Performance of rice (Oryza sativa (L.)) under AWD irrigation practice-A brief review

Alternate wetting and drying (AWD) is an irrigation technique where water is applied to the field a number of days after disappearance of ponded water. This is in contrast to the traditional irrigation practice of continuous flooding (i.e., never letting the ponded water disappear). This means that the rice fields are not kept continuously submerged but are allowed to dry intermittently during the rice growing stage. The number of days where the field is allowed to be "non-flooded" before irrigation is applied can vary from 1 to more than 10. The underlying premise behind this irrigation technique is that the roots of the rice plant are still adequately supplied with water for some period (due to the initial flooding) even if there is currently no observable ponded water in the field. There is a specific form of AWD called "Safe AWD" that has been developed to potentially reduce water inputs by about 30%, while maintaining yields at the level of flooded rice. In Safe AWD, the ponded water on the field (also called "perched water") is allowed to drop to 10-15 cm below the soil surface before irrigation is applied. The depth of perched water is monitored using a perforated or punctured water tube embedded in the soil. With the threshold of 10-15 cm, roots are still able to extract water from the perched water table and no stress to the plants develops.