Mid-season drainage involves the removal of surface flood water from the rice crop for about seven days towards the end of tillering. The duration of the dry period must be long enough for rice plant to experience visible moisture stress. 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.
Introduction
Mid-season drainage aerates the soil, interfering with anaerobic conditions and thereby interrupting CH4 production. Mid-season drainage of a rice crop involves withholding flood irrigation water for a period until the rice shows symptoms of stress. It involves ridge and furrow cultivation technology, where some moisture still exists in the soil even after the toe furrow is drained. It is essential to check when the crop has used most of the available water. The degree of soil cracking will depend on the soil type and on the spatial distribution of the rice cultivars. The cumulative evapotranspiration of the crop varies from 77-100mm during the time water is removed depending on crop vigour and soil types. The field is then re-flooded as quickly as possible. It is necessary to cover the soil surface with water so that the plants start recovery. Water depth then can be gradually increased to that required for protection of the developing plant canopy from damaging high temperatures during anthesis.
Mid-season drainage reduces methane emissions of paddy fields, with reductions ranging from 7 to 95% (figure 1).
Figure 1: Reductions in methane emissions due to various water management practices compared to continuous flooding (with organic amendments). WS = wet season, DS = dry season. (source: Wassman et al., 2000).
However, rice is also a significant anthropogenic source of N2O. Mid-season drainage or reduced water use creates unsaturated soils conditions, which may promote N2O production. Mid-season drainage is an effective option for mitigating net global warming potential although 15-20% of the benefit gained by decreasing methane emission was offset by increasing N2O emissions. Little N2O emission occurred when fields were continuously flooded (Zou et al., 2005). Mid-season drainage, however, caused intense emissions of N2O, which contributed greatly to the seasonal amount. After the midseason drainage, on the other hand, no recognizable N2O was observed when the field was frequently waterlogged by the intermittent irrigation. In contrast, large N2O emissions were observed when the field was moist but not waterlogged by the intermittent irrigation. Thus, N2O emissions during intermittent irrigation periods depended strongly on whether or not waterlogging was present in the fields. Different water regimes cause changes to N2O emissions from rice paddies (Zou et al., 2005).
Feasibility of technology and operational necessities
Farmers fear potential adverse effects on yield both from observing the visible stress and from the delay in harvest time. They need to be educated on the benefits that outweigh the potential losses. A large part of the benefits are towards GHG mitigation, which do not accrue any financial return to the farmers.
Status of the technology and its future market potential
Advantages
- Methane emission reductions associated with mid-season drainage in rice field range from about 7 to 95% (Table 3.6) with little effect on rice grain yield.
- Draining stimulates root development and accelerates decomposition of organic materials in the soil making more mineralised nitrogen available for plant uptake.
- Mid-season drainage saves water, which could be used for other purposes.
- Mid-season drainage inhibits ineffective tillers and improves root activities.
Disadvantages
- Drainage has the unintended effect of increasing nitrous oxide emissions. However, mid-season drainage can help mitigation of N2O if a field was frequently water logged by intermittent irrigation.
- Intermittent drying or drainage of soil is not feasible on terraced rice fields because drying could cause cracking of the soil leading to water losses, or in extreme cases, complete collapse of the terraced construction.
- Field drainage also induces weeds and thereby reduces the rice grain yield.
- Mid-season drainage delays the development of crop. Flowering is generally delayed by 3-4 days and harvest/maturity may be delayed by 7-10 days.
- Mid-season drainage may increase plant height, and this will make the crop more prone to lodging especially when grain yield is high.
How the technology could contribute to socio-economic development and environmental protection
According to Wassmann and Pathak (2007), mid-season drainage a profitable mitigation technology due to low labour cost and low yield risk. The cost of the technology was around US$20 per t CO2e saved. Nelson et al., (2009) observed that by one mid-season drying, net revenue dropped less than 5% while GHG emissions dropped by almost 75 million metric tons of CO2e (approximately 4,000 tonnes CO2e ha-1).
The technologies of conservation tillage, mid-season drainage and alternate flooding reduced GHG emissions without extra expenditure. Higher net return with these technologies suggests the tremendous potential scope of their adoption by farmers.
Water management is often considered a good strategy to mitigate methane emissions from rice fields. Water saving technologies can reduce methane emissions in a given area of rice land. The saved water will then be used to irrigate more land and new crops in future seasons. Rice is grown on more than 140 million hectares worldwide. Ninety per cent of rice fields are temporarily flooded, providing scope for better water management to reduce water consumption, related energy and electricity consumption, and fertiliser consumptions. These reductions would result in methane mitigation and could then be included for claiming carbon credits.
Financial requirements and costs
Farmers fear potential adverse effects on yield both from observing the visible stress and from the delay in harvest time. They need to be educated on the benefits that outweigh the potential losses. A large part of the benefits are towards GHG mitigation, which do not accrue any financial return to the farmers.
References
- Nelson, G.C., Robertson, G., Msangi, S., Zhu, T., Liao, X. and Jawagar, P.(2009): Greenhouse Gas Mitigation: Issues For Indian Agriculture: Int. Food. Pol. Res. Inst. Pp 1-60.
- 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.
- Zou J, Huang Y, Jiang J Zheng X and Sass RL (2005): A 3-year field measurement of methane and nitrous oxide emissions from rice paddies in China: Effects of water regime, crop residue, and fertiliser application. Global biogeochemical cycles, vol 19 GB2021, doi:10.1029/2004GB002401, 2005.