Gravity Surveys in the vertical as a Tool for monitoring long-term Water Storage Changes in the Vadose Zone

The vadose zone plays a key-role for a comprehensive understanding of hydrological states and processes at the interfaces of atmosphere, soil, vegetation and groundwater. Yet it is the most difficult hydrological compartment to observe water storage and fluxes due to limited accessibility and high heterogeneity. Terrestrial gravimetry represents a potentially useful monitoring method for this compartment. Its non-invasive and integrative nature provides many advantages compared to traditional hydrological field methods. Nevertheless, these benefits go along with some methodological downsides: vadose zone water storage changes, for instance, can only be disentangled from integrative measurements if all undesired signal components are known. A setup of two gravimeters can overcome this challenge as these components cancel out when calculating the differences between gravity observations. This approach was applied in the presented study.

Monthly relative gravity surveys are performed at a forest site in the TERENO Observatory (North-East Germany) using 2 CG-6 gravimeters. The surveys are carried in and around a 170 years old water well shaft with a total depth of 13 m. The well shaft is located at a distance of about 50 m to a superconducting gravimeter that is operated continuously in a field enclosure. The groundwater table is closely below the well bottom and continuously monitored. Gravimetric survey data range from May 2019 to November 2022. During the monthly surveys, we performed repeated gravity measurements on 3 pillars: one next to the iGrav, one next to the well shaft on the terrain surface and one on the bottom of the well shaft. After survey-based network-adjustment, the data is compared and chronologically connected to the continuous recordings of the iGrav. Gravity changes at each pillar are furthermore compared to physically-based theoretical modeling of different support volumes which depend on the instruments position in the landscape. Differences of the CG-6 gravity measurements between top and bottom of the well provide a unique dataset for describing the water storage variations in the vadose zone.

This contribution represents an update and advancements of data and methods presented at EGU 2022 [https://doi.org/10.5194/egusphere-egu22-7200]