GAS-EXCHANGE AT RIVER CASCADES - FIELD EXPERIMENTS AND MODEL-CALCULATIONS

A model describing gas exchange, at river cascades, which considers di rect gas exchange over the free surface as well as gas exchange via ai r bubbles entrained into the water is presented. The superiority of th is model over simpler models that consider only one of the two gas-exc hange processes is demonstrated by an application to experimental data for five compounds at four different cascades. Using the calibrated m odel, it is shown that the relative importance of the two gas-exchange processes depends strongly on the physicochemical properties (especia lly on the Henry coefficient) of the compound exchanged. A special con sequence of this finding is that, in rivers in which the water-level d ifference over hydraulic controls contributes significantly to the tot al water-level difference, gas transfer of volatile organic compounds cannot be estimated from oxygen exchange and evaporation data alone us ing the two film models of gas exchange.