Search

• Of the three gaseous fluxes, the CH 4 flux suffered the largest high frequency loss. • The aerodynamic model yielded the best results for the CO 2 and CH 4 fluxes. • The footprint with flux-gradient method was smaller than that of the eddy covariance. • A hybrid scheme is the most appropriate flux observation method for small ponds. Despite their small overall area, small ponds play a large role in the greenhouse gas budgets of inland water bodies. This study aims to evaluate the performance of the flux-gradient (FG) and the eddy covariance (EC) method for measuring the fluxes of CO 2 , CH 4 , and H 2 O at two small fish ponds (fetch < 120 m) in subtropical climate conditions. The EC fluxes were subject to two sources of error: high frequency flux loss and footprint contamination. Of the three gaseous fluxes, the CH 4 flux suffered the largest high frequency loss (18%) due to a combination of low EC instrument height and long optical path of the CH 4 analyzer. Despite the low measurement height, the EC fluxes were influenced by sources outside the boundary of the target fish ponds, with the footprint contamination most severe on the CO 2 flux and least severe on the CH 4 flux. With regards to the FG method, one major uncertainty lies in the eddy diffusivity calculation. Of the three eddy diffusivity models evaluated [the aerodynamic (AE) model deploying the full Obukhov stability correction, the modified Bowen-ratio model using H 2 O as a tracer, and the wind profile model for neutral stability], the AE model yielded the best results for the CO 2 and CH 4 fluxes. Our results support Horst's (1999, Boundary-Layer Meteorology 90, 171) theoretical prediction that the footprint of the AE flux based on a two-level concentration profile measurement should be much smaller than that of the gradient flux footprint and the EC flux footprint at the geometric mean of the two heights. We conclude that the most appropriate micrometeorological method for measuring fluxes from small water bodies is a hybrid scheme, whereby an EC system is deployed to measure the eddy diffusivity and a precision gas analyzer is used to obtain the concentration gradient of the target gas. [ABSTRACT FROM AUTHOR]

• Methane fluxes from freshwater aquaculture ponds were measured using eddy covariance. • Ebullition was the main transport way accounting for 70%±4% of the total CH 4 flux. • Water temperature was the primary driver of the CH 4 flux across multiple time scales. • Large spatial variations were observed in the total and ebullitive CH 4 flux with the FC and IF method. • Dredging plays a more important role in regulating the CH 4 emission than aeration. Aquaculture ponds are important anthropogenic methane (CH 4) sources to the atmosphere. Currently large uncertainties still remain regarding the emission strength of this source type and its relationship with aquacultural farming practices. In this study, the methane flux was measured continuously for four years with eddy covariance (EC) in an aquaculture pond complex in the Yangtze River Delta, China. These ponds have never been dredged and were aerated during part of the aquacultural season. Additionally, floating chambers and inverted funnels were used to investigate spatial heterogeneity of the CH 4 flux and to quantify the contribution via ebullition to the flux. The results showed that the daily CH 4 flux ranged from 0.1 to 16.7 μg m−2 s−1, with an average value of 4.10 ± 3.08 µg m−2 s−1. Water temperature was the primary driver of the CH 4 flux across multiple time scales (half-hourly, daily, and monthly scale). Ebullition was the main transport way accounting for 70% ± 4% of the total CH 4 flux. The annual flux in this study was about three times the median flux reported by other researchers for similar freshwater aquaculture ponds. A statistical analysis of our data together with the published flux data reveals that ponds with dredging have much lower CH 4 emission flux than those without dredging and suggests that dredging may have a much larger influence on the emission flux than aeration. [ABSTRACT FROM AUTHOR]