Reduced groundwater use and increased grain production by optimized irrigation scheduling in winter wheat-summer maize double cropping system-A 16-year field study in North China Plain

Optimizing irrigation strategies to increase water utilization efficiency and achieve higher yield is vital for balancing groundwater use and improving food security during water shortage in the North China Plain (NCP). Based on a 16-year field experiment (2003-2018) using seven irrigation schedules from W0M0 to W4M3 (numbers are irrigation times in wheat (W) and maize (M) season, 75 mm each) in the winter wheat-summer maize double cropping system, we analyzed annual total water consumption (ETa) and groundwater table change in terms of net groundwater depletion, annual total grain yield, water productivity (WP), irrigation water productivity (IWP) and marginal benefit of the whole wheat-maize system. Relationship between yield or WP and irrigation or ETa were also revealed. Results showed that (1) total ETa increased as irrigation input increased, ranging from 427.3 mm (Rainfed, W0M0) to 891.0 mm (W4M3). Soil water storage contributed nearly 30% to ETa for winter wheat under water deficit conditions. Pre-sowing soil water storage played an important role in improving the annual yield and WP of both wheat and maize by promoting germination, seedling emergence and root growth; (2) the rainfed treatment (W0M0) was best for mitigating the groundwater table decline (0.1 m yr(-1)), followed by W1M1 (0.5 m yr(-1)) and W2M1 (0.8 m yr(-1)). Groundwater table decline in M2W2 almost overlapped the observed data at the station (1.1 m yr(-1)). In W3M2, the farmers' traditional practice, the groundwater table declined by 1.4 m yr(-1), obviously over exploitation, while W4M2 and W4M3 declined by almost 2.0 m yr(-1); (3) the relationship between total annual yield and irrigation (or ETa) followed a quadratic curve. Total annual yield significantly increased from W0M0 to M1W1 (25%) to M2M1 (5%) and then kept stable. Average annual WP decreased as irrigation increased, from 2.4 kg m(-3) (W0M0) to 1.6 kg m(-3) (W4M3). Average annual IWP and marginal benefit also declined as irrigation increased. These results over 16 years indicated that the W2M1 is the most balanced irrigation regime for wheat- maize rotation to mitigate groundwater decline, maintain grain production, and improve water use efficiency in the NCP.