O2 diffusion rate

This is the molecular value and should not be changed. You can choose different reaeration formulas, but I have a feeling this will not help you. Is the O2 concentration you are trying to reproduce above saturation (I suspect it is)? If so, then the ONLY way observed concentrations can be produced is through photosynthetic production of oxygen. You can find out if reaeration is possibly the culprit by choosing a LAKE reaeration type, choosing a reaeration equation of 1, and then setting the wind speed to 0 for some time before the comparison date. With a zero windspeed, there will be no water/atmosphere exchange of oxygen. Generally what is happening to cause this behavior in the model is that the algae are growing at some very small fraction of their maximum growth rate due to nutrient/light limitation. This is generally phosphorus that is <= detection limit in the data and in the model. This is a fundamental problem with all current eutrophication formulations for algal growth, and a lot of research needs to be done in this area to determine how phosphorus can be below detection levels, but photosynthesis is still producing supersaturated oxygen levels.[addsig]