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Wetland water flows and interfacial gas exchange


The flow of water in wetlands may exert significant influence on wetland biogeochemistry, and specifically interfacial gas exchange. Measuring currents in wetlands requires caution. The acoustic Doppler velocimeter (ADV) is widely used for the characterization of water flow and turbulence. However, deployment of ADVs in low-flow environments is hampered by a unique source of bias related to the ADV's mode of operation. The extent of this bias is revealed by Particle image velocimetry (PIV) measurements of an ADV operating in quiescent fluid. Image-based flow measurement techniques such as PIV may provide improved accuracy in low-flow environments like wetlands. Such techniques were applied to observe wind-driven flows in a wetland with emergent vegetation and investigate the effects of the wind shear on gas transfer across the air-water interface. Wind speed is the parameter most often used to model interfacial gas exchange in other aquatic environments. In wetlands with emergent vegetation, the emergent vegetation will attenuate wind speed above the water surface, modify fluid shear at the water surface, and influence stirring beneath the water surface. Direct measurements of gas transfer in a model wetland in the laboratory indicated that unless wind speeds are extreme, interfacial gas transfer in wetlands is typically dominated by another physical force: surface cooling-induced thermal convection. In an application of these lab results, gas transfer across the air-water interface due to thermal convection in the water column is shown to account for a sizable portion of total methane fluxes from a restored marsh in California's Sacramento-San Joaquin Delta.

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