## 4 Conclusions  
In this study, a process was established to link a watershed management goal and a site-scale control policy. Nonpoint source pollution relies on onsite control methods,  
Figure 8: The relationship between the area of bioretention cells and the water quality achievement rate in the Jinggualiao stream. either structural or nonstructural measures. However, what level of control measures is needed for site-scale management or what size of structural measures should be designed is difficult to determine. From the process presented in this study, the optimal size of bioretention cells for a tea farm can be determined, and the optimal size is based on the watershed management goal. In addition to the team farm presented, the process was applied to more than 30 tea farms in this watershed. A 1 m\({}^{2}\) bioretention cell could reduce TP loads by 55 g/year, and an average of 13 m\({}^{2}\) bioretention cells were suggested for 1 ha tea farm to match the 85% goal. When considering the loss from delivery, which means that pollutants exported from tea farms will not all reach waterbodies, a LID area smaller than 13 m\({}^{2}\)/ha might be used. At least 10 m\({}^{2}\) of bioretention cells for each 1 ha tea farm is suggested. Therefore, a ratio of 1 to 1000 for the area of bioretention cells to the area of a tea farm is recommended as a control rule. Considering the onsite properties of tea farms, the optimal size might be different. The SWMM model with the LID module is a useful tool. Before application to different cases, model calibration and verification are necessary. Once the model is verified, the determination process can be used for different cases and different LID facilities. This process was applied to the Jinggualiao watershed for three years, and a total of 89 m\({}^{2}\) of bioretention cells distributed in 49 tea farms were built. While applying this process to real cases, realistic problems occurred: The land area might not be sufficient, or the landowners might not be able to provide the area for building control facilities. In these cases, nonstructural measures such as reducing fertilizer application are needed. The unit reduction loads were determined for tea farms, and they were 270 g/ha-year for the 85% goal and 326 g/ha-year for the 90% goal. If structural facilities are insufficient, a level of nonstructural measures can also be designed based on unit reduction loads. Therefore, with a quantitative determination process, policy measures can be better assessed and designed. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app12052661/s1](https://www.mdpi.com/article/10.3390/app12052661/s1)]([https://www.mdpi.com/article/10.3390/app12052661/s1](https://www.mdpi.com/article/10.3390/app12052661/s1)), Figure S1: Flow simulation results. The upper part shows flow calibration using data from 2015 to 2016, and the bottom part shows flow verification using data from 2017. Figure S2: TP simulation results. The upper part shows TP calibration using data from 2015 to 2016, and the bottom part shows TP verification using data from 2017. Figure S3: The simulation results at the site-scale, including runoff discharge, TP concentration and TP concentration in the LID outflow. Table S1: The results of SWMM model calibration and verification for the Jinggualiao stream watershed. Table S2: The calibrated model parameters for the watershed-scale SWMM model. Table S3: The calibrated model parameters and dimensions of the bioretention cell used in the model. Conceptualization, C.-F.C. and C.-C.H. ; methodology, C.-F.C. and C.-C.H. ; software, C.-F.C. ; validation, C.-C.H. and H.-F.L. ; resources, H.-F.L. ; writing--original draft preparation, C.-F.C. ; writing--review and editing, C.-F.C. and C.-C.H. ; supervision, C.-C.H. ; project administration, H.-F.L. ; funding acquisition, H.-F.L. All authors have read and agreed to the published version of the manuscript.