In alternate wetting and drying (AWD), irrigation water is applied to obtain flooded conditions after a certain number of days have passed after the disappearance of ponded water. AWD is also called intermittent irrigation or controlled irrigation.
The number of days of nonflooded soil in AWD before irrigation is applied can vary from 1 day to more than 10 days.
A practical way to implement AWD is to monitor the depth of the water table on the field using a simple perforated field water tub. After an irrigation application, the field water depth will gradually decrease in time.
When the water level (as measured in the tube) is 15 cm below the surface of the soil, it is time to irrigate and flood the soil with a depth of around 5 cm. Around flowering, from 1 week before to one week after the peak of flowering, ponded water should be kept at 5 cm depth to avoid any water stress that would result in potentially severe yield loss.
The threshold of 15 cm is called “Safe AWD” as this will not cause any yield decline since the roots of the rice plants will still be able to take up water from the saturated soil and the perched water in the rootzone.
The field water tube helps farmers see this “hidden” source of water. In Safe AWD, water savings may be relatively small, in the order of 15%, but there is no yield penalty.
After creating confidence that Safe AWD does not reduce yield, farmers may experiment by lowering the threshold level for irrigation to 20, 25, 30 cm, or even deeper.
Some yield penalty may be acceptable when the price of water is high or when water is very scarce.