## 2 Materials and Methods
### Optimization Process
Load allocation should consider economic effectiveness such that point pollution sources are usually controlled before nonpoint sources. In this study, point sources had already been collected and treated by wastewater treatment plants; therefore, the study focused on nonpoint source pollution from tea farms. Tea farms are the obvious and major nonpoint source in the case study, and other pollution sources were not considered in this study. Step 5: Determine the unit reduction loads for tea farms  
This is the final output in the first stage. In the first stage, the unit reduction loads for tea farms (in this study, tea farms were the target pollution sources) is the total reduction in pollution loads divided by the total tea farm area, and the unit reduction load per tea farm area can be obtained and can serve as the target goal for the management of each tea farm. If more than one pollution source exists, the pollution load allocation needs more discussion, such as on effectiveness and equality. Step 6: Apply the verified tea farm model  
In this step, a verified tea farm model should be prepared. This model aims to be applied to site-scale tea farms, and the watershed-scale model in the first stage is not feasible. The site characteristics can be adjusted in the site-scale model. Because the site-scale model is different from the above watershed-scale model, the site-scale model needs to be tested through calibration and verification processes. When such a site-scale model is constructed, the unit reduction loads obtained in Step 5 can be used as the target loads for a tea farm. Step 7: Determine the optimal size of a controlling facility  
With the target allowable loads for a tea farm and the verified model, the next step is to find the optimal size of a controlling facility, I such as bioretention cells or another LID facility from the SWMM LID modules, required to match the target load in the tea farm case. This is an iterative process. An initial facility size is first input into the model and the loading difference before and after this facility design is checked. If the reduction load does not match the requirement, the size is adjusted until the required reduction load is achieved. The optimal size is based on the target load in this study and does not consider other factors, such as land space, cost and benefit or farmers' will. The optimal size obtained from the procedure can serve as a specific and quantitative guide to know how many controlling facilities are needed. If space or budget is insufficient, the realistic size might be less than the optimal suggestion, and nonstructural Best Management Practices (BMPs) can be introduced to compensate for the required reduction loads. Nonstructural BMPs, such as increasing the frequency of cleaning or reducing the amount of fertilizers,are considered source management for reducing the pollution accumulations on the land surface. Nonstructural BMPs are not discussed in this study, but many official manuals, such as [30], are available. The procedure demonstrates the optimal LID size for site-scale tea farms, and the optimal size is based on the required reduction loads allocated to tea farms. This is similar to the concept of the TMDL policy in the U.S. This procedure can be regarded as a universal procedure for linking the pollution sources within a watershed and the water quality in a receiving waterbody, and it can be used for different pollutants, sources or BMPs. In the procedure, a final step is the subsequent monitoring and feedback. Onsite monitoring data are crucial, and the data provide evidence for evaluating the performance of the policy. The observation data are very good feedback for rechecking the management goal or updating the simulation of the modeling tools.