Gas exchange at river cascades: field experiments and model calculations

A model describing gas exchange, at river cascades, which considers direct gas exchange over the free surface as well as gas exchange via air bubbles entrained into the water is presented. The superiority of this model over simpler models that consider only one of the two gas-exchange processes is demonstrated by an application to experimental data for five compounds at four different cascades. Using the calibrated model, it is shown that the relative importance of the two gas- exchange processes depends strongly on the physicochemical properties (especially on the Henry coefficient) of the compound exchanged. A special consequence of this finding is that, in rivers in which the water-level difference over hydraulic controls contributes significantly to the total water-level difference, gas transfer of volatile organic compounds cannot be estimated from oxygen exchange and evaporation data alone using the two film models of gas exchange.