Theory and methodology of critical control levels of groundwater: a case study of Tianjin, China

Groundwater is one of the most important freshwater resources worldwide. However, long-term over-abstraction of groundwater has resulted in deteriorating groundwater functions, including gradually decreasing resources function and geo-environmental function and subsequent adverse geo-environmental issues. In this study, the concept of critical control levels of groundwater including blue line levels and red line levels, which play an important role in groundwater resource management, is presented for the first time. In addition, the theory of critical control levels of groundwater is established based on the groundwater geo-environmental function and resources function. A coupled numerical model of groundwater flow field and land subsidence is then established for Tianjin based on the conceptual model and analysis of the hydrological conditions of the study area. Finally, the critical control levels of groundwater for the subarea are presented based on results obtained from a numerical model by the model space division and time discretization, hydrogeological parameters processing, model calibration and validation. The results presented herein will be useful for groundwater resource management.