Using a crop/soil simulation model and GIS techniques to assess methane emissions from rice fields in Asia. IV. Upscaling to national levels

The process-based crop/soil model MERES (Methane Emissions from Rice EcoSys tems) was used together with daily weather data, spatial soil data, and ric e-growing statistics to estimate the annual methane (CH4) emissions from Ch ina, India, Indonesia, Philippines, and Thailand under various crop managem ent scenarios. Four crop management scenarios were considered: (a) a 'basel ine' scenario assuming no addition of organic amendments or field drainage during the growing season, (b) addition of 3,000 kg DM ha(-1) of green manu re at the start of the season but no field drainage, (c) no organic amendme nts but drainage of the field for a 14-d period in the middle of the season and again at the end of the season, and (d) addition of 3,000 kg DM ha(-1) of green manure and field drainage in the middle and end of the season. Fo r each scenario, simulations were made at each location for irrigated and r ainfed rice ecosystems in the main rice-growing season, and for irrigated r ice in the second (or 'dry') season. Overall annual emissions (Tg CH4 yr(-1 )) for a province/district were calculated by multiplying the rates of CH4 emission (kg CH4 ha(-1) yr(-1)) by the area of rice grown in each ecosystem and in each season obtained from the Huke and Huke (1997) database of rice production. Using the baseline scenario, annual CH4 emissions for China, I ndia, Indonesia, Philippines, and Thailand were calculated to be 3.73, 2.14 , 1.65, 0.14, and 0.18 Tg CH4 yr(-1), respectively. Addition of 3,000 kg DM ha(-1) green manure at the start of the season increased emissions by an a verage of 128% across the five countries, with a range of 74-259%. Drainage of the field in the middle and at the end of the season reduced emissions by an average of 13% across the five countries, with a range of -10% to -39 %. The combination of organic amendments and field drainage resulted in an increase in emissions by an average of 86% across the five countries, with a range of 15-176%. The sum of CH4 emissions from these five countries, com prising about 70% of the global rice area, ranged from 6.49 to 17.42 Tg CH4 yr(-1), depending on the crop management scenario.