Eddy correlation flux measurements: The sediment surface area that contributes to the flux
Limnol. Oceanogr., 52(4), 2007, 1672-1684 | DOI: 10.4319/lo.2007.52.4.1672
ABSTRACT: We investigated the size and shape of the area on the sediment surface, the so-called footprint, that contributes to the flux in subaqueous eddy correlation measurements. Tracer tracking simulations were performed for a dissolved conservative tracer released from the sediment surface into a current-driven flow not affected by density stratifications and surface waves. Simulations revealed that the footprint length (l) can be calculated as l = -2.783 - 158.7h + 159.2h2 - 120.8h log(z0) (all units in m) for eddy correlation measurements heights (h) between 0.05 and 0.3 m above the sediment surface and for sediment surface roughness parameter (z0) values between 7.04 x 10-6 and 0.01 m. The upstream distance (xmax) to the location that contributes the strongest flux signal can likewise be estimated as xmax = -0.09888 - 11.53h + 10.25h2 - 6.650h log (z0). Because vertical turbulent mixing scales with mean current velocity, l and xmax are independent of current velocity. The footprint width (w) can be calculated as w = 6.531h. These expressions were developed for water depths (H) of H > 27h. In the depth interval 6.7h > H > 27h, l can be calculated by multiplying the length, as given above, by the factor 1 + 8.347exp(-0.2453 H/h), whereas xmax is independent of H. For H > 6.7h, the tracer transfer rate over the air-water interface controls the size and shape of the footprint. All expressions are valid for isotropic turbulence, but as a first-order estimate, the expressions for l and xmax also hold for anisotropic conditions. In contrast, w scales with sqrt(Ey/Ez), where Ey and Ez are the transverse and the vertical eddy diffusivity, respectively. Finally, we describe how site-specific values of z0 and levels of anisotropy in a turbulent near-bottom flow can be extracted directly from eddy correlation measurements.