Carbonaceous Biological Oxygen Demand

The carbonaceous oxygen demand (CBOD) of the water is the amount of oxygen required to decompose the organic material in the water. CBOD is added to the stream with loadings from surface runoff or point sources. Within the stream, two processes are modeled that impact CBOD levels, both of which serve to reduce the carbonaceous biological oxygen demand as the water moves downstream. The change in CBOD within the stream on a given day is calculated:

$\Delta cbod=-(\kappa_1*cbod+ \kappa_3*cbod)*TT$ 7:3.4.1

where $\Delta cbod$ is the change in carbonaceous biological oxygen demand concentration (mg CBOD/L), $\kappa_1$ is the CBOD deoxygenation rate (day$^{-1}$ or hr$^{-1}$), $cbod$ is the carbonaceous biological oxygen demand concentration (mg CBOD/L), $\kappa_3$ is the settling loss rate of CBOD (day$^{-1}$ or hr$^{-1}$), and $TT$ is the flow travel time in the reach segment (day or hr). The calculation of travel time is reviewed in Chapter 7:1.

The user defines the carbonaceous deoxygenation rate at 20C. The CBOD deoxygenation rate is adjusted to the local water temperature using the relationship:

7:3.4.2

where is the CBOD deoxygenation rate (day or hr), is the CBOD deoxygenation rate at 20C (day or hr), and is the average water temperature for the day or hour (C).

The user defines the settling loss rate of CBOD at 20C. The settling loss rate is adjusted to the local water temperature using the relationship:

7:3.4.3

where is the settling loss rate of CBOD (day or hr), is the settling loss rate of CBOD at 20C (day or hr), and is the average water temperature for the day or hour (C).

Table 7:3-4: SWAT+ input variables used in in-stream CBOD calculations.