Estimating Co2 Source/sink Distributions within a Rice Canopy Using Higher-order Closure Models

Source/sink strengths and vertical flux distributions of carbon dioxide within and above a rice canopy were modelled using measured mean concentration profiles collected during an international rice experiment in Okayama, Japan (IREX96). The model utilizes an Eulerian higher-order closure approach that permits coupling of scalar and momentum transport within vegetation to infer sources and sinks from mean scalar concentration profiles; the so-called ‘inverse problem’. To compute the required velocity statistics, a Eulerian second-order closure model was considered. The model well reproduced measured first and second moment velocity statistics inside the canopy. Using these modelled velocity statistics, scalar fluxes within and above the canopy were computed and compared with CO2 eddy-correlation measurements above the canopy. Good agreement was obtained between model calculations of fluxes at the top of the canopy and measurements. Close to the ground, the model predicted higher respiratory fluxes when the paddy was drained compared to when it was flooded. This is consistent with the floodwater providing a barrier to diffusion of CO2 from the soil to the atmosphere. The Eulerian sources and flux calculations were also compared to source and flux distributions estimated independently using a Lagrangian Localized Near Field theory, the first study to make such a comparison. Some differences in source distributions were predicted by these analyses. Despite this, the calculated fluxes by the two approaches compared well provided a closure constant, accounting for the influence of ‘near-field’ sources in the Eulerian flux transport term, was given a value of 1.5 instead of the value of 8 found in laboratory studies.