Dynamic assessment of the impacts of global warming on nitrate losses from a subsurface-drained rainfed-canola field

The impact of global warming on water and nitrate losses from a rainfed-canola cropping system under various artificial drainage systems was assessed using an integrated field-modeling approach. Four subsurface drainage systems with different drain depths (D-x) and spacings (L-y), including D0.90L30, D0.65L30, D0.65L15, and Bilevel (with a drain spacing of 15 m and alternate drain depths of 0.65 and 0.90 m), were considered. The HYDRUS (2D/3D) model was first calibrated and validated using data collected for all drainage systems during the 2015-2016 and 2016-2017 canola cropping cycles, respectively, and then applied to simulate water/nitrate losses for different drainage systems under meteorological conditions predicted assuming expected future global warming. Future weather data were downscaled from 20 general circulation models and four RCP scenarios for the mid 21st century (for 2041-2070). The model capability of representing experimental field data was evaluated using the mean bias error (MBE), the normalized root mean square error (nRMSE), and the model efficiency (EF). The HYDRUS (2D/3D) model provided reliable description of soil water contents (MBE= -0.5 % to 0.2 %, nRMSE = 0.005 - 0.034%, and EF = 0.73 - 0.99), drainage fluxes (MBE= -21.7 x 10(-3) to 24.9 x 10(-3) mm( )d(-1), nRMSE = 0.23 - 0.37%, and EF = 0.69 - 0.85), soil nitrate concentrations (MBE= -0.002 to 1.00 mg cm(-3), nRMSE = 0.08 - 0.18%, and EF = 0.51 - 0.88), and nitrate fluxes (MBE= -0.97 to 0.72 mg cm 4 nRMSE = 0.35 - 0.57%, and EF = 0.77 - 0.87). The modeling results indicate that climate change will cause an increase of up to 148 % in average daily drainage fluxes and up to 125 % in average daily nitrate fluxes compared to the base case. This will result in an increase of 4-125 % in seasonal nitrate losses from various drainage systems, with the lowest and highest projections for the D0.65L15 and D0.65L30 systems, respectively. The HYDRUS-simulated results indicate that the D0.65L15 system is environmentally safer than the other evaluated drainage systems for predicted global warming conditions concerning water/nitrate losses.