Sensitivity of methane estimates for irrigated rice in Thailand: A comparison of Tier-2 factors and empirical models regarding water and residue management

Thailand is a major national source of methane (CH₄) from irrigated rice, yet its national greenhouse gas inventory applies fixed IPCC Tier-2 seasonal emission factors. Tier-2 follows the IPCC scaling-factor framework but replaces default parameters with country- or region-specific values derived from local data. For irrigated rice in Thailand’s Central/Southern region, the inventory applies fixed seasonal factors of 143 and 71.8 kg CH₄ ha⁻¹ season⁻¹ for the main and second crops, respectively, which may not reflect heterogeneity in farm management and soils. Using survey and soil data from irrigated farms in Central Thailand, we evaluated the sensitivity of estimated seasonal CH₄ emissions to water regime, residue management, and soil organic carbon (SOC) by comparing Thailand’s Tier-2 reference with four estimation approaches:  IPCC, (2019) and the empirical/statistical models of Yan et al. (2005), Wang et al. (2018), and Nikolaisen et al. (2023). For the analysed farms, multiple drainage water management was common (54.0% in the main crop; 52.8% in the second crop), while straw incorporation occurred in 38.7% of farms before the main crop and 27.8% before the second crop. Sensitivity was quantified using a structured scenario framework. Scenario 0 was a counterfactual baseline (continuous flooding, no residue inputs). Water effects were isolated as Scenario A−0, residue effects as Scenario B−0, and combined effects under current practices as Scenario C−0. In the main season, drainage effects were negative for 149 of the 150 farms. Residue effects produced large upper-quartile increases (Q3 = 295–390 kg CH₄ ha⁻¹ season⁻¹ across models). Under current practices, the median net effect remained negative, but high-emitting cases persisted. Across seasons, empirical/statistical models produced higher medians and wider farm-to-farm distributions than the Thai Tier-2 reference factors. SOC further structured variability under current practices, with farms >2% SOC disproportionately represented among the highest estimated emitters. These results indicate that fixed seasonal emission factors can mask management and soil heterogeneity, and that more detailed activity data on drainage techniques, residue incorporation, and soil carbon status are necessary to improve Thai rice methane inventories.

Reference

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