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Abstract: Serious water deficits and deteriorating environmental quality are threatening agricultural sustainability in the North China Plain (NCP). This paper addresses spatial and temporal availability of water resources in the NCP, identifies the effects of soil management, irrigation timing and amounts, and crop genetic improvement on water use efficiency (WUE), and then discusses knowledge gaps and research priorities to further improve WUE. Enhanced irrigation and soil nutrient (mainly nitrogen) management are the focal issues in the NCP for enhancing WUE, which are shown to increase WUE by 10–25% in a wheat–maize double cropping system. Crop breeding has also contributed to increased of WUE and is expected to play an important role in the future as genetic and environmental interactions are understood better. Agricultural system models and remote sensing have been used to evaluate and improve current agronomic management practices for increasing WUE at field and regional scales. The low WUE in farmer''s fields compared with well-managed experimental sites indicates that more efforts are needed to transfer water-saving technologies to the farmers. We also identified several knowledge gaps for further increasing WUE in the NCP by: (1) increasing scientific understanding of the effects of agronomic management on WUE across various soil and climate conditions; (2) quantifying the interaction between soil water and nitrogen in water-limited agriculture for improving both water and nitrogen-use efficiency; (3) improving irrigation practices (timing and amounts) based on real-time monitoring of water status in soil-crop systems; and (4) maximizing regional WUE by managing water resources and allocation at regional scales. [Copyright &y& Elsevier]

Abstract: In the North China Plain (NCP), while irrigation using groundwater has maintained a high-level crop productivity of the wheat–maize double cropping systems, it has resulted in rapid depletion of groundwater table. For more efficient and sustainable utilization of the limited water resources, improved understanding of how crop productivity and water balance components respond to climate variations and irrigation is essential. This paper investigates such responses using a modelling approach. The farming systems model APSIM (Agricultural Production Systems Simulator) was first calibrated and validated using 3 years of experimental data. The validated model was then applied to simulate crop yield and field water balance of the wheat–maize rotation in the NCP. Simulated dryland crop yield ranged from 0 to 4.5tha−1 for wheat and 0 to 5.0tha−1 for maize. Increasing irrigation amount led to increased crop yield, but irrigation required to obtain maximum water productivity (WP) was much less than that required to obtain maximum crop yield. To meet crop water demand, a wide range of irrigation water supply would be needed due to the inter-annual climate variations. The range was simulated to be 140–420mm for wheat, and 0–170mm for maize. Such levels of irrigation applications could potentially lead to about 1.5myear−1 decline in groundwater table when other sources of groundwater recharge were not considered. To achieve maximum WP, one, two and three irrigations (i.e., 70, 150 and 200mmseason−1) were recommended for wheat in wet, medium and dry seasons, respectively. For maize, one irrigation and two irrigations (i.e., 60 and 110mmseason−1) were recommended in medium and dry seasons, while no irrigation was needed in wet season. [Copyright &y& Elsevier]