A question about calculating surface mass transfer coefficient s

According to EFDC+ theory of version 10.2, in order to calculate surface mass transfer coefficient s, I need to know the magnitude of overlying water oxygen concentration DO(0). My model were divided into six layers in the vertical direction. So, Could you tell me whether the magnitude of overlying water oxygen concentration is equal to the dissolved oxygen at the bottom layer or average value of dissolved oxygen in the whole water column? Besides, are there any other ways to calculate surface mass transfer coefficient s?