Water management in irrigated rice

79 Mill. ha of irrigated lowland rice provide 75% of the world‘s rice production.
Rice can be raised in a separate seedbed and subsequently transplanted into the rice field or established by direct wet seeding (broadcasting pregerminated seeds onto wet soil) or direct dry seeding (broadcasting dry seeds onto dry or moist soil).
Before crop establishment, the field is prepared under wet conditions: soaking, plowing and puddling (i.e., harrowing or rotavating under shallow submerged conditions). Puddling is done to control weeds, to reduce soil permeability, and to ease transplanting.
Total seasonal water input to rice fields (rainfall plus irrigation) can be up to 2-3 times more than for other cereals such as wheat or maize (Tuong et al. 2005, in: Bouman et al. 2007).

One possibility to save water in lowland rice is the „Alternate Wetting and Drying (AWD)-Technology“.
In AWD the soil is nonflooded for 1 to more than 10 days before irrigation water is applied again.
A practical way to implement AWD is to monitor the depth of ponded water on the field using a „field water tube“. When the ponded water has dropped to 15 cm below the surface of the soil, irrigation should be applied to re-flood the field with 5 cm of ponded water.
From 1 week before to 1 week after flowering, ponded water should always be kept at 5 cm depth.
Impact on N fertilizer application: Urea- and NH4-based fertilizer should be applied preferably on the dry soil just before irrigation. NO3-based fertilizer should be applied after flooding.

Water management in irrigated rice. Coping with water scarcity. IRRI, Los Banos, Philippines. (Bouman et al, 2007)