Repeated vertical relative gravity measurements in a well shaft for monitoring 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. This can be a challenge in particular for observations with a single gravimeter. However, using two gravimeters may open up new possibilities as the undesired signal components may cancel out when calculating the differences of the gravity observations of both devices. The latter approach was applied in the presented study.

We carried out monthly relative gravity campaigns in the TERENO Observatory (Mueritz National Park, North-East Germany) using 2 Scintrex CG-6 gravimeters (#58, #69). On this site, we have an iGrav (#33) continuously operating since end of 2017. In May of 2019 we started with the monthly campaigns in an about 170 years old water well shaft, located at a distance of about 50 m from the iGrav. This well shaft has a diameter of roughly 2 m and a total depth of 13 m. The groundwater table is one to two meters below the well bottom and continuously monitored. During the campaigns in each month, 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. This monthly data is compared and chronologically connected to the continuous recordings of the iGrav. 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 of 13 m thickness. Additionally, we set these observations into the context of meteorological and near-surface soil moisture time series monitored at the site.