Monitoring Groundwater Storage Changes in a Karst Aquifer using Superconducting Gravimeter OSG-066 at the Lijiang Station in China

Monitoring groundwater storage changes is one of the most important topics in the sustainable management of groundwater resources. The sensitivity of gravity to changes in mass allows one to obtain an estimate of groundwater storage changes through gravity observations. The karst aquifer in the Black Dragon Pool is the main water resource for Lijiang City. As a result of overexploitation of groundwater resources, the groundwater level has declined since 2008, although it started to rise with the implementation of water resource management practices in 2017. In this study, we assess the local groundwater storage changes in a karst aquifer by using a more than 4-year record (2017-2021) of the superconducting gravimeter OSG-066 located at the Lijiang station. Firstly, the records were preprocessed by calibrating the data and removing interferences such as earthquakes, spikes, and steps. The gravity effects of the tides, atmosphere, and polar motion and the linear trend term were then subtracted to obtain the preliminary residual gravity signal. After correcting for the effect of terrestrial water storage and land uplifting, the final residual gravity signal was obtained, which was analyzed by comparison with the groundwater level change. We find that the topographic effect on gravity must be considered due to the complex topography around the Lijiang Basin. We note that the amplitude of residual gravity is dominated by the groundwater level change in the Black Dragon Pool, with a phase lag between the final residual gravity and groundwater level change. The final residual gravity clearly displays characteristics of seasonality and fluctuation, with variable amplitude. From the joint analysis of gravity records and well and model data, we demonstrate that both the residual gravity and groundwater level exhibit a downward trend after 2018. At the same time, however, gravity observation reflects the effectiveness of water resource management practices implemented in 2017.