# 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.


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