Estimating COo2 source/sink distributions within a rice canopy using higher-order closure model

Source/sink strengths and vertical flux distributions of carbon dioxide wit hin and above a rice canopy were modelled using measured mean concentration profiles collected during an international rice experiment in Okayama, Jap an (IREX96). The model utilizes an Eulerian higher-order closure approach t hat permits coupling of scalar and momentum transport within vegetation to infer sources and sinks from mean scalar concentration profiles; the so-cal led 'inverse problem'. To compute the required velocity statistics, a Euler ian second-order closure model was considered. The model well reproduced me asured first and second moment velocity statistics inside the canopy. Using these modelled velocity statistics, scalar fluxes within and above the can opy were computed and compared with CO2 eddy-correlation measurements above the canopy. Good agreement was obtained between model calculations of flux es at the top of the canopy and measurements. Close to the ground, the mode l predicted higher respiratory fluxes when the paddy was drained compared t o when it was flooded. This is consistent with the floodwater providing a b arrier to diffusion of CO2 from the soil to the atmosphere. The Eulerian so urces and flux calculations were also compared to source and flux distribut ions estimated independently using a Lagrangian Localized Near Field theory , the first study to make such a comparison. Some differences in source dis tributions were predicted by these analyses. Despite this, the calculated f luxes by the two approaches compared well provided a closure constant, acco unting for the influence of 'near-field' sources in the Eulerian flux trans port term, was given a value of 1.5 instead of the value of 8 found in labo ratory studies.