Alternate wetting and drying (AWD) technique has been widely reported to reduce methane (CH₄) emission from irrigated rice paddies. However, little is known about its feasibility in an environment that has distinct weather conditions involving tropical wet season (WS). To investigate the AWD’s feasibility in terms of reducing greenhouse gas (GHG) emissions both in dry season (DS) and WS, 3-year field experiments were conducted in Central Luzon, Philippines. Three treatments of water management were continuous flooding (CF), flooding when surface water level naturally declines to 15 cm below the soil surface (AWD), and site-specific AWD that modified the criteria of soil drying (AWDS). Methane and nitrous oxide (N₂O) fluxes were measured using a closed chamber method, and the global warming potential (GWP) of the two GHGs was calculated. Rice grain yield did not significantly differ among treatments. In accordance with the previous findings, the seasonal total CH₄ emission was significantly greater in WS than in DS. The effect of treatment was significant, but the reduction rate by AWD was just 1.7% compared to CF. The seasonal total N₂O emission was significantly affected by cropping season and treatment. The AWD increased the N₂O emission by 97%, especially in DS. The resultant GWP did not significantly differ among three treatments. The results indicate that the AWD and AWDS with the current settings were insufficient to reduce the annual GHG emissions in this site. However, fragmentary results obtained in the last DS suggest that an earlier rice residue incorporation and keeping dry soil conditions in the preceding fallow season is effective in reducing CH₄ emission in combination with an earlier implementation of AWD.

Reducing methane (CH₄) emission from paddy rice production is an important target for many Asian countries in order to comply with their climate policy commitments. National greenhouse gas (GHG) inventory approaches like the Tier-2 approach of the Intergovernmental Panel on Climate Change (IPCC) are useful to assess country-scale emissions from the agricultural sector. In paddy rice, alternate wetting and drying (AWD) is a promising and well-studied water management technique which, as shown in experimental studies, can effectively reduce CH₄ emissions. However, so far little is known about GHG emission rates under AWD when the technique is fully controlled by farmers. This study assesses CH4 and nitrous oxide (N₂O) fluxes under continuous flooded (CF) and AWD treatments for seven subsequent seasons on farmers’ fields in a pumped irrigation system in Central Luzon, Philippines. Under AWD management, CH₄ emissions were substantially reduced (73% in dry season (DS), 21% in wet season (WS)). In all treatments, CH₄ is the major contributor to the total GHG emission and is, thus, identified as the driving force to the global warming potential (GWP). The contribution of N₂O emissions to the GWP was higher in CF than in AWD, however, these only offset 15% of the decrease in CH₄ emission and, therefore, did not jeopardize the strong reduction in the GWP. The study proves the feasibility of AWD under farmers’ management as well as the intended mitigation effect. Resulting from this study, it is recommended to incentivize dissemination strategies in order to improve the effectiveness of mitigation initiatives. A comparison of single CH₄ emissions to calculated emissions with the IPCC Tier-2 inventory approach identified that, although averaged values showed a sufficient degree of accuracy, fluctuations for single measurement points have high variation which limit the use of the method for field-level assessments.

Combination of a pre-season wet soil condition and rice straw incorporation just before transplanting, which is typical for a tropical rice double cropping, can induce a flash of methane (CH₄) emission shortly after the transplanting. The conventional practice of alternate wetting and drying (AWD) irrigation technique that typically starts at 21 days after transplanting (DAT) can hardly reduce this emission because the soil become methanogenic before the onset of AWD treatment. Field experiments were conducted in Central Luzon, Philippines, during the 2014–2017 dry rice seasons to examine the effects of the timing of rice straw/stubble incorporation on the efficacy of AWD in reducing the CH₄ emission. Two treatments of the timing of stubble incorporation were stubbles incorporated during the start of wet land preparation (S1) and stubbles incorporated during the dry fallow tillage (S2). For the water management, we compared two treatments: continuous flooding (CF) and AWD with – 15 cm threshold for irrigation. The AWD under S2 was implemented earlier at 10 DAT. We observed a significant interaction (p < 0.01) between effects of AWD and straw management on CH₄ emissions; the seasonal total CH₄ emission was reduced by AWD compared with CF by 73% under S2, while the reduction was <20% under S1. The AWD significantly (p < 0.05) increased the nitrous oxide (N₂O) emissions by 47 and 48% relative to CF under S1 and S2, respectively. The global warming potential (GWP, CH₄ + N₂O) and yield-scaled GWP were still substantially lower by 62 and 59%, respectively, in AWD than in CF under S2, but the reduction was not realized under S1 due to the relatively smaller CH₄ reduction and increased N₂O emission. The results confirm that pre-season aerobic stubble decomposition and earlier implementation of AWD enhanced AWD’s mitigation potential in reducing substantially the CH₄ emission from the tropical rice double-cropping system.

Alternate Wetting and Drying (AWD) is a well-known low-cost water-saving and climate change adaptation and mitigation technique for irrigated rice. However, its adoption rate has been low despite the decade of dissemination in Asia, especially in the Philippines. Using cross-sectional farm-level survey data, this study empirically explored factors shaping AWD adoption in a gravity surface irrigation system. We used regression-based approaches to examine the factors influencing farmers’ adoption of AWD and its impact on yield. Results showed that the majority of the AWD adopters were farmers who practiced enforced rotational irrigation (RI) scheduling within their irrigators’ association (IA). With the current irrigation management system, the probability of AWD implementation increases when farmers do not interfere with the irrigation schedule (otherwise they opt to go with flooding). Interestingly, the awareness factor did not play a significant role in the farmers’ adoption due to the RI setup. However, the perception of water management as an effective weed control method was positively significant, suggesting that farmers are likely to adopt AWD if weeds are not a major issue in their field. Furthermore, the impact on grain yields did not differ with AWD. Thus, given the RI scheduling already in place within the IA, we recommend fine-tuning this setup following the recommended safe AWD at the IA scale.