The truth about alternate wetting and drying

This AWD field at the Bangladesh Agricultural Development Corporation's Madhupur Farm received only 5 irrigations and produced paddy yield of 8.4 tons per hectare. Meanwhile, the nearby conventionally flooded field had 9 irrigations and yielded 8.1 tons per hectare. (Photo by M.A. Mannan)

In alternate wetting and drying (AWD), the field is allowed to dry for a certain number of days before applying irrigation water. The number of days that the soil is left dry 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 in the field using a perforated or punctured water tube. After an irrigation application, the field water depth will gradually decrease in time. When the water level (as measured in the tube) is 15 centimeters below the surface of the soil, it is time to irrigate and flood the soil with a depth of around 5 centimeters. Around flowering,
from 1 week before to 1 week after the peak of flowering, ponded water should be kept at a 5-centimeter depth to avoid any water stress that would result in potentially severe yield loss.

The threshold of 15 centimeters is called safe AWD, because this will not cause yield decline since the roots of the rice plants will take up water from the saturated soil and the perched water in the roots. The field water tube helps farmers see this “hidden” source of water.

In safe AWD, water savings may be relatively small, around 15%, but yield will not be smaller. After creating confidence that safe AWD does not reduce yield, farmers may experiment by lowering the threshold level for irrigation to 20, 25, or 30 centimeters, or even deeper. A decrease in yield may be acceptable when the price of water is high or when water is very scarce.

In safe AWD, the following rules should be observed. AWD irrigation can be used from a few days after transplanting (or a 10-centimeter-tall crop after direct seeding) until first heading. In the period of first heading to 1 week after flowering, keep the field flooded at a 5-centimeter depth. After that, during grain filling and ripening, apply AWD again. When many weeds are present in the early stages of crop growth, the implementation of AWD can be postponed for 2–3 weeks until weeds have been suppressed by the ponded water.

Under safe AWD, no special nitrogen (N) management
routine is needed and local recommendations for flooded rice can be used. Apply N fertilizer preferably on the dry soil just before irrigation is applied.

Bas Bouman (b.bouman@cgiar.org)

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