Carbonaceous Biological Oxygen Demand

Carbonaceous biological oxygen demand (CBOD) defines the amount of oxygen required to decompose the organic matter transported in surface runoff. The SWAT+ loading function for CBOD is based on a relationship given by Thomann and Mueller (1987):

$cbod_{surq}=\frac{2.7*orgC_{surq}}{Q_{surf}*area_{hru}}$ 4:5.2.1

where $cbod_{surq}$ is the CBOD concentration in surface runoff (mg CBOD/L), $orgC_{surq}$ is the organic carbon in surface runoff (kg $orgC$), $Q_{surf}$ is the surface runoff on a given day (mm H$_2$O), and $area_{hru}$ is the area of the HRU (km$^2$).

The amount of organic carbon in surface runoff is calculated:

$orgC_{surq}=1000*\frac{orgC_{surf}}{100}*sed*\varepsilon_{C:sed}$ 4:5.2.2

where $orgC_{surq}$ is the organic carbon in surface runoff (kg $orgC$), $orgC_{surf}$ is the percent organic carbon in the top 10 mm of soil (%), $sed$ is the sediment loading from the HRU (metric tons), and $\varepsilon_{C:sed}$ is the carbon enrichment ratio.