A root zone model for estimating soil water balance and crop yield responses to deficit irrigation in the North China Plain

• A new root zone model was proposed to quantify drainage out of root zone by simplification of the unsaturated flow equation.
• The model could satisfactorily simulate field water balance of winter wheat–summer maize crop rotation.
• The cumulated drainage accounted for −27% to 19% of the applied irrigation and precipitation.
• The optimal irrigation schedules of three typical hydrologic years were obtained by the model.
• The developed model is useful for irrigation scheduling and reasonable utilization of groundwater resources in semi-arid region.

The development of water-conserving and sustainable agricultural management practices is essential and significant to alleviate the rapid depletion of groundwater resources in the North China Plain (NCP). Estimating drainage out of the root zone and improving water use efficiency could provide a basis to assist in reasonable utilization of groundwater resources for agricultural irrigation in the NCP. This study proposed a new soil water balance model to quantify drainage out of the root zone by incorporating the Darcy's law. This model was connected with the Jensen crop water production function to simulate soil water components and relative crop yield under deficit irrigation. Field experiments with the winter wheat and summer maize crop rotation were conducted in Beijing area in the NCP (2007–2009) to evaluate the model. The model could give quite reasonable predictions of soil water content in the root zone with the average root mean square error (RMSE), mean relative error (RE) and model efficiency (EF) of 0.02 cm3 cm−3, 6.69% and 0.78, respectively. The predicted soil water flux through the bottom of root zone agreed well with the measured ones supported by the values of RMSE (0.10 mm d−1) and EF (0.92). The simulations indicated that the accumulated drainage out of root zone accounted for −27% to 19% of the applied water (irrigation and precipitation) among different crop seasons. As an application, the model was used to obtain the optimal irrigation management schedules for the hydrologic years of 75%, 50%, and 25% in the study area. The average amount of irrigation saving and reduction of water losses through drainage under optimal irrigation alternative were about 175 mm and 101.9 mm, respectively. This study shows that the developed root zone model has minimal input requirement, robust physical meaning and satisfactory simulation performance, which is more applicable and feasible for agricultural water management in the semi-arid area.