Eddy correlation flux measurements: The sediment surface area that contributes to the flux

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 − 158.7 h + 159.2 h 2 − 120.8 h log( z 0 ) (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 ( z 0 ) values between 7.04 × 10 −6 and 0.01 m. The upstream distance ( x max ) to the location that contributes the strongest flux signal can likewise be estimated as x max = −0.09888 − 11.53 h + 10.25 h 2 ‐ 6.650 h log ( z 0 ). Because vertical turbulent mixing scales with mean current velocity, l and x max are independent of current velocity. The footprint width ( w ) can be calculated as w = 6.531 h . These expressions were developed for water depths ( H ) of H > 27 h . In the depth interval 6.7 h > H > 27 h , l can be calculated by multiplying the length, as given above, by the factor 1 + 8.347exp(−0.2453 H / h ), whereas x max is independent of H . For H > 6.7 h , 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 x max also hold for anisotropic conditions. In contrast, w scales with √( E y /E z ), where E y and E z are the transverse and the vertical eddy diffusivity, respectively. Finally, we describe how site‐specific values of z 0 and levels of anisotropy in a turbulent near‐bottom flow can be extracted directly from eddy correlation measurements.