Multi-year water and carbon flux contrasts between high-yielding and conventional rice cultivars

• Canopy-scale fluxes of two rice varieties were compared under field conditions for three years. • The high-yielding variety showed +24 % GPP with only ≈+10 % increase in seasonal ET. • Greater WUE was driven by early LAI development and extended canopy photosynthesis. • Rice varietal choice, along with associated management differences, may shift water–carbon flux contrasts. • Flux-based varietal comparisons offer insights for irrigation planning and climate adaptation. Cultivation of high-yielding indica rice varieties has been spreading in Japan. This trend raises concerns about their potential impacts on water use and the carbon cycle. To this end, eddy covariance flux measurements were conducted from transplanting to harvest over three years for a high-yielding rice variety, Oonari, and compared with a conventional japonica rice variety, Koshihikari, in farmers’ fields under actual management practices. Evapotranspiration (ET) was higher for Oonari in 2018 and 2019, being 9.6 % higher than Koshihikari (2018: Oonari, 549.6 mm; Koshihikari, 501.2 mm; 2019: Oonari, 472.0 mm; Koshihikari, 430.5 mm). In 2020, ET for Oonari was 6.0 % smaller due to a shorter growth period (Oonari, 426.5 mm; Koshihikari, 453.9 mm). Gross primary production (GPP) was significantly higher for Oonari, averaging 24 % more than Koshihikari (Oonari: 1175.2±76.5 g C m –2 , Koshihikari: 946.5±32.7 g C m –2 ). Higher ET and GPP of Oonari were reflected in greater crop coefficients and radiation use efficiency. These characteristics are mainly due to Oonari's higher leaf area index and extended photosynthetic period, highlighting the importance of field-scale evaluations throughout the entire growth period. Although both ET and GPP were greater in Oonari than in Koshihikari, the more pronounced difference in GPP indicates that cultivating Oonari has a greater impact on the carbon cycle than on water use.