Source/sink distributions of heat, water vapour, carbon dioxide and methane in a rice canopy estimated using Lagrangian dispersion analysis

Abstract

Source distributions for heat, water vapour, CO 2 and CH 4 within a rice canopy were derived using measured concentration profiles, a prescribed turbulence field and an inverse Lagrangian analysis of turbulent dispersion of scalars in plant canopies. Measurements were made during IREX96, an international rice experiment in Okayama, Japan. Results for the cumulative fluxes of heat, water vapour and CH 4 at the canopy top were satisfactory once their respective concentration profiles were smoothed using simple analytic functions. According to the inverse analysis, water vapour was emitted relatively uniformly by each of five equi-spaced layers within the canopy, whereas sensible heat fluxes were small (<100 W m −2) and of either sign. Methane fluxes were predicted to be emitted most strongly in the lower 50% of the canopy, as expected from the distribution of micropores along leaves and leaf sheaths, the major pathway for CH 4 loss from the soil–crop system. No smoothing was required for CO 2 concentration profiles and the inverse analysis provided close correspondence between the turning point in the concentration profile is the changeover from respiration by the soil/paddy water and lower canopy to net photosynthesis by the upper canopy. These results could only be obtained by including both the near- and far-field contributions of sources to the total concentration profile. Neglect of the near-field contribution in the inverse analysis led to spurious source distributions. Excellent agreement was obtained between cumulative fluxes of heat, water vapour, CO 2 and CH 4 at the top of the canopy from the inverse analysis and direct eddy covariance measurements when the friction velocity u *>0.1 m s −1, and atmospheric stability was approximately neutral. Nocturnal fluxes of CO 2 and CH 4 from the inverse method exceeded micrometeorological measurements above the canopy by a factor of 2–3 when u *<0.1 m s −1 and stable atmospheric conditions prevailed within and above the canopy. Neglect of these stability effects will lead to an underestimate of the dispersion coefficients (dimension of resistances) in the transport model and hence an overestimate of the fluxes. Further work is required to establish the correct procedure for incorporating stability effects into the inverse analysis.