Impacts of alternate wetting and drying and delayed flood rice irrigation on growing season evapotranspiration

•Measured ET in AWD and conventional DF irrigated rice fields was similar.•As canopy developed through the growing season, ET observations became more similar regardless of irrigation treatment.•The PM-AET method was able to suitably estimate ET across irrigation regimes. As rice production is water intensive, establishing an accurate field–scale water budget is paramount for sustainable use of local water resources. This study’s goal was to quantify and characterize half-hourly and seasonal evapotranspiration (ET) in two commercial, zero–grade rice fields in the U.S. Mid–South over three growing seasons. During each growing season, irrigation regimes for the studied fields differed between alternate wetting and drying (AWD) and delayed flooding (DF). The 2015 growing season enabled a direct comparison of the effects of AWD and DF on ET, while during the 2016 and 2017 seasons both fields were simultaneously under AWD and DF, respectively. The DF method is the region’s most common irrigation practice, and it prescribes a continuous flood to be maintained for the majority of the growing season after the plants have reached the 5-leaf growth stage (40–50 days after planting). In contrast, after holding this initial flooding for 3 weeks, AWD allows for field drying to promote the capture of seasonal rains to reduce irrigation water withdrawal and associated water pumping costs. In this study, ET was estimated using gap–filled eddy covariance observations and two variations of the Penman–Monteith equation. These methods determined growing season ET values between 560 mm and 636 mm. This study found that there were no significant differences in cumulative ET or yield when comparing AWD to DF practices. Furthermore, AWD elicited no change in ET during periods of drying when compared to DF. By this metric, AWD did not induce drought stress within the plants. We conclude that the main benefit of the AWD practice is to take advantage of seasonal rainfall to offset pumping costs and pressure on irrigation water requirements while maintaining yields comparable to conventional irrigation practices.