Constraining groundwater recharge estimates with gravity measurements

Measuring gravity over time provides direct information about subsurface mass changes and, consequently, changes in groundwater storage. Gravity residuals can be used to estimate the specific yield of aquifers or to calibrate hydrogeological models.
As part of our research project, a high-precision absolute gravimeter will be developed for field applications. The absolute gravimeter is expected to offer significant advantages over existing high-precision instruments, such as a simple setup and low drift. In parallel, we will build a mesh-based physical hydrogeological model simulating both unsaturated and saturated zone processes in a study area experiencing declining groundwater levels. Mass changes in each mesh cell lead to gravity changes, which can be calculated by accounting for local subsurface properties. Model parameters will be constrained by fitting the modeled gravity response to the observed data. While previous studies have successfully calibrated saturated zone models using gravity data, including the unsaturated zone will improve the accuracy of subsurface mass change estimates and enhance the understanding of processes such as groundwater recharge. A comparison with repeated measurements of surface nuclear magnetic resonance will further assess the method’s validity for capturing unsaturated zone processes.