Evapotranspiration and crop coefficients of sprinkler-irrigated aerobic rice in southern Brazil using the SIMDualKc water balance model

Flooding rice (Oryza sativa L.) is commonly used in lowlands of Southern Brazil. The sustainability of this production system is threatened by an increase in the production costs, excessive water use and an incipient plateau in yield due to biophysical limitations. Sprinkler irrigation is considered a feasible strategy to improve crop and water productivity but requires proper assessment since advances in irrigation scheduling and crop performance are not yet conclusive. A study was, therefore, developed to evaluate water use and overall growth performance of two sprinkler-irrigated rice cultivars. Field experiments from 2020/21 to 2022/23 growing seasons compared three treatments: (i) daily irrigation (DI), (ii) irrigation in alternate days (AI), and (iii) rainfed (RF). The season water applied for the DI treatments were 532 mm in 2020/21, 514 mm in 2021/22 and 614 mm in 2022/23, and for AI they were 490 mm, 474 mm and 621 mm for the same crop seasons. Soil water content was maintained close to field capacity in the irrigated plots. Soil moisture was daily measured using FDR sensors. Measurements of crop growth parameters were taken throughout the growing seasons. The SIMDualKc model was used to assess evapotranspiration and derive crop coefficients for each treatment. The model was calibrated with the 2020/21 DI treatment and validated with all other sets of the three growing seasons. The model accurately simulated the soil water dynamics in the root zone, with RMSE < 4.4 mm and normalized RMSE < 7%. The calibrated basal crop coefficients adjusted to climate were 0.15, 1.08(± 0.01) and 0.80(± 0.01), while the single crop coefficients (K_c) were 0.72(± 0.04), 1.13(± 0.01), and 0.95 (± 0.01) for the initial, mid-season, and end-season stages, respectively. Grain yield was 7600 (± 1400) and 7200 (± 1800) kg ha⁻¹, with daily and alternate days irrigation, respectively, which are comparable with flooding irrigation. Results showed that sprinkler irrigation may be a proficient water-saving technology, without decreasing yields and contributing to improve the sustainability of rice systems in Southern Brazil.