Evaporation and CO₂ fluxes in a coastal reef: an eddy covariance approach

Introduction: We conducted season-long observations of evaporation and carbon flux at the Gulf of Aqaba coast, northern Red Sea. We used the eddy-covariance method with a two-tower setup to measure evaporation rates over land and sea and the advection between them. Using a three-dimensional mass balance approach, we calculated total evaporation as the sum of two main components in our site: horizontal advection and turbulent vertical flux, with half-hourly change of water vapor storage and horizontal flux divergence found to be negligible.

Outcomes: Average evaporation rates were 11.4 [mm/day] from April through May (early summer) and 10.5 [mm/day] from June through August (summer). The coastal reef was a CO₂ sink over the period of measurements, significantly higher in June through August than in April through May. The main environmental drivers of CO₂ flux were humidity, water temperature, sensible heat flux, and wind speed.

Discussion: The rates of evaporation near the shore were considerably higher than values reported in other studies typically used to represent the mean for the whole Gulf area. We found that evaporation rates computed by common bulk models approximate the mean values of evaporation but have poor representativeness of the intra-daily temporal variation of evaporation. There was a significant correlation between CO₂ flux and evaporation attributed to common environmental drivers of gas diffusion, turbulent fluxes, and horizontal transport.

Conclusion: We conclude that observations of fluxes in coastal waters need to use at least a two-tower system to account for the effect of horizontal advection on the total flux.