Modeling Water and Nitrogen Balance of Different Cropping Systems in the North China Plain

The North China Plain (NCP) is experiencing serious groundwater level decline and groundwater nitrate contamination due to excessive water pumping and application of nitrogen (N) fertilizer. In this study, grain yield, water and N use efficiencies under different cropping systems including two harvests in 1 year (winter wheat−summer maize) based on farmer (2H1Y)^FP and optimized practices (2H1Y)^OPT, three harvests in 2 years (winter wheat−summer maize−spring maize, 3H2Y), and one harvest in 1 year (spring maize, 1H1Y) were evaluated using the water-heat-carbon-nitrogen simulator (WHCNS) model. The 2H1Y^FP system was maintained with 100% irrigation and fertilizer, while crop water requirement and N demand for other cropping systems were optimized and managed by soil testing. In addition, a scenario analysis was also performed under the interaction of linearly increasing and decreasing N rates, and irrigation levels. Results showed that the model performed well with simulated soil water content, soil N concentration, leaf area index, dry matter, and grain yield. Statistically acceptable ranges of root mean square error, Nash−Sutcliffe model efficiency, index of agreement values close to 1, and strong correlation coefficients existed between simulated and observed values. We concluded that replacing the prevalent 2H1Y^FP with 1H1Y would be ecofriendly at the cost of some grain yield decline. This cropping system had the highest average water use (2.1 kg m^−3) and N use efficiencies (4.8 kg kg^−1) on reduced water (56.64%) and N (81.36%) inputs than 2H1Y^FP. Whereas 3H2Y showed insignificant results in terms of grain yield, and 2H1Y^FP was unsustainable. The 2H1Y^FP system consumed a total of 745 mm irrigation and 1100 kg N ha^−1 in two years. When farming practices were optimized for two harvests in 1 year system (2H1Y)^OPT, then grain yield improved and water (18.12%) plus N (61.82%) consumptions were minimized. There was an ample amount of N saved, but water conservation was still unsatisfactory. However, considering the results of scenario analyses, it is recommended that winter wheat would be cultivated at <200 mm irrigation by reducing one irrigation event.