{n}\) = 365 per Year)** \\ \hline
2015 & 17.92 & 15.25 & 75.0\% \\
2016 & 21.08 & 12.05 & 25.0\% \\
2017 & 16.33 & 13.09 & 75.0\% \\ \hline Average & 18.44 & 13.46 & 58.3\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the observed and simulated average TP concentrations and the achievement rate from 2015 to 2017.  
can set a unit of a bioretention cell and denote how many of these facilities are placed within the watershed. The model will generate runoff discharge and water quality after LID placement. Therefore, an initial area of bioretention could be set, and a trial process could be used to determine the number of facilities needed to reach the management goal. Figure 6 shows the trial process, in which the TP concentration was simulated under different areas of bioretention cells denoted along the Jingqualiao stream. With every test, the achievement rate was calculated. According to the figure, the peak concentration was obviously reduced with increasing bioretention cell area, implying the effectiveness of this control facility on nonpoint source pollution. For example, in Table 3, 300 m\({}^{2}\) of bioretention cells increases the achievement rate from 79.93% (current) to 83.95%, and 350 m\({}^{2}\) and 600 m\({}^{2}\) will reach the goals of 85% and 90%, respectively. The simulated water flow and water quality can be used to calculate the loads. The current average TP load per year is 534.74 kg. The reduction load needed is 16.52 kg/year to match the 85% goal, and 26.38 kg/year to match the 90% goal. In this step, the reduction loads and the required area of the bioretention cells to reduce the loads were obtained. Figure 6: TP simulation results using different areas of bioretention cells. The upper panel shows 350 m\({}^{2}\) of bioretention cells, and the bottom panel shows 600 m\({}^{2}\) of bioretention cells. Step 4: Allocate loads  
The reduction loads should be allocated to all pollution sources, and they should be allocated to point sources prior to nonpoint sources due to the cost issue. However, in this case, the point sources had already been well controlled. As a result, the required reduction loads were allotted to nonpoint sources. In the Jinggualiao watershed, tea farms are the dominant nonpoint sources of pollution, and the required reduction loads were allocated to these tea farms. Step 5: Determine the unit reduction loads for tea farms  
The tea farms are mostly located downstream in the Jinggualiao watershed, where the total area of the tea farms is 61.18 ha. If the total reduction loads of TP are allocated to these tea farms, the unit reduction load is 270 g/ha-year for the 85% goal and 326 g/ha-year for the 90% goal. These unit reduction loads do not consider the specific properties of each tea farm and regard their impact on stream water quality as equal. This value is critical and serves as the management goal of the tea farms for the subsequent control measure design. The unit TP reduction load for the tea farms are summarized in Table 4.  
Step 6: Apply the verified tea farm model  
When the unit TP reduction loads were obtained, the control measures for each tea farm were designed based on this requirement. In this study, bioretention cells were selected as the control measure, and other BMPs were not considered. A verified site-scale model tool, the SWMM model with the LID module, was used. Here, one tea farm was presented to show its application. The demonstrated tea farm, which has three drainage systems and three outflows, is shown in Figure 7. The area of this tea farm is 1.987 ha, and the slope ranges from 6 to 10%. According to the unit reduction loads, this tea farm must reduce 534 g of TP per year to reach the 85% goal and reduce 648 g of TP per year to achieve the 90% goal.