Managing food and bioenergy crops with declining groundwater levels in the North China Plain

Managing the complex demands on available energy, food and water is a fundamental challenge for a world with increasingly tenuous resource pools. The North China Plain's groundwater crisis has become increasingly serious due to decades-long over-exploitation for irrigation to pursue high grain yield. This combined challenge motivated investigation of ways to mitigate the groundwater table decline while fostering the use of renewable bioenergy resources. To provide insights about tradeoffs among water, food and energy, we performed a six-year experiment from 2010 to 2015 in the North China Plain where groundwater use, biomass and biofuel production and water use efficiency were compared for four cropping systems. They are winter wheat-summer maize (WM, grains, 1-year cycle), ryegrass-sweet sorghum (RS, forage plus bioenergy crops, 1-year cycle), winter wheat-summer maize -> ryegrass-sweet sorghum (WMRS, forage, bioenergy plus grain crops, 2-year cycle), and perennial switchgrass (SG, bioenergy crop). Actual evapotranspiration (ETa) and recharge to groundwater were calculated and validated using observed soil moisture contents. Results showed ETa of the cropping systems were statistically similar ranging from 694 to 730 mm yr(-1) with the exception of switchgrass which was significantly less at 504 mm yr(-1). The water table was projected to decline by about 1.3 m yr(-1) for all three multi-crop rotations and 0.4 m yr(-1) for switchgrass. In terms of bioethanol production per unit land and per unit ETa, the ryegrass-sweet sorghum (RS) rotation performed best with 25.0 m(3) ha(-1) and 5.7 L m(-3) outperforming the yearly winter wheat-summer maize (WM) rotation (11.7 m(3) ha(-1) and 3.3 L m(-3)) and switchgrass (11.4 m(3) ha(-1) and 2.6 L m(-3)). The multiple year rotation of winter wheat-summer maize -> ryegrass-sweet sorghum (WMRS) yielded 16.9 m(3) bioethanol per hectare and 4.3 L IT1-3 bioethanol per m(3) ETa. When optimizing bioethanol against net groundwater depletion, RS and switchgrass outperformed the two grain-based rotations with 15.5 L and 14.4 L of bioethanol per m(3) of depleted groundwater, ahead of WMRS at 10.9 L m(-3) and WM at 9.1 L m(-3). These results are significant for setting governmental policies on sustainable use of groundwater by various crop mixes.