Modelling groundwater level dynamics under different cropping systems and developing groundwater neutral systems in the North China Plain

Over-exploitation of groundwater for irrigation has led to a series of ecological and environmental problems in the North China Plain (NCP). Identifying the water consumption and groundwater level dynamics under different cropping systems can help to develop groundwater neutral system in the NCP. The WHCNS (soil Water, Heat, Carbon and Nitrogen Simulator) model was applied to quantify the effects of different cropping systems (2H1Y, two harvests in one year; 3H2Y, three harvests in two years; and 1H1Y, one harvest in one year) on groundwater use and crop growth, and to explore the trade-offs of possible scenarios on the decline of groundwater level and cereal yield. Results showed that WHCNS performed well in simulating soil water content, leaf area index, dry matter and crop yield, as well as groundwater level dynamics, with the Nash and Sutcliffe Efficiency > 0.4 and index of agreement > 0.8. The simulated results indicated that the groundwater levels of 2H1Y decreased faster than those of other cropping systems, at a decline rate of 0.33 m yr⁻¹. Irrigation of 300 mm yr⁻¹ for the remaining high yield of winter wheat mainly resulted in the decline of groundwater level in the NCP. Scenario analyses showed that the groundwater levels would stop decreasing when the current planting area of winter wheat decreased by 76%. However, the reduction of wheat planting area (scenario 1) will also decrease the annual yield by 27% (from 13,547 to 9909 kg ha⁻¹). Fallowing (scenario 2) may reduce annual yield by 50% (from 13,547 to 6834 kg ha⁻¹) in order to maintain groundwater level. The SNWT (South to North Water Transfer) project (scenario 3) may have to provide 50% of irrigation water (130 mm yr⁻¹), to prevent groundwater decline while maintaining the current yield. Scenario 3 could be better than scenario 1 only if the water price was less than 8 ￥m⁻³. In the future, reducing winter wheat planting area (especially for low-yield cropland) may be a good option to mitigate groundwater decline while maintaining relatively high yield and income for local farmers in the NCP.