# Dissolved Oxygen Rainfall is assumed to be saturated with oxygen. To determine the dissolved oxygen concentration of surface runoff, the oxygen uptake by the oxygen demanding substance in runoff is subtracted from the saturation oxygen concentration. $$Ox\_{surf}=Ox\_{sat}-\kappa\_1*cbod\_{surq}*\frac{t\_{ov}}{24}$$ 4:5.3.1 where $$Ox\_{surf}$$ is the dissolved oxygen concentration in surface runoff (mg $$O\_2$$/L), $$Ox\_{sat}$$ is the saturation oxygen concentration (mg $$O\_2$$/L), $$\kappa\_1$$ is the CBOD deoxygenation rate (day$$^{-1}$$), $$cbod\_{surq}$$ is the CBOD concentration in surface runoff (mg CBOD/L), and $$t\_{ov}$$ is the time of concentration for overland flow (hr). For loadings from HRUs, SWAT+ assumes $$\kappa\_1$$ = 1.047 day$$^{-1}$$. --- # Agent Instructions: 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/theoretical-documentation/section-4-erosion/water-quality-parameters/dissolved-oxygen.md?ask= ``` 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.