Modelling the effects of climate variability and water management on crop water productivity and water balance in the North China plain

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 4.5 t ha-1 for wheat and 0 to 5.0 t ha-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¿420 mm for wheat, and 0¿170 mm for maize. Such levels of irrigation applications could potentially lead to about 1.5 m year-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 200 mm season-1) were recommended for wheat in wet, medium and dry seasons, respectively. For maize, one irrigation and two irrigations (i.e., 60 and 110 mm season-1) were recommended in medium and dry seasons, while no irrigation was needed in wet season