Irrigated rice is the largest user of precious global water reserves. Adoption of water-saving irrigation practices is limited by the associated increased labor demand compared to flooded rice cultivation. Automated gravity surface irrigation systems have shown the potential to deliver significant labor savings in traditional flooded rice; however, widespread adoption does not seem apparent. Furthermore, previously designed systems have not been capable of irrigation control during both ponded and non-ponded periods. This study aimed to evaluate the performance of an automated irrigation system for rice with features not previously developed, provide direction for future systems and analyze the opportunity cost (the value of other on- or off-farm activities that could be conducted with that time) of time associated with automated irrigation. The automated irrigation system was found to successfully control 23–31 flush-irrigation events per bay per season in a 9-bay border-check aerobic rice field for 2 seasons. In addition, successful water control was achieved in a traditional drill-sown field with 4 flush irrigations followed by 15 weeks of permanent flooding. Labor savings of 82–88% during the flush-irrigation events and 57% during the ponding period were achieved with automation when compared to manual irrigation. However, the opportunity cost of the saved time was found to comprise the greatest benefit. Changing the analysis from using a flat “cash” cost of time to using opportunity cost of time reduced the payback period from seven to four years at the traditional ponded-rice site. In the more labor-intensive aerobic rice site, the payback period was reduced from three years to one year when accounting for the opportunity cost of time as opposed to only the direct costs. Whilst the payback period is site-dependent and cultivation method-dependent, these case studies demonstrate that automated gravity surface irrigation can enable novel water-saving practices in rice and provide substantial economic benefits.

Declining water availability is pressing rice growers to adopt water-saving irrigation practices such as aerobic rice to maintain profitability per megalitre (ML) of water input. Irrigators require well-defined irrigation thresholds to initiate irrigation to maximise water productivity. Such thresholds do not exist for temperate rice regions. Adopting a strategy that has been reported to succeed in non-temperate environments may fail in temperate climates, and therefore, needs investigation. This study aimed to investigate, in a temperate Australian environment, the effect of increasing soil moisture deficit during the rice vegetative period on crop physiological development, grain yield and water productivity. The study was conducted in a commercial farm using a randomised complete block design in the 2020/21 and 2021/22 growing seasons. Automated gravity surface irrigation technologies were adopted to enable high-frequency irrigation. Extending soil moisture deficit beyond 15 kPa was found to significantly delay panicle initiation by at least 13–14 days, exposing rice to cold temperatures in Year 1 during the cold-sensitive early pollen microspore period. This reduced yield by up to 55% (4.5 t/ha) compared to the 15 kPa treatment that was not impacted by cold sterility. In the absence of cold sterility, irrigated water productivity and total water productivity ranged between 1.02 and 1.61 t/ML, and 0.84 and 0.93 t/ML, respectively. The highest yields (8.1 and 7.5 t/ha) were achieved irrigating at a soil tension of 15 kPa in growing seasons 2020/21 and 2021/22. This research demonstrates that sound water productivity can be achieved with aerobic rice cultivation in temperate climates, providing cold temperatures during early pollen microspore are avoided. The quantification of the delay in crop development caused by increasing soil moisture deficit provides rice farmers greater confidence in determining the irrigation strategy and timing of pre-emergent irrigation in regions at risk of cold sterility. However, due to the high labour demand associated with aerobic rice, the adoption of aerobic rice at a commercial scale in this Australian environment is unlikely without adopting automated irrigation technology.

Aerobic rice cultivation offers the potential to reduce irrigated water use. A multitude of challenges, such as cold sterility, drought stress, and labor shortages, limit its adoption in temperate rice-growing regions. Increasing the duration and extent of soil moisture tension between irrigation events has been demonstrated to slow crop development. Delaying panicle initiation (PI) beyond the optimal window can expose rice to cold nighttime temperatures during the cold sensitive early pollen microspore, severely reducing yield. Tools to assist Australian temperate farmers and researchers in the irrigation management of aerobic rice to ensure PI occurs during the optimal window do not yet exist. Using data collected from an aerobic rice experiment conducted in temperate Australia in 2020–2021 and 2021–2022, a predictive model was built to assist in forecasting PI based on the timing of irrigation. Estimation of the area on an hourly basis of the cumulative evapotranspiration with rainfall subtracted from pre-emergent irrigation to PI, defined as the irrigation deficit integral, was used to account for the frequency, duration, and extent of soil moisture deficit between irrigation events. The relationship between the irrigation deficit integral and the number of days from pre-emergent irrigation to PI (R² = 0.91) was used to build a model to predict PI with a root mean square error of 1.8 days for the validating data set. Furthermore, an example is provided of how the model can be used as a decision support tool to assist researchers and growers to schedule irrigation of aerobic rice to ensure PI occurs in a timely manner. This will increase the likelihood of high-yielding aerobic rice and may enhance the adoption of water-saving rice cultivation.