## 2 Materials and Methods
### Optimization Process  
The optimization process comprises two stages. The first stage is for the watershed scale. During this stage, the management goal is established and the carry capacity is determined; the management goal is also regarded as the required reduction load. The second stage is for the site scale. During this stage, the optimal LID facility size for each tea farm site is determined based on the unit reduction loads obtained from the first step. In the two-stage process, a total of seven steps are included. Steps 1 to 5 are for Stage 1 and Steps 6 to 7 are for Stage 2. In the two stages, two verified modeling tools are needed to provide quantitative data. The flow chart is shown in Figure 3. This flow is a universal process, and the modeling tools, the SWMM model and its LID module, are not limited to the use of this study. The core concept of the process is the connection between watershed-scale goals and site-scale design, and the idea of optimization is to accomplish the goal. The components of the process are explained below. Figure 2: Total phosphorous (TP) concentration of the Jinggualiao stream from 2015 to 2018. Step 1: Set the management goal  
For any policy making, the first step is to decide the management goal. All subsequent policy design falls under this goal. In this step, as much background data as possible are needed to understand the state of the water, especially the water quality trend. Considering the current water quality, future changes in the environment and discussions with stakeholders, the management goal can be formed. A commonly used management goal for water quality is the legislated water quality standard, which is usually based on a concentration. A water quality concentration standard or the percentage of time that the water quality meets the standard (or achievement rate) can be used as the management goal. In addition, pollution loads, such as the Total Maximum Daily Loads (TMDL) policy in the U.S., are also good indicators. The choice of indicators can be determined by data sufficiency. If the number of observed water quality is few, the average concentration can be used. If long-term observed water quality data are available, then the achievement rate can be the indicator. If not only water quality but also water flow data are available, then the total reduction loads based on natural carry capacity would be a management goal. Step 2: Apply the verified watershed model  
A verified tool for demonstrating the complete water quality and discharge of a watershed is very important. Monitoring data are usually limited to specific locations and times. A modeling tool used after the verification process can serve as a useful tool to show  
Figure 3: The two-stage optimization process for watershed and tea farm management. more complete data about the water state. A quantitative tool to link the pollution sources, including the point and nonpoint sources, and the resulting water quality data are also necessary. Many watershed models are available to choose from, and a recommended rule for choosing a suitable one is that the model has been applied successfully and verified in similar regions and that the required data and model parameters are available for the specific case. Step 3: Find the required reduction loads  
This step uses the verified watershed model to calculate the required reduction loads under the management goal. With the verified modeling tool, it can simulate the water quality and discharge, and the total pollution loads can be obtained. Therefore, the total required pollutant loads under either the water quality standard concentration or the carry capacity can be calculated. The reduction loads required to match the management goal are then determined. Step 4: Allocate loads  
The required reduction loads of the watershed is then allocated to all pollution sources as their quantitative goal for control measures. Pollution sources include point sources and nonpoint sources.