## 1 to 1000 Policy: Controlling Phosphorous Pollution from Tea Farms with Bioretention Cells  
[PERSON] 1, [PERSON] 2,* and [PERSON] 3  
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Footnote 27: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))  
Footnote 28: [[https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci)]([https://www.mdpi.com/journals/applsci](https://www.mdpi.com/journals/applsci))the carry capacity of a receiving waterbody as the foundation of watershed management. The carry capacity of the environment refers to the natural ability to self-purify pollution without damaging its natural function. Some studies have used watershed model tools to link the relationship between pollution sources, including point and nonpoint sources in watersheds, and water quality [18; 19; 20; 21]. The Total Maximum Daily Loads (TMDL) policy in the U.S. is also based on the carry capacity of a receiving waterbody and allocates the loads to different pollution sources [22]. While the carry capacity is determined as a top-down policy, another gap is found in the design of onsite control facilities. Without a quantitative goal for each site, building effective onsite BMPs is difficult. Thus, the question is, how can a watershed-scale goal be implemented into a site-scale design? This is a very practical question. When a structural BMP aims to reduce nonpoint source pollution, the level of treatment required to match the watershed management goal must be decided. This is referred to as the optimal design for BMPs in this study. The optimal design is based on the requirement of the watershed, not the site itself. Therefore, a top-down linkage is important, and all of the control measures are designed according to the water quality goal. A process for determining an optimal onsite BMP facility is demonstrated in this study. Data from the watershed, receiving waterbody and pollution sources as well as policy-making considerations are logically integrated in this process. The process demonstrated in this study can be applied to different water quality goals, pollution sources or control measures. The Jinggualiao stream is one of the inflow streams of the Feitsui Reservoir, Taipei, Taiwan, and it was used as a case study. The reservoir is the main water supply for Taipei and New Taipei city. To maintain the water quality of the reservoir, the inflow stream quality was considered. To avoid possible eutrophication and algae blooming, the excess nutrients in these inflow streams need to be controlled. In this reservoir watershed, nonpoint source pollution dominates the water quality because the point sources are mostly collected and treated in domestic sewage treatment plants. According to previous investigations, tea farms are regarded as the main nonpoint source pollution generators, and phosphorous (P) is the target pollutant [23; 24]. The government implemented a distributed policy rather than a concentrated treatment policy to control nonpoint source pollution and built small-scale onsite treatment facilities at each tea farm. Bioretention cells were chosen within this watershed as onsite treatment BMPs.