National and global emissions assessments

The Task Force on National Greenhouse Gas Inventories (TFI) of the IPCC produces from time to time guidelines for the preparation of the inventories of national greenhouse gas emissions that have to be submitted annually by the signatories to the United Nations Framework Convention on Climate Change (UNFCCC). Emissions from agriculture are inherently more difficult to assess than those from other economic sectors, for example energy production or transport, no matter whether they relate to nitrous oxide or CH4 emissions, because of the variability of environmental conditions in which they occur -many of the factors involved have been discussed above. In many countries there are no - or insufficient - experimental data to determine directly the emissions of CH4 from rice-growing systems, and so methodologies and a series of 'default' factors have been developed to provide guidance for calculation.

The latest IPCC Guidelines (IPCC, 2007a) provide two basic equations (Equations 6 and 7) for calculating national inventories of CH4 emissions from rice cultivation. Because the natural conditions and agricultural management of rice production may be highly variable within a country, it is recommended that this variability should be accounted for by disaggregating national total harvested area into sub-units (for example harvested areas under different water regimes or organic amendment: conditions 'i, j and k' in Equation 6 below). Then, total annual emissions are equal to the sum of emissions from each sub-unit of harvested area:

Emissions from rice cultivation = 2ijk (EFijk • tijk • Aijk) (6)

where EFijk is a daily emission factor, tijk is the rice cultivation period in days, and Aijk is the annual harvested area of rice. Emissions for each sub-unit are adjusted by multiplying a baseline default emission factor for continuously flooded fields without organic amendments (EFc) by various scaling factors as shown in Equation 7:

where EFi is the adjusted daily emission factor for a particular harvested area and SFw, SFp, SFo and SFsr are scaling factors for the water regime during the cultivation period, that in the pre-season, the organic amendment applied, and others (soil type, rice cultivar, etc.), respectively.

A value for the default baseline emission factor EFc of 1.30 (with error range of 0.80-2.20) kg CH4 ha-1 day-1 is provided by IPCC (2007a); it was estimated via a statistical analysis of available field measurement data (Yan et al, 2005). Table 8.3 shows examples of the default scaling factors to account for the differences in water regime during the cultivation period.

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