Diel cycle of lake‐air CO2 flux from a shallow lake and the impact of waterside convection on the transfer velocity

Abstract

AbstractTwo years of eddy covariance measurements of lake carbon dioxide (CO2) fluxes reveal a diel cycle with higher fluxes during night. Measurements of partial pressure in the air (pCO2a) and in the water (pCO2w), during 4 months, show that the high nighttime fluxes are not explained by changes in the difference between pCO2a and pCO2w. Analyzing the transfer velocity (k600,meas), which is a measure of the efficiency of the gas transfer, with respect to wind speed, shows that variations in wind speed do not explain the diel cycle. During nighttime, when the fluxes are high, the wind is normally low. Thus, a solely wind‐based parameterization of the transfer velocity (ku,CC) results in large errors compared to k600,meas, especially for wind speeds lower than 6 m s−1. The mean absolute percentage error between ku,CC and k600,meas is 79%. By subtracting ku,CC from k600,meas, we investigate how waterside convection influence k600,meas. Our results show that the difference (k600,meas − ku,CC) increases with increasing waterside convection. Separating the transfer velocity parameterization in two parts, one depending on the wind speed and one depending on waterside convection, the mean absolute percentage error compared to the measurements reduces to 22%. The results in this paper show that the high nighttime CO2 fluxes can, to a large extent, be explained by waterside convection and that a transfer velocity parameterization based on both wind speed and waterside convection better fits the measurements compared to a parameterization based solely on wind speed.