## 3 Results and Discussion
### Optimization Results for the Jinggualiao Watershed (Step by Step)
Step 7: Determine the optimal size of a controlling facility  
This example tea farm has three subwatersheds; thus, the runoff from the three subwatersheds should be calculated separately. This step requires time to determine the number of bioretention cells needed to match the required TP loads. In the model, the size of the bioretention unit is set, including its vertical properties, and the process is iterative. Taking one of the subwatersheds as an example, the area of the S1 subwatershed is 0.890 ha, and the required TP reduction loads are 240 and 290 g/y for the 85% and 90% \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Achievement Rate** & \begin{tabular}{c} **Average TP Loads** \\ **(kg/Year)** \\ \end{tabular} & \
\begin{tabular}{c} **Required Reduction** \\ **Load (kg/Year)** \\ \end{tabular} &
\begin{tabular}{c} **Unit Reduction Load** \\ **for Tea Farms (g/ha)** \\ \end{tabular} \\ \hline
79.93\% (current) & 534.74 & - & - \\
85\% & 518.22 & 16.52 & 270 \\
90\% & 508.36 & 26.38 & 326 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The unit TP reduction loads of tea farms required to match the target goal: 85% and 90% water quality achievement rates. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Year**} & \multicolumn{8}{c}{**Bioretention Cell Area (m2)**} \\ \cline{2-10}  & **50** & **100** & **200** & **300** & **350** & **375** & **400** & **450** & **600** \\ \hline
2015 & 83.2\% & 84.1\% & 84.6\% & 85.44\% & **85.2\%** & 86.81\% & 87.09\% & 87.64\% & **90.4\%** \\
2016 & 79.8\% & 80.3\% & 82.2\% & 84.70\% & **87.4\%** & 87.98\% & 8  
goals, respectively. Table 5 shows the simulation results. When the bioretention cell area was set to 3 m\({}^{2}\), the reduction load was 62 g/y, less than the required loads of 240 g/y and 290 g/y. After applying the process and interpolation, areas of 11.8 and 14.3 m\({}^{2}\) were found to achieve the required loads for the S1 subwatershed. Following this process, the optimal area of the bioretention cells for the other two subwatershedswere were obtained (Table 6). The final optimal LID size for this tea farm is 26 m\({}^{2}\) to achieve the 85% goal and 32 m\({}^{2}\) to achieve the 90% goal. This is very useful information for control policy design, and the required TP reduction and suggested LID size were determined. In addition to this tea farm case study, the optimization process was applied to 30 other tea farms in this watershed. Reduction goals and individual models were established for \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Tea Farm Case Study** & **Unit** & **S1** & **S2** & **S3** & **Total** \\ \hline Area & ha & 0.890 & 0.580 & 0.517 & 1.987 \\ \hline Original TP export & g/y & 2118 & 1381 & 1242 & 4741 \\ \hline Required TP reduction loads for 85\% goal & g & 240 & 157 & 140 & 537 \\ Required TP reduction loads for 90\% goal & g & 290 & 189 & 169 & 648 \\ \hline The optimal LID size for 85\% goal & m\({}^{2}\) & 11.8 & 7.7 & 6.7 & 26.2 \\ The optimal LID size for 90\% goal & m\({}^{2}\) & 14.3 & 9.4 & 8.2 & 31.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The optimal LID size suggested for the tea farm case study. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Bioretention Cell Area (m\({}^{2}\))** & **TP Loads with Bioretention Cells (g/y)** & **Reduction Loads (g/y)** & **Reduction Rate (\%)** \\ \hline
3 & 2055 & 62 & 2.9\% \\
6 & 1992 & 126 & 5.9\% \\
9 & 1931 & 186 & 8.8\% \\
12 & 1871 & 247 & 11.6\% \\
18 & 1756 & 362 & 17.1\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Determination process for the S1 subwatershed in the case tea farm. Figure 7: Example tea farm within the Jinggualiao watershed. This tea farm has three subwatersheds and 3 outfalls. each tea farm. Due to the different sizes and geographical characteristics, the optimal sizes of bioretention cells were different. The average area of the bioretention cells was 13 m\({}^{2}\) for each hectare of tea farm.