Assuming that the volume of the water body remains constant over time, the processes described above (inflow, settling, outflow) can be combined into the following mass balance equation for a well-mixed water body:
8:3.2.1
where is the volume of the system (m HO), is the concentration of nutrient in the system (kg/m HO), is the length of the time step (1 day), is the amount of nutrient entering the water body during the day (kg/day), is the rate of water flow exiting the water body (m HO/day), is the apparent settling velocity (m/day), and is the area of the sediment-water interface (m).