Abstract
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 (Dx) and spacings (Ly), 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 × 10−3 to 24.9 × 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-1 d-1, 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.
Highlights
The need to feed the growing global population on the one hand, and increasing global scarcity of blue water (WWAP, 2009; Hoekstra et al, 2012) on the other hand, indicate that it is essential to expand rainfed agriculture in the world
Field experiments and modeling analyses involving a subsurfacedrained field of rained-canola, as a winter crop, were carried out to evaluate the integrated influence of subsurface drainage and global warming on probable future water and N losses
Our quantitative assessment successfully evaluated the capability of the HYDRUS model to predict soil water contents, soil nitrate concentrations, and drainage/ nitrate fluxes for various drainage systems
Summary
The need to feed the growing global population on the one hand, and increasing global scarcity of blue (fresh) water (WWAP, 2009; Hoekstra et al, 2012) on the other hand, indicate that it is essential to expand rainfed agriculture in the world In this regard, humid regions may be of higher importance since they receive a sufficient quantity of precipitation to fully supply crop’s water requirements by green (from rainfall) water (Shahsavari et al, 2019). Waterlogging threatens the year-round cropping, resulting in considerable areas either going out of production or experiencing reduced yields (Darzi-Naftchali et al, 2013) Under such circumstances, installing subsurface drainage systems may help in providing suitable conditions for winter-crops-based cropping systems and improving the annual productivity of these lands. Subsurface drainage systems can speed up the water table drawdown and provide better aeration during the growing season
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