Interfacial gas transfer plays a significant role to preserve water quality in creek and rivers. However direct measurements of gas transfer velocity and reaeration coefficient are still hard, and therefore a reliable prediction model is required to be developed. The present paper gives physical interpretations to a modified surface divergence (SD) model which authours proposed in previous works, and examines applicability of the modified SD model together with PIV measurements focus on small aspect ratio conditions in which secondary currents influence remarkably streamwise velocity profile. Gas flux properties are also considered in frequency fields on the basis of power spectrum of normalized surface divergence, and low frequency fluctuations include secondary currents are found to govern the interfacial gas transfer. As the contributions of such a low-frequency and large-scale eddies are installed in the modified SD model, our practical formula could predict the gas transfer velocity in wide-range hydraulic conditions.

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