Wetting and Drying: Reducing Greenhouse Gas Emissions and Saving Water from Rice Production
Creating a Sustainable Food Future, Installment Eightby , , , and -
Installment 8 of Creating a Sustainable Food Future explores the potential to improve water management in rice production in order to reduce agricultural greenhouse gas emissions and save water. Through a series of case studies, we examine the opportunities and challenges of mitigating emissions through water management, and close with a series of recommendations for how to scale up adoption of improved water management techniques in rice production.
A sustainable food future will require reductions in greenhouse gas emissions from agriculture even as the world produces substantially more food. The production of rice, the staple crop for the majority of the world’s population, emits large quantities of methane, a potent greenhouse gas. According to various governments, global rice production emits 500 million tons of greenhouse gases (carbon dioxide equivalent) per year—or at least 10 percent of total agricultural emissions. The figure may be closer to 800 million tons when adjusted for new estimates by the Intergovernmental Panel on Climate Change of the sustained warming effect of methane. Although uncertain, there is evidence that increasing levels of carbon dioxide in the atmosphere could also increase future rice-related emissions substantially through its effect on soil microbes.
Most of the world’s rice grows in inundated conditions, and one of the most promising techniques for reducing rice-related emissions is to reduce or interrupt the periods of flooding. The production of rice in flooded paddies produces methane because the water blocks oxygen from penetrating the soil, creating conditions conducive for methane-producing bacteria. Shorter flooding intervals and more frequent interruptions of flooding lower bacterial methane production and thus methane emissions.
Techniques for reduced or interrupted flooding include (a) a single drawdown of water during the mid-season; and (b) alternate wetting and drying (AWD), which repeatedly interrupts irrigation, so that water levels modestly decline below the soil level before reflooding. Other techniques include dry seeding instead of transplanting rice into flooded fields, and various “aerobic rice” systems, in which rice is grown in well-drained soil. Evidence indicates that all of these techniques substantially reduce greenhouse gas emissions. Perfect water management can theoretically reduce emissions by up to 90 percent compared to full flooding. Numerous field experiments also suggest that if properly employed, these practices will at least maintain rice yields, and sometimes increase them. Many of the world’s rice-producing regions also face water shortages, underscoring the need for higher water use efficiencies at the field level for stabilizing yields.
But despite the potential benefits, our case studies from China, India, the Philippines, and the United States indicate mixed practical potential to adopt these water management techniques without improvements to irrigation or drainage systems. Farmers need reliable control over irrigation water to implement these measures, and generally also need small, well-leveled fields to assure that water levels do not drop too far in parts of the field, which would impact rice yields. Where farmers irrigate by pumping groundwater in India, the southern United States, and some parts of the Philippines, they generally have the technical ability to apply water saving techniques, at least during periods without substantial rainfall.
In the Philippines and many other Asian countries, farmers have limited technical ability to drain their fields during the rainy season, so full-scale AWD is probably not feasible. During the dry season, farmers who rely on surface irrigation systems tend to be reluctant to interrupt irrigation when water is available because of doubts that water will be available later when needed to refill the field. In some of these locations, dry seeding may be an effective means of reducing methane emissions, and in others, a single drawdown may still be feasible, but the technical opportunities remain generally unexplored.
Our case studies also reveal unexplained discrepancies in the observed impacts of water management techniques in different environments. Although farmers in China and Japan widely practice a single mid-season drainage because of a common understanding that it improves their yields, researchers have found no similar yield gains in the United States. There are many studies finding yield gains from AWD, but there are also studies showing losses. No one today can explain these differences fully. The scope of potential water savings associated with these water management techniques is also uncertain. AWD has been shown to reduce water use at the farm level, but the assessment of the actual water balance at the level of an entire irrigation system is more complex and remains an open question. Overall, apart from our relatively broad analysis in these case studies, little information exists about precisely where, and under what conditions, these measures really present a benefit to farmers. Similarly, there is a lack of information about the relative cost-effectiveness of implementing these water management techniques in major rice-growing areas. Put simply, there are individual farm studies of the impacts and benefits of water management, and broad global analyses that require many assumptions, but the knowledge in between is mostly lacking. These challenges remain serious barriers to the wide-scale adoption of improved water management practices.
Rice farmers currently have only limited incentives to improve water management. In regions where farmers irrigate by pumping groundwater, improved water use efficiency can directly translate into reductions in fuel used for pumping water, and therefore lower production costs, if pumping is unsubsidized. In general, these are the prime areas with immediate opportunities to implement improved water management, and reducing water or pumping subsidies could help encourage these changes. Broader incentives will be necessary to encourage farmers in other areas to implement these practices at the necessary scale.
To fully realize the opportunities for water management benefits, we recommend that research organizations and government aid agencies fund coordinated assessments of the practical potential to implement different water management techniques at the irrigation district level. These organizations should also fund an improved global assessment of yield, greenhouse gas and water saving effects of these techniques based on a series of pilot projects. We also recommend that governments reform water and energy subsidies, and develop new affirmative incentives for water management, especially in water-stressed areas. Taken together, these measures have the potential to substantially reduce the environmental impacts of the world’s most important staple crop—and could constitute a significant step toward a sustainable food future.
Research organizations and government aid agencies should fund coordinated assessments of the practical potential to implement different water management techniques at the irrigation district level.
These organizations should also fund an improved global assessment of yield, greenhouse gas and water saving effects of these techniques based on a series of pilot projects.
Governments should reform water and energy subsidies.
Governments should strengthen incentives for water management, especially in water-stressed areas.