Alternate wetting drying (AWD) is a water management technique, practiced in cultivating irrigated lowland rice with much less water than the usual system of maintaining continuous standing water in the crop field. It is a technique of controlled and intermittent irrigation.
Alternate wetting and drying is also known as intermittent flooding. AWD is the practice of flooding and drying the field. It was first developed at the International Rice Research Institute (IRRI). This method has become a recommended practice in water scarce irrigated rice areas in South and Southeast Asia.
IRRI has been promoting alternate wetting and drying as a smart water saving technology for rice cultivation through national agricultural research and extension in different countries.
A periodic drying and re-flooding irrigation scheduling approach is followed in which the fields are allowed to dry for few days before re-irrigation, without stressing the plants. This method decreases water demand for irrigation and greenhouse gas emissions without reducing crop yields.
As a rice flood managing practice, AWD is used to maximize rainfall capture and reduce irrigation pumping while maintaining grain quality and yield. AWD includes of flooding a field to a reasonable depth and allowing the flood to naturally subside to the soil surface via infiltration and evapotranspiration.
The timing, regularity, and extent of the wetting and drying cycles depend on rice growth stage, prevailing weather, field conditions and grower experience with the practice. After holding the initial flood for 3 weeks, it is common to refrain from applying a flood for five or more days between wet-dry cycles when using AWD.
Again, flood is maintained at panicle initiation (green ring) and at flowering, when rice is most sensitive to water stress. Properly managed AWD should not affect nutrient management in regard to rates and timings of fertilizer application.
Management of alternate wetting drying technique
A continuous flood should preserve well saturated soils for a full three weeks following reflood N application to ensure efficient N uptake by rice plants. Both methane gas emissions and arsenic levels in grain are reduced when AWD flooding is practiced where the soil becomes aerobic for a short period of time.
Potential risks reduced grain yield or quality may result from water stress or reduced control of pests, particularly grasses and diseases. Water stress will occur if the field is allowed to dry too much or if the flood is not re-established in a timely manner, as can occur with undersized wells, irrigation system failure and human error.
Late-planted rice (late May and June) is susceptible to disease and should not be managed using AWD flooding. A water tube/pipe made of PVC is usually used to practice AWD method. The main purpose of the tube is to monitor the water depth. The tube allows measuring water accessibility in the field below the soil surface.
The usual practice is to use a pipe of 7–10 cm diameter and 30-40 cm long, with perforations in bottom 20 cm. The pipe is fitted in such a way that the bottom 20 cm of perforated portion remains below the soil surface and the non-perforated 10 cm above the surface.
The perforations permit the water to come inside the tube from the soil, where a scale is used to measure water depth below the soil surface. However, there are variations in preparing the tube/pipe for the implementation of AWD. Some farmers use a bamboo pipe instead of PVC pipe. Some farmers use a 30-cm tube with 15 cm perforated at the bottom.
After the irrigation in the crop field the water depth gradually declines because of evapotranspiration, seepage, and percolation. Because of the fixed tubes in the field, it is possible to monitor the water depth below the soil surface up to 15–20 cm. When the water level drops 15- 20 cm below the soil surface, irrigation should be applied in the field to re-flood to a depth of 5 cm.
During flowering stage of rice
During the flowering stage of the rice, the field should be kept flooded. After flowering, during the mid-season and late season (grain filling and ripening stages), the water level is allowed to drop below the soil surface to 15 cm before re-irrigation.
To suppress the growth of weeds in the rice field, AWD method should be followed 1–2 weeks after the transplantation. In the case of many weeds in the field, AWD needs to be started after three weeks of transplantation. Application of nitrogen fertilizer is preferable on dry soil just before re-irrigation.
To ensure a similar dry or wet condition throughout the crop field, which is essential to maintain good yield, it is important to level the rice field properly.
AWD method can save water by about 30-40% without adversely disturbing rice yield. This method increases water productivity by 15-20% compared with continuously flood irrigation. High-yielding rice varieties developed for continuously flood irrigation rice system still produce high yield under safe AWD.
This method can even increase grain yield because of enhancement in grain-filling rate, root growth and remobilization of carbon reserves from vegetative tissues to grains. AWD can reduce the cost of irrigation by reducing pumping costs and fuel consumption.
This method can also reduce the labor costs by improving field conditions at harvest, allowing mechanical harvest. AWD leads to firmer soil conditions at harvest, which is suitable to operate machines in the field. Therefore, AWD increases net return for farmers.
Several studies also show that AWD reduces methane (CH4) emissions. CH4 is formed by the anaerobic decomposition of the organic material in the flooded paddy field. Allowing to drop water level below soil surface removes the anaerobic condition for some time till re-flooded and pauses the production of CH4 from the rice field for several times and, hence, reduce the total amount of CH4 released during the rice growing season.
This method has been invented to reduce CH4 emissions by an average of 48% compared to continuous flooding in the 2006 IPCC methodology.
Alternate wetting and moderate soil drying reduce cadmium and arsenic accumulation in rice grains. This method can also reduce insect pests and diseases. Periodic soil drying may reduce the incidence of fungal diseases. So, in nut sell AWD not only saves dwindling water resources also environment friendly, economic and socially acceptable for common farmer.
This article is jointly written by Muhammad Ishfaq*, Khadija Murtza, Muhammad Ahmad Arain, and Nadeem Akbar from University of Agriculture, Faisalabad. Corresponding Author can be reached at Ishfaq2727@gmail.com