Development of a micrometeorological model for the estimation of methane flux from paddy fields: Validation with standard direct measurements

This work has focussed on the development of an indirect method for estimating methane fluxes from paddy fields and wetlands. A micrometeorological model, based on an analytical solution of the Eulerian advectionediffusion equation for vertical diffusion, has been used; model parameters include the location of the methane analyser and standard surface layer scaling factors. Flux chambers, which are commonly used for measuring methane fluxes from agricultural sources, are usually mechanically operated with a rated induced-draft fan and as such cannot replicate the real world atmospheric conditions. The results are not very reliable due to leakages along the piping and at fittings, especially when these chambers are used over a relatively rough surface like an agricultural field or a wetland. The results of the model have been compared with those from the direct method. The seasonal average methane flux calculated by the indirect method, for the cultivar type ‘‘Sundari’’, is 7.13Eþ05 g/ha, while cultivar type ‘‘Shatabdi’’ gives a little lower value of 5.22Eþ05 g/ha. In case of the direct chamber method also, the seasonal average methane flux for the cultivar type ‘‘Sundari’’ (6.20Eþ05 g/ha) is more than cultivar type ‘‘Shatabdi’’ (4.84Eþ05 g/ha). When the two methods of assessment were compared, season SeptembereDecember 2004 gave r2 ¼ 0.91, RMSE ¼ 0.16 and MNB ¼ 0.13 while we got r2 ¼ 0.94, RMSE ¼ 1.22 and MNB ¼ 0.06 for the season SeptembereDecember 2005. In very few experiments we could cover a huge aerial plot instead of a huge number of experiments necessary for the direct chamber method.