Effect of alternate wetting and drying technology and seedling age on grain yield and water productivity of irrigated lowland rice (Oryza sativa L.): an experimental and modeling study

To help farmers cope with increasing water scarcity in rice production, developing strategies that increase water productivity is imperative. Alternate wetting and drying (AWD) is a mature water saving technology that saves water use without significant yield loss. Also, by shortening the crop growth duration in the main yield, water input is reduced. While most studies on water saving technology quantify the reductions in water input, very little research has been done to quantify the impact of AWD on the different outflows in the rice fields. This study combined field experiment and modeling approach to evaluate and simulate the effect of AWD at different threshold levels and the seedling age on grain yield, water productivity and water balance components of irrigated lowland rice. The experiment was set up in split-plot design on clay soil with four water regimes (continuously flooded CCF), AWRDIS, AWD25,AWD30) and 3 seedling ages (14,21, and 30 days) during the 2010 dry season in PhilRice Philippine Rice Research Inst.], Munoz, Nueva Ecija [Philippines]. The crop growth model ORYZA2010 was used to support the analysis of the field experiment and to perform scenario analysis at increasing groundwater depth. Under shallow groundwater conditions in PhilRice, grain, yield was not affected by water regimes but by seedling age and the interacting effect of the treatments (6,410 to 8670 kg/cu m). Twenty-one day seedlings (21 D) increased grain yield by 6 to 14% among treatments.The combination of AWD25 at 21 D gave the highest grain yield (8,320 kg/cu m) while AWD 30 at 30 D gave the lowest (6,410 kg/cu m). Compared with CF, water savings using AWD irrigation ranged from 43 to 54% without significant yield loss. Compared with 14D and 21D, water savings using 30D ranged from 11 to 19% with yield loss (6 to 14%). Water productivity in all the AWD water regimes (1.06 to 1.34 kg/cu m) was higher than CF (0.63 kg/cu m). Among seedling ages, 21 D gave the highest water productivity (1.18 kg/cum)and 30 D gave the lowest (0.93 kg/cu m). ORYZA2000 was calibrated for the variety used, and supported the analysis of the field experiment. The results of the modeling showed that with deep groundwater depth, grain yield slightly decreased by 10-11% due to reduced transpiration while water input did not change significantly. The simulated water balance components of rice revealed that evaporation in the AWD condition versus CF decreased by 23 to 25% while transpiration did not change among water regimes. The seasonal combined net contribution of stored soil water, capillary rise and deep percolation (deltaS) was higher in CF (491 to 513 mm) than all the AWD water regimes(260 to 283 mm). This study concludes that more water savings can be achieved by increasing the threshold level for irrigation of the Safe AWD (-15 cm) up to -25cm without significant yield loss, especially when this irrigation technique is combined with 21 day-old seedlings for transplanted rice. Increasing threshold level for irrigation in AWD and using younger seedlings of medium maturing variety for transplanted rice enhanced water productivity while maintaining high yield.