  Wetland biogeochemical transformations are affected by flow and mixing in
wetland surface water. We investigate the influence of wind on wetland water
flow by simultaneously measuring wind and surface water velocities in an
enclosed freshwater wetland during one day of strong-wind conditions. Water
velocities are measured using a Volumetric Particle Imager while wind
velocities are measured via sonic-anemometer. Our measurements indicate that
the wind interacting with the vegetation canopy generates coherent billows and
that these billows are the dominant source of momentum into the wetland water
column. Spectral analysis of velocity timeseries shows that the spectral peak
in water velocity is aligned with the spectral peak of in-canopy wind velocity,
and that this peak corresponds with the Kelvin-Helmholtz billow frequency
predicted by mixing layer theory. We also observe a strong correlation in the
temporal pattern of velocity variance in the air and water, with high variance
events having similar timing and duration both above and below the air-water
interface. Water-side variance appears coupled with air-side variance at least
down to 5 cm, while the theoretical Stokes' solution predicts momentum transfer
down to only 2 mm assuming transfer via molecular viscosity alone. This
suggests that the wind-driven flow contributed to significant mixing in the
wetland water column.
