Outflow COD in your question is highly affected by temperature, the aqueous medium mainly holds the chemical oxygen more at low temperatures and low humidity..
good question Neji, in addition to the temperature of the out flow, the sensitivity and precision of the results depend on the technique used.... is it open reflux method or instrumental?? and what is the difference in numbers..
Perhaps it is significant that extraction from the wetland area will introduce into the effluent water organic species extracted from the natural soil matrices. These will have their own COD demand. Examples include the soluble humic acids and the soluble fulvic acids.