Quantifying winter wheat evapotranspiration and crop coefficients under sprinkler irrigation using eddy covariance technology in the North China Plain

Sprinkler irrigation has been widely used for winter wheat crops in the North China Plain (NCP) to maintain high crop yield and enhance water use efficiency because water resources are scarce. Determining the accurate crop evapotranspiration (ETc) and crop coefficient (Kc) of winter wheat is critical for optimizing sprinkler irrigation management. In this study, an eddy covariance system was deployed in a sprinkler irrigation field to measure the energy balance components (net radiation Rn, soil heat flux G, sensible heat H, latent heat LE) and calculate ETc and Kc. The wheat growth parameters and yield were also investigated. The results showed that the energy balance closure rate, the rate of daily (LE + H) to (Rn -G), was 0.89, indicating a sufficient energy balance closure. Seasonal total LE flux accounted for 67% of the total available energy (Rn -G). The seasonal ETc values were 434 and 396 mm during the 2019-2020 and 2020-2021 growing seasons, respectively, which are com-parable to those (400-470 mm) under surface irrigation. The mean daily ETc was 0.87, 0.46, 3.24, 4.81, and 3.94 mm day-1 in the early growth, overwintering, rapid growth, middle, and late growth stages, respectively. The corresponding Kc values were 0.77, 0.47, 1.09, 1.14 and 0.70. The Kc values were close to those reported in the FAO 56 paper. The Kc is positively related to the leaf area index (LAI), and their relationship was fitted as Kc = 0.87 + 0.14ln(LAI) (R2 = 0.96) for an LAI range of 1-7, where ln is a natural logarithm function. The measured wheat grain yield was 9100 and 9857 kg ha-1, and the water productivity (WP) was 2.07 and 2.49 kg m- 3 in the two seasons, respectively. Both factors are approximately 30% higher than those (7200 kg ha-1, 1.68 kg m-3) reported by other studies. Considering the high wheat yield and comparable crop ETc, sprinkler irrigation is recommended for application on the NCP. The seasonal ETc of 400-430 mm and the obtained Kc values in this study can be used to make sprinkler irrigation scheduling for winter wheat on the NCP.