Assessing Climate Change Impacts on Second-Crop Soybean Yield and Irrigation Requirements Using AquaCrop in a Mediterranean Climate

This study assesses the response of second-crop soybean to future climate change by modeling yield and water-related indicators under three irrigation methods—drip, subsurface drip, and furrow—considering limited water availability. Field experiments were conducted at the International Agricultural Research and Training Center in western Türkiye during 2018 and 2019. The 2018 dataset was used for AquaCrop model calibration and the 2019 dataset for validation. The FAO AquaCrop model demonstrated strong performance (calibration: R² = 0.89, RMSE = 124.9 kg ha⁻¹, NRMSE = 3.65%, EF = 0.79, d = 0.94; validation: R² = 0.93, RMSE = 86.6 kg ha⁻¹, NRMSE = 2.42%, EF = 0.87, d = 0.97). Future projections for mid-century (2041–2060) and late-century (2061–2080) were based on climate inputs from HADGEM2-ES, MPI-ESM-MR, and GFDL-ESM2M global circulation models under RCP4.5 and RCP8.5 scenarios. Results indicate notable temperature increases and variable precipitation trends across models and periods. Irrigation choice significantly affected net irrigation requirements (NIR), which increased by 5–27% under RCP4.5 and 10–33% under RCP8.5. Crop evapotranspiration (ETc) rose by 6–15% under RCP4.5 and 12–20% under RCP8.5, while water productivity (WP) improved across all irrigation methods. WP gains ranged from 8 to 22% under RCP4.5 and 12–28% under RCP8.5, with subsurface drip irrigation achieving the highest efficiency. Overall, although climate change will elevate soybean water demand, enhanced CO₂ concentrations may improve WP, partially offsetting these impacts. Subsurface drip irrigation appears to be the most effective adaptation strategy for maintaining high yields and optimizing water use under future climatic conditions.