Alternative cropping systems for groundwater irrigation sustainability in the North China Plain
Introduction
Sustainability of agriculture is being challenged by the growing demand for food production accompanied by population growth as well as the natural resources and environmental boundaries (Godfray et al., 2010, Davis et al., 2019). Water shortage has increasingly become a threat for irrigation agriculture, which produces 46% of the world’s food on 28% of the cropland (Molden, 2007). Moreover, groundwater depletion has occurred in some highly intensified groundwater-fed agricultural regions, e.g., High Plains and Central Valley in the United States, Indo-Gangetic Plains in India, and the North China Plain (Scanlon et al., 2012, Gleeson et al., 2012). Sustainable crop production in these regions can be achieved in the long term only if sufficient water is available.
The North China Plain (NCP) contributes to more than two-thirds of wheat and one-third of maize production in China (NBSC, 2018). However, it is important to note that over-exploitation of groundwater is imposing restrictions on such achievements due to the gap between annual precipitation (500–600 mm) and the water requirement (850–900 mm) of intensive winter wheat – summer maize (WWSM) double cropping system (van Oort et al., 2016, Zhao et al., 2017, Wang et al., 2019). Continuous groundwater over-exploitation has resulted in a 1.0–1.5 m yr−1 water table decline and led to the largest groundwater depression worldwide (Kong et al., 2016). Among water-saving agricultural practices, optimization in irrigation regime and cropping sequence are more promising options to mitigate groundwater crisis and ensure food security.
Stage-based deficit irrigation with optimized timing and amount greatly reduced groundwater use without significant yield loss (Zhang et al., 2013, Sun et al., 2015, Kang et al., 2017). However, based on a two-year field experiment, Xu et al. (2016) advocated that the optimal irrigation strategy for wheat is one irrigation (60 mm) applied at stem elongation, whereas Xu et al. (2018) recommended two supplemental irrigations at stem elongation and anthesis (75 mm for each irrigation), based on another two-year field experiment. These inconsistent irrigation strategies may arise from the large inter-annual precipitation variability. Moreover, the amount of irrigation reduction induced by regime optimization is not equal to the amount of water-saving, real water-saving can be realized only when ET is reduced (Zhang, 2018). Thus, it is vital to evaluate the impact of irrigation strategy optimization on real water-saving considering the effect of weather variability. Continuous monocropping of early maize (EM) (also called spring maize in China) is being proposed as a possible alternative to the current high-intensity WWSM cropping system to conserve groundwater in the NCP (Meng et al., 2012, Cui et al., 2018). Although EM has a high grain yield potential (up to 16.5 Mg ha−1) in the NCP (Meng et al., 2013), there is still a yield loss range from 14% to 63% when compared with WWSM (Sun et al., 2011, Gao et al., 2015, Yan et al., 2016, Liang et al., 2019). Evapotranspiration (ET) reduction of EM ranged from 20 mm under the rainfed condition to 490 mm under conventional irrigation compared with WWSM (Meng et al., 2012, Yan et al., 2016, Liang et al., 2019). The large difference in grain yield and ET is mainly due to the large variability in weather, management, and soil type across the NCP. In the Southern NCP, for example, the grain yield of EM was lower than summer maize even with a longer growing period due to high temperature stress during the grain-filling stage (Yan et al., 2017, Gao et al., 2018). Therefore, there is a need for further exploration of the grain yield and water productivity of EM in the long-term period.
Process-based crop simulation models have unique advantages in evaluating the effects of agricultural management scenarios under long-term inter-annual climate variability (Jeong and Zhang, 2020). APEX is a physically based model for simulating agricultural management impacts on crop production and environmental quality on multiple spatially distributed fields (Williams et al., 2008, Osorio Leyton, 2019). The APEX model has been widely and successfully used to simulate crop production (Wang et al., 2008, Meki et al., 2013, Luo and Wang, 2019), irrigation regime (Zhang et al., 2016a, Zhang et al., 2016b, Zhang et al., 2018, Zhao et al., 2019), conservation agriculture (Assefa et al., 2018), and water and nitrogen management (Talebizadeh et al., 2018, Tadesse et al., 2018, Tadesse et al., 2019, Timlin et al., 2019) for major crops across the world. In addition, recent model improvements such as the variable saturation hydraulic conductivity methods greatly enhanced the capability and application of APEX (Doro et al., 2017). Overall, APEX is capable of evaluating the impacts of climate, soil, and management practices on hydrology and crop productivity. To our knowledge, no studies have calibrated and validated the APEX model for maize and wheat production in the NCP, especially comparison analysis for WWSM and EM cropping systems under different irrigation regimes.
Therefore, two field experiments of WWSM (2015–2018) and EM (2012–2018) were conducted to calibrate and validate the APEX model. Then, the validated model was used to assess the effect of irrigation strategies on water use and crop productivity of WWSM and EM using historical weather data. We aimed to (1) assess the applicability of APEX on mimicking crop growth, the yield of maize and wheat, and VMC of WWSM, (2) quantify grain yield, water stress days (WS), ET, water use efficiency (WUE), and net water use (NWU) of WWSM and EM under four irrigation schedules, and (3) propose optimal irrigation strategies and potential alternative cropping systems in the NCP.
Section snippets
Experimental site
The field experiments were conducted at the Wuqiao Experimental Station of China Agricultural University (37.62°N, 116.43°E; a.s.l. 18–21 m). The study site is located in the Southern part of Hebei Province which is typically representative of the overall agricultural production in the NCP (Fig. 1). The study area has a warm temperate and continental monsoon climate. The average annual precipitation over the last 35 years is 547 mm, 70% of which occurred in the summer monsoon months (June to
APEX calibration and validation
In general, simulated values of LAI, ABIOM, and grain yield for winter wheat (WW), summer maize (SM), and early maize (EM) were in good agreement with the field observations (Table 3). Evaluation indicators for LAI and ABIOM indicated that model performance was satisfactory (Moriasi et al., 2007, Wang et al., 2012) during calibration and validation periods with R2 that ranged between 0.68 and 0.98, PBIAS < 25%, and d >0.85 (Table 3). The trend lines of the simulated LAI and ABIOM matched
APEX model evaluation
Crop growth and water balance modules of the APEX model were evaluated using measured grain yield, daily growth, and soil water content data collected from field study in the NCP. APEX simulations of LAI, ABIOM, and grain yield for maize and wheat were considered reasonable with R2 ≥ 0.68, PBIAS<21%, and d values close or higher than 0.9 (Moriasi et al., 2007, Wang et al., 2012). Although VMC under WWSM was better estimated under high irrigation volume than lower irrigation amount, APEX
Conclusion
For the first time, the APEX model was calibrated and validated for early maize, summer maize, and winter wheat, with reasonable accuracy in the North China Plain. With further simulation of winter wheat – summer maize (WWSM) and early maize (EM) cropping system under different irrigation regimes, our findings demonstrated that ET and net water use of WWSM under CI were reduced by 7% and 28%, without yield loss, compared with that under FI. Moreover, less intensive irrigation and shifting
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
We thank the English editing by Nicole Norelli from Texas A&M University. We acknowledge funding from the National Key Research and Development Program of China (2016YFD0300205-01) and the National Natural Science Foundation of China (31671640; 31901470).
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