Estimation of Actual Evapotranspiration and Its Components at Hourly and Daily Scales Using Dual Crop Coefficient Method for Water-Saving Irrigated Rice Paddy Field

Accurately partitioning actual evapotranspiration ETc act into soil evaporation Es and plant transpiration Tc act is crucial for improving water use efficiency and devising precise irrigation schedules. In water-saving irrigated rice fields, ETc act, Es and Tc act were estimated using a dual crop coefficient method based on three approaches: FAO56 adjusted, locally calibrated and leaf area index LAI-based coefficients. Continuous measurements of hourly and daily ETc act, Es and Tc act with weighing lysimeters were used to validate these coefficients. Results showed that hourly ETc act, Es and Tc act exhibited a distinct inverted “U” shape single-peak trend. Daily ETc act and Tc act, along with the corresponding crop coefficients Kc act and basal crop coefficients Kcb act, initially increased and then decreased throughout the rice growth stages, while daily Es and soil evaporation coefficient Ke act were high during the initial stage and gradually decreased as the development stage progressed. FAO56 adjusted coefficients consistently underestimated both hourly and daily ETc act, Es and Tc act. Locally calibrated basal crop coefficients Kcb Cal were determined as 0.28, 1.17 and 1.09 for the initial, mid-season and end-season stages, respectively, and locally calibrated turbulent transport coefficient of water vapor Kcp Cal (recommended as 1.2 by FAO) was determined to be 1.59. Based on these calibrated coefficients, estimates of hourly and daily evapotranspiration ETc Cal, soil evaporation Es Cal and plant transpiration Tc Cal performed poorly during the initial stage but showed improved accuracy during subsequent growth stages. Hourly and daily evapotranspiration and its components based on LAI-based coefficients exhibited similar performance in estimating measurements, albeit slightly inferior to FAO56 calibrated coefficients. Overall, both the FAO56 calibrated coefficients and LAI-based coefficients are recommended for estimating evapotranspiration and its components at daily and hourly scales. These research findings provide valuable insights for optimizing irrigation regimes and improving water use efficiency in rice cultivation.