Ecological restoration by afforestation may increase groundwater depth and create potentially large ecological and water opportunity costs in arid and semiarid China

Water scarcity is a global environmental problem that jeopardizes human safety and socioeconomic development. Since 1952, China has implemented a large-scale tree-planting program in the country's arid regions to combat desertification. However, there is a serious risk this program will exacerbate water shortages and lower the groundwater table: the trees selected for this program were not chosen based on local environmental constraints, and their evapotranspiration exceeds the regional precipitation. However, no data on the afforestation's effects on the groundwater table is available. This is problematic because any loss of groundwater will severely constrain socioeconomic development in China's arid and semiarid regions, which already face severe water shortages. The economic concept of a groundwater opportunity cost could be used to guide socioeconomic activities to improve the sustainability of groundwater use and mitigate the potential problems caused by the Chinese afforestation program. In this paper, seven evapotranspiration models were used to provide the first estimate of the opportunity cost created by afforestation's adverse effects on the groundwater resource. The results showed that the groundwater table declined yearly throughout the study area, with mean water opportunity costs in 2011 ranging from 0.04 x 10(9) RMB to 11.1 x 10(9) RMB under nine provinces in arid and semiarid China. This study provides the first solid evidence that afforestation in arid and semiarid northern China will exacerbate the groundwater decline while creating enormous opportunity costs. Thus, the afforestation program must be urgently reassessed and the water-use efficiency of vegetation must be considered when planning future ecological restoration both to make the restoration more cost-effective and to protect the ecosphere. (C) 2016 Elsevier Ltd. All rights reserved.