Evaluating the effects of limited irrigation on crop water productivity and reducing deep groundwater exploitation in the North China Plain using an agro-hydrological model: II. Scenario simulation and analysis

The east-central North China Plain (NCP) is a globally representative area that presents a sharp contradiction between deep groundwater exploitation and grain production. Evaluating the effects of limited irrigation on crop water productivity (WP) and deep groundwater exploitation reduction in this area is important for developing sustainable agricultural practices. In this study, the 11 limited irrigation scenarios were simulated and analyzed using the distributed Soil-Water-Atmosphere-Plant-WOrld-FOod-STudy (SWAP-WOFOST) model that was established, calibrated and validated in a companion paper; the recommended irrigation timing for maximizing the yield or WP of winter wheat was selected; the irrigation scheme for minimizing evapotranspiration constrained by 14 winter wheat yield reduction thresholds was optimized separately; and further the effects of the optimized irrigation scheme on saving water and reducing deep groundwater exploitation were evaluated. The results showed that the recommended irrigation timing was predominantly in the pre-sowing, booting to anthesis and early grain-filling stages for three irrigations, the pre-sowing and booting to anthesis stages for two irrigations, and the booting to anthesis stage for one irrigation. Adopting the optimized irrigation scheme with the threshold of 60% could achieve the goal of reducing deep groundwater exploitation by 6.05 x 10(8) m(3) but reduced winter wheat yield by approximately 50%. The contribution index of reducing exploitation (CIRE) was defined to identify the contribution of implementing the optimized irrigation scheme to reducing current deep groundwater exploitation. At the related water resource subregional level, the CIRE values showed that this contribution was relatively higher in the related Tuhai and Majia Rivers plain (THMJHP), the related plain of the Zhangwei River basin (ZWHP) and the related plain of the Ziya River basin (ZYHP). Following the framework of simulation-optimization-evaluation, the three results at the manageable county level, namely, the optimized irrigation scheme under the constraint of 14 thresholds, the crop yield variation compared with the current irrigation schedule and the CIRE values, provide decision-making references for implementing limited irrigation for winter wheat under local conditions to achieve the target of deep groundwater exploitation reduction.