Reducing water productivity gap by optimizing irrigation regime for winter wheat-summer maize system in the North China Plain

Over-irrigation to ensure high crop yields in North China Plain (NCP) has led to a sharp groundwater table decline over the past decades. We conducted a scenario analysis from1980 to 2017 with 12 irrigation strategies from T1W0M0 to T12W3M2 (T is treatment and numbers of irrigation times in wheat (W) and maize (M) seasons, 75 mm each) for the dominant winter wheat-summer maize rotation using the soil-water-atmosphere-plant (SWAP) model in the NCP. After model calibration and validation with 4-year experimental data, we analyzed the long-term simulated annual crop yield, actual evapotranspiration (ETa), water productivity (WP), ground-water table change and economic benefit to the different irrigation practices. The optimized irrigation strategy is based on an integral analysis for different precipitation year-types. Results showed that annual yield and WP increased with increased irrigation until these reached a plateau. Treatment T4W2M1 was the turning point with an annual yield of 15335 kg.ha-1, significantly higher than T2W1M1 (26 %) and T1W0M0 (63 %), but not significantly different from 4 to 5 irrigations in wet and normal years. ETa increased with increased irrigation from 456 to 644 mm yr-1. The annual WP followed a similar trend as annual crop yield for all irrigation schedules. T4W2M1 had an annual WP of 2.47 kg cm-3, significantly higher by 15 % than T2W1M1 and 24 % higher than T1W0M0, with no significant difference seen with 4-5 irrigations. The rainfed treatment mitigated groundwater table decline the best, even reversing the trend during the past four decades to 0.11 m yr-1 rise. This was followed by two irrigation treatments (-0.30 m yr-1), three irrigations (-0.55 m yr-1), four irrigations (-0.72 m yr-1), and five irrigations (-0.86 m yr-1). The latter corresponds to typical farmer practice. Using the productivity indicators, including irrigation WP, marginal benefit and economic index, T4W2M1 showed the best performance. Therefore, we recommend T4W2M1 (two irrigations at pre-wintering and jointing stages for wheat and one irrigation at maize sowing stage) as the best irrigation strategy for the wheat-maize rotation to mitigate groundwater table decline, and sustain grain yields and water productivity in the NCP; another anthesis irri-gation for winter wheat is recommended during extreme dry years to sustain grain yield.