The International Rice Research Institute (IRRI) in the Philippines has developed a new mitigation technology for methane known as alternate wetting and drying (AWD) (IRRI, 2009). AWD is a watersaving and methane mitigation technology that lowland (paddy) rice farmers can use to reduce their water consumption in irrigated fields. Rice fields using this technology are alternately flooded and dried. The number of days of drying the soil in AWD can vary according to the type of soil and the cultivar from 1 day to more than 10 days. Rice cultivation is responsible for 10% of GHG emissions from agriculture. In developing countries, the share of rice in GHG emissions from agriculture is even higher, e.g., it was 16% in 1994.
AWD is also called controlled irrigation or intermittent irrigation. The number of days of non-flooded soil can vary from 1 to more than 10 days. A practical way to implement AWD technology is by monitoring the depth of the water table in the field using a simple perforated field water tube. When the water level is 15cm below the surface of the soil, it is time to flood the soil to a depth of around 5cm at the time of flowering, from 1 week before to 1 week after the maximum flowering. The water in the rice field needs to be kept at 5cm depth to avoid any water stress that would result in severe loss in rice grain yield. The threshold of water level at 15cm is called ‘safe AWD’, as this will not cause any yield decline because the roots of the rice plants will still be able to take up water from the saturated soil and move it to root zone. The field water tube used in this technology will help to measure the water level in the field so that incipient water stress in the rice plants can be anticipated (figure 1). Thus, this alternate wetting and drying technology will not only save water but can greatly reduce emissions of methane. Water-saving technologies such as alternate wetting and drying reduce the amount of time rice fields are flooded and can reduce the production of methane by about 60% (figure 2) or even up to 90% (IRRI, 2009).
Starting from about 15 days after transplanting, farmers using AWD stop irrigating until the water table goes 15 cm below the ground level. A 20cm hole is dug in the rice field, and a perforated plastic pipe is installed to monitor the level of the water table after each irrigation. This practice is continued until flowering starts. At that time, it is necessary to keep 2-4cm of standing water from flowering to dough stage.
The practice requires that the irrigation systems must accommodate precise control of the timing of the irrigations and the depths of water in the paddies. Therefore, farmers need to be trained in its use. The benefits towards GHG mitigation do not accrue any financial return to the farmers.
This technology is very common in countries such as China, India and the Philippines (IRRI, 2002).
- Large reductions in methane emissions are possible compared to continuous flooding (Table 3.6).
- It will help the economic use of water during rice cultivation.
- The drying phase of rhizosphere will help root growth and its sustainability for water transport to rice plants even under low soil moisture conditions.
- Farmers will be able to know the status of water of their rice growing fields and would be able to balance irrigation with achieving minimum methane emissions.
- The savings of irrigation water will have impact on environment because of reduced withdrawal of ground water and a reduction in consumption of diesel for water pumps.
- Protection of water levels of ground water may also reduce arsenic contamination in rice grain and straw.
- Occasionally, rice productivity is reduced using AWD technology if moisture stress condition is induced. However, the reduction of yield was less compared to the yield reduction due to the direct moisture stress effect.
- N2O emissions are increased.
AWD technology can reduce the number of irrigations significantly compared to farmer’s practice, thereby lowering irrigation water consumption by 25 per cent, reducing diesel fuel consumption for pumping water by 30 liters per hectare, and producing 500kg more rice grain yield per hectare.
The visible success of AWD has dispelled the concept of yield losses under moisture stress condition in non-flooded rice fields. Adoption of AWD technology reduced water use and methane emissions, and it increased rice productivity. It can reduce methane emissions by 50% as compared to rice produced under continuous flooding.
The cost of AWD was found to be US$20 per t CO2e saved in Haryana, India, whereas in Ilocos Norte, Philippines and Zhejiang, China, this cost became greater than US$45 per t CO2e saved (Wassmann and Pathak, 2007).
IRRI (2002): Potentials of Water-saving Technologies in Rice Production: An Inventory and Synthesis of Options at the Farm Level. Available at http://www.iwmi.cgiar.org/assessment/FILES/word/proposals/Project%20Prop... 
IRRI (2009): Every drop counts. Rice Today, Vol 8, No. 3;16-19.
Wassman R., Lantin R.S., Neue H. U., Buendia L.V., Corton T.M. and Lu Y.(2000): Characterization of methane emissions from rice fields in Asia. III. Mitigation options and future research needs. Nutrient Cycling in Agroecosystems 58: 23–36.
Wassmann R and Pathak H. (2007): Introducing greenhouse gas mitigation as a development objective in rice-based agriculture: II. Cost- benefit assessment for different technologies, regions and scales. Agricultural Systems 94:826-840.