Alternate Wetting and Drying (AWD)

Alternate Wetting and Drying, or AWD, is a way of watering rice fields. Instead of keeping the field under standing water all season, the farmer floods it, lets the water slowly go down until it is below the soil surface, and only then floods it again. This "wet, then dry, then wet" cycle saves water and sharply cuts the methane gas that flooded rice fields give out, usually without lowering the harvest.

Why do flooded rice fields release methane?

When a field stays under water, air (oxygen) cannot reach the soil. In soil without oxygen, a group of tiny microbes called methanogens (they belong to a group of microbes called archaea) break down dead roots, straw and other organic matter. As they do this, they produce methane. Most of this methane does not bubble up through the water.

Rice plants have hollow air channels in their stems and roots, called aerenchyma, and these work like tiny chimneys. Field studies have found that around 90% of the methane leaving a paddy field escapes through the rice plants themselves.

Why does AWD exist?

Rice is traditionally grown in standing water because water controls weeds and protects the plant. But this has two big costs:

• Water: Rice is one of the thirstiest crops. In places like Punjab and Haryana, paddy fields are filled using tubewells, which drains groundwater.
• Climate: Flooded fields are a steady methane factory. Rice cultivation accounts for roughly 10% of global methane emissions.

AWD tackles both problems at once. When the field dries for a few days, oxygen re-enters the soil. Methanogens cannot work well with oxygen around, so methane production drops.

Where did it come from?

Farmers in parts of Asia have long drained fields in the middle of the season, especially in China and Japan. The modern, tested version called "Safe AWD" was developed and promoted by the International Rice Research Institute (IRRI), based in Los Baños, Philippines, along with national research bodies in countries like Bangladesh, Vietnam and the Philippines. In India, the ICAR-National Rice Research Institute (NRRI) in Cuttack, Odisha (earlier called the Central Rice Research Institute) and state agricultural universities have tested and promoted it.

How does AWD work, step by step?

The farmer needs one simple tool: a field water tube (also called a "pani pipe"). This is a plastic or bamboo pipe with holes in its side, pushed into the soil so the farmer can see the water level under the ground.

1. The field is flooded as usual after transplanting.
2. The farmer stops watering and lets the water level fall. Water seeps down and evaporates.
3. The farmer checks the tube. When the water inside has dropped to about 15 cm below the soil surface, the field is flooded again, to about 5 cm of standing water.
4. This cycle repeats through the season.
5. The field is kept flooded around flowering time (about a week before to a week after the main flowering), because the plant is most sensitive to water stress then.

The 15 cm level is called "safe" because rice roots can still reach the wet soil just below. So the plant does not suffer, and yields are generally unchanged.

The key numbers

• Methane cut: The 2006 IPCC Guidelines for national greenhouse gas inventories assume that fields drained more than once cut methane by about 48% compared with continuous flooding. Field results vary with soil and climate; some studies report 30% to 70% cuts.
• Water saving: Usually about 15% to 30% less irrigation water.
• Yield: Generally unchanged under Safe AWD; sometimes slightly higher because roots grow deeper and stronger.
• Methane strength: Over 100 years, one tonne of methane causes about 27 to 30 times the warming of one tonne of carbon dioxide (IPCC Sixth Assessment Report). So even a small methane cut has a big climate value.

India's position and Indian examples

India is the world's largest rice exporter (since 2012) and one of the two largest producers. Rice production was estimated at about 149 million tonnes in 2024-25 (third advance estimates of the Ministry of Agriculture). Rice is grown on over 43 million hectares. So the way India waters rice matters a lot for both groundwater and methane.

• AWD is promoted under climate-smart farming programmes of ICAR and state governments, and has been tested in states such as Andhra Pradesh, Tamil Nadu, Punjab and Odisha.
• It fits with the National Mission for Sustainable Agriculture (NMSA), one of the missions under India's National Action Plan on Climate Change, which promotes water-use efficiency ("more crop per drop").
• Private companies and carbon-credit projects now pay some farmers for the methane they avoid by using AWD. These are voluntary carbon markets (markets where companies buy credits to balance their own emissions).

Commonly confused concepts

• AWD vs Direct Seeded Rice (DSR): AWD is about how you water the field after the crop is in. DSR is about how you plant: seeds are sown directly in the field instead of growing a nursery and transplanting seedlings into a puddled (muddy, flooded) field. DSR also saves water and labour and cuts methane, because the field is not flooded for so long. Punjab offered an incentive of ₹1,500 per acre for DSR from 2022. The two methods can be combined.
• AWD vs System of Rice Intensification (SRI): SRI is a full package of practices: young single seedlings planted widely apart, little water (soil kept moist, not flooded), and frequent weeding that churns the soil. It was developed in Madagascar in the early 1980s by French Jesuit priest Henri de Laulanié. AWD is only the water part; SRI includes AWD-style watering plus planting changes.
• AWD vs mid-season drainage: Mid-season drainage is a single dry spell in the middle of the season. AWD is repeated cycles of drying throughout the season.
• Methane vs nitrous oxide from rice: Flooded fields mainly give out methane. Drier fields can give out more nitrous oxide (N2O), another greenhouse gas that comes from fertiliser.

Issues, criticism and the way forward

• Trade-off with nitrous oxide: When soil dries, some fertiliser nitrogen turns into nitrous oxide. Studies show that the overall climate benefit still stays large, but fertiliser use must be managed carefully.
• Needs control over water: AWD works only where the farmer can decide when to flood. In canal systems with fixed water turns, or in rain-fed areas, it is hard to time.
• Weeds: Less standing water means more weeds, so labour or weedicide costs may rise.
• Free or cheap electricity: Where farmers get free power for tubewells, they have little money reason to save water.
• Fear and habit: Many farmers believe that a dry field means a poor crop. Training and visible demonstrations are needed.
• Grain quality: Some studies find drying lowers arsenic in grain but may raise cadmium in some soils. More local research is needed.
• Way forward: Experts suggest rewarding farmers for avoided methane (through carbon credits or government payments), better water pricing and metering, training through Krishi Vigyan Kendras, and linking AWD with DSR and good fertiliser management.

Concepts to Know

• Methane (CH4): A colourless gas made of carbon and hydrogen. It is the main part of natural gas. In the air it traps heat much more strongly than carbon dioxide, but it breaks down in about 12 years.
• Anaerobic conditions: A state with no oxygen. Flooded soil becomes anaerobic because water blocks the air.
• Methanogens: Tiny single-celled microbes that live without oxygen and produce methane as a waste product. They also live in cow stomachs and landfills.
• Aerenchyma: Air-filled spaces inside the stems and roots of water plants like rice. They carry oxygen down to the roots, but also let methane escape up into the air.
• Global Warming Potential (GWP): A score that tells how much heat a gas traps compared with the same weight of carbon dioxide over a fixed time (usually 100 years). Carbon dioxide has a GWP of 1.
• CO2-equivalent (CO2e): A way to add up different greenhouse gases in one unit. For example, 1 tonne of methane is about 27 to 30 tonnes CO2e.
• Puddling: Ploughing a flooded field to make soft mud before transplanting rice seedlings.
• Carbon credit: A certificate showing that one tonne of CO2e has been avoided or removed. Companies can buy these to balance their own emissions.