Sustainability and efficiency of water-land-energy-food nexus based on emergy-ecological footprint and data envelopment analysis: Case of an important agriculture and ecological region in Northeast China

Ecological security and efficient utilization of resources in the water-land-energy-food (WLEF) nexus are important strategic needs. Therefore, this study developed an emergy ecological footprint using the super-efficiency Slacks-Based Measure-Data Envelopment Analysis (SBM-DEA) to comprehensively evaluate the ecological security and efficiency of the WLEF nexus from 2006 to 2019 in the Songnen-Sanjiang Plain in Northeast China. Our findings indicated that: 1) Ecological security had improved from 2006 to 2019, with an average ecological carrying capacity supply and demand balance index of 3.301 and 2.723 in 2006 and 2019, respectively. 2) The average concordant index in the Songnen Plain and Sanjiang Plain was 1.258 and 1.358 respectively, indicating that the Sanjiang Plain had better ecological security than the Songnen Plain. 3) The efficiency of the water and energy subsystems demonstrated an increasing trend over time, with the overall land subsystem increasing before 2016 but decreasing afterwards. 4) The efficiency of the Songnen-Sanjiang Plain's water, land, energy, and food subsystems varied greatly on temporal and spatial scales. The water efficiency in the Songnen-Sanjiang Plain was higher in the middle and low all around. 5) From 2006 to 2019, the WLEF nexus efficiency in the Sanjiang Plain was higher than in the Songnen Plain, and in 2019, the WLEF nexus efficiency in 50% of the cities did not reach the Songnen-Sanjiang Plain production frontier. Therefore, to achieve regional sustainable development, additional efforts are required to ameliorate the ecological conditions of the middle part of the Songnen Plain and improve the resource utilization efficiency of each subsystem, especially the water resources of the WLEF nexus.