A1 Flooded Land Remaining Flooded Land

This section provides information on how to estimate CH4 emissions from Flooded Land Remaining Flooded Land. This information is drawn from available literature and is intended to be useful to countries that wish to develop preliminary estimates of CH4 emissions from this source. Countries with potentially significant CH4 emissions from Flooded Land seeking to report these emissions should consider the development of country-specific emission factors to reduce overall uncertainty. Guidance on the development of such factors is provided in Box 2a. 1 in Appendix 2.

3A.1.1 CH4 emissions from Flooded Land Remaining Flooded Land methodological issues

Post-flooded CH4 emissions can occur via the following pathways:

• Diffusive emissions, due to molecular diffusion across the air-water interface;

• Bubble emissions, or gas emissions from the sediment through the water column via bubbles; this is a very important pathway for CH4 emissions, especially in temperate and tropical regions;

• Degassing emissions, or emissions resulting from a sudden change in hydrostatic pressure, as well as the increased air/water exchange surface after reservoir waters flow through a turbine and/or a spillway (Helie, 2004; Soumis et al., 2004; Delmas et al., 2005); this is a very important pathway for CH4 emissions from young tropical reservoirs.

The Tier 1 approach only covers diffusive emissions. Tier 2 includes a term for estimating CH4 bubble emissions, and if applicable, separate consideration of ice-free and ice-covered periods. Tier 3 methods refer to any detailed measurement-based approach that includes an estimate of all relevant CH4 fluxes from Flooded Land, which also includes degassing emissions, and considers the depth, the geographical localization and water temperature of the reservoir for its entire life-time. Tier 3 methods are not outlined further in this chapter, but countries should refer to Box 2a. 1 in Appendix 2 on the derivation of country-specific emission factors as a resource for implementing a Tier 3 approach. Table 3 a. 1 summarizes the coverage of the three tiers and CH4 emission pathway.

Table 3a.1

Summary of methods and emissions coverage

CH4

Tier 1

• Diffusive emissions

• Bubble emissions

Tier 3

• All emissions

The following section describes the Tier 2 and Tier 1 approaches for CH4 emissions. CHOICE OF METHOD

Methane can be emitted from flooded lands through release of bubbles, by diffusion and by degassing. The decision tree in Figure 3a. 1 guides inventory compilers through the processes of selecting an appropriate approach for CH4 emissions from Flooded Land. Tier selection and the level of spatial and temporal disaggregation implemented by inventory compilers will depend upon the availability of activity data and emission factors, as well as the importance of reservoirs as contributors to national greenhouse gas emissions. Country-specific scientific evidence and data are always preferable to Tier 1 default data.

Tier 1

The Tier 1 method for estimating CH4 emissions from Flooded Land includes only diffusion emissions during ice-free period. Emissions during the ice-cover period are assumed to be zero. Equation 3a.1 can be used with measured emissions provided in Table 3a.2 and country-specific total area of flooded land:

Equation 3a.1 CH4 emissions from flooded lands (Tier 1)

CH4Emissionwwflood = P• E(CH 4)dlff • Aflood total_surface *10-6

Where:

CH4 emissionsww flood = total CH4 emissions from Flooded Land, Gg CH4 yr-1

P = ice-free period, days yr-1 (usually 365 for annual inventory estimates, or less in country with ice-cover period))

E(CH4)diff = averaged daily diffusive emissions, kg CH4 ha-1 day-1

Aflood, totai surface = total flooded surface area, including flooded land, lakes and rivers, ha

Figure 3a.1 Decision Tree for CH4 Emissions from Flooded Land Remaining Flooded

Land

Figure 3a.1 Decision Tree for CH4 Emissions from Flooded Land Remaining Flooded

Land

Note:

1: See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

2: A subcategory is significant if it accounts for 25-30% of emissions/removals for the overall category.

Tier 2

A Tier 2 approach for CH4 emissions requires country-specific emission factors for diffusive and bubble emissions, and if applicable, accounts for different rates of diffusion and bubble emissions during the ice-free and ice-covered periods. Flooded land area may also be disaggregated by climatic zone, or any relevant parameter listed in Box 2a. 1 in Appendix 2. This approach is described in Equation 3a.2.

Equation 3a.2 CH4 emissions from flooded lands (Tier 2)

CH 4 Emissionswwflood =

Pi • {Ei (CH4 ) diff + Ei (CH4 ) bubble ) • A flood,surface

CH4 emissionsWW flood = total CH4 emissions from Flooded Land per year, kg CH4 yr-1

Pf = ice-free period, days yr-1

Pi = period with ice cover, days yr-1

Ef(CH4)diff = averaged daily diffusive emissions from air water-interface during the ice-free period, kg CH4 ha-1 day-1

Ef(CH4)bubble = averaged daily bubbles emissions from air water-interface during the ice-free period, kg CH4 ha-1 day-1

Ei(CH4)diff = diffusive emissions related to the ice-cover period, kg CH4 ha-1 day-1 Ei(CH4)bubble = bubbles emissions related to the ice-cover period, kg CH4 ha-1 day-1 Aflood, surface = total flooded surface area, including flooded land, lakes and rivers, ha

CHOICE OF EMISSION FACTORS Tier 1

The key default values for Tier 1 are emission factors for CH4 via the diffusion pathway. Table 3a.2 provides measured emissions for various climate zones. To the extent possible given available research, these measured emissions integrate spatial (intra reservoir and regional variations) and temporal variations (dry/rainy and other seasonal, inter-annual variations) in the emissions from reservoirs. Default emission factors should be used in Tier 1 for the ice-free period only. During complete ice-cover period, CH4 emissions are assumed to be zero. When default data are not available, countries should use the closest default emission factors value (emissions of the most similar climatic region).

Tier 2

Under Tier 2, country-specific emission factors should be used instead of default factors to the extent possible. Additional estimates of winter emissions and CH4 bubble emissions are also needed, which will require the development of country-specific emission factors. It is anticipated that a mix of default values and country-specific emission factors will be used when the latter do not cover the full range of environmental and management conditions. The development of country-specific emission factors is discussed in Box 2a. 1 in Appendix 2. The derivation of country-specific factors should be clearly documented, and published in peer reviewed literature.

CHOICE OF ACTIVITY DATA

Several different types of activity data may be needed to estimate flooded land emissions, depending on the tier being implemented and the known sources of spatial and temporal variability within the national territory. These activity data types correspond to the same data required for CO2 emissions as described in Section 7.3.2.

Flooded land area

Country-specific data on flooded land area are required for all tiers to estimate diffusive and bubble emissions. Alternatively, countries can obtain an estimate of their flooded land area from a drainage basin cover analysis, from a national dam database, from the International Commission on Large Dams (ICOLD, 1998) or from the

World Commission on Dams report (WCD, 2000). Since flooded land area could change rapidly, countries should use updated and recent data. Tier 2 and Tier 3 approaches preferably rely on a national database to track reservoirs surface area. This database should also include other parameters as reservoir depth, year of flooding, reservoir localization (see Box 2a.1 in Appendix 2).

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