> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/bacteria-transport/bacteria-in-surface-runoff.md).

# Bacteria in Surface Runoff

Due to the low mobility of bacteria in soil solution, surface runoff will only partially interact with the bacteria present in the soil solution. The amount of bacteria transported in surface runoff is:

          $$bact\_{lp,surf}=\frac{bact\_{lpsol}*Q\_{surf}}{\rho\_b*depth\_{surf}\*k\_{bact,surf}}$$                                                             4:4.1.1

         $$bact\_{p,surf}=\frac{bact\_{psol}*Q\_{surf}}{\rho\_b*depth\_{surf}\*k\_{bact,surf}}$$                                                               4:4.1.2

where $$bact\_{lp,surf}$$is the amount of less persistent bacteria lost in surface runoff(#cfu/m$$^2$$), $$bact\_{p,surf}$$ is the amount of persistent bacteria lost in surface runoff (#cfu/m$$^2$$), $$bact\_{lpsol}$$ is the amount of less persistent bacteria present in soil solution (#cfu/m$$^2$$), $$bact\_{psol}$$ is the amount of persistent bacteria present in soil solution (#cfu/m$$^2$$), $$Q,*{surf}$$ is the amount of surface runoff on a given day (mm H$$*2$$O), $$\rho\_b$$ is the bulk density of the top 10 mm(Mg/m$$^3$$) (assumed to be equivalent to bulk density of first soil layer), $$depth*{surf}$$ is the depth of the “surface” layer (10 mm), and $$k*{bact,surf}$$ is the bacteria soil partitioning coefficient       (m$$^3$$/Mg). The bacteria soil partitioning coefficient is the ratio of the bacteria concentration in the surface 10 mm soil solution to the concentration of bacteria in surface runoff.

Table 4:4-1: SWAT+ input variables that pertain to bacteria in surface runoff.

| Variable Name | Definition                                                              | Input File |
| ------------- | ----------------------------------------------------------------------- | ---------- |
| SOL\_BD       | $$\rho\_b$$: Bulk density(Mg/m$$^3$$)                                   | .sol       |
| BACTKDQ       | $$k\_{bact,surf}$$: Bacteria soil partitioning coefficient (m$$^3$$/Mg) | .bsn       |


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://swatplus.gitbook.io/io-docs/~/changes/GiVD5ID0WVyRIAumaMy2/bacteria-transport/bacteria-in-surface-runoff.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.