Assessing the spatial representativeness of water storage variations from superconducting gravimeter residuals by regional CG-6 surveys

Regularly updated information about states, trends and dynamics of water storage in different spatio-temporal scales has gained increasing importance, especially with a perspective on hydrological extreme events as well as water management issues. Monitoring these storage dynamics is challenging due to the spatial heterogeneity and the contribution of different storage compartments (e.g., near-surface soil moisture, deep unsaturated zone, groundwater). A promising monitoring technique is gravimetry, well suited for the integral observation of different storage compartments. While satellite gravimetry (GRACE, GRACE-FO) provides information on storage variations at a spatially large scale with low spatial and temporal resolution, the opposite is true for terrestrial gravimetry. Ways to combine both satellite and terrestrial gravimetry are addressed and evaluated within the German Collaborative Research Centre TerraQ. For the terrestrial approach, several gravimeters were deployed for continuous monitoring at different locations within Germany. The work presented here takes as an example a forest site within the TERENO observatory of north-eastern Germany, with continuous observations  of a superconducting gravimeter (iGrav 033) since 2017.
The signal footprint of such a gravimeter typically covers a radius of 0.5 to 2 km, depending on local topography, although most of the signal originates from the direct vicinity of the instrument. Also, the device can sense mass changes beyond this distance, depending on their magnitude (e.g., tides, atmosphere or global hydrological effects). In hydro-gravimetric studies, all non-desired signals are typically removed, resulting in residuals that are representative for the local hydrological effects only. Towards comparing and combining these terrestrial measurements with satellite products, one open question is how representative the terrestrial gravity residuals are in a regional context. With the goal to assess this spatial representativeness, we conducted seasonal relative gravity surveys with 2 CG-6 gravimeters in an extent of roughly 25 by 30 km around the iGrav installation. The survey data were combined with spatial information about topography and land-use. Water storage changes could thus be attributed to each survey point. A joint analysis with the continuous measurements of the superconducting gravimeter at the permanent installation site allowed for mapping the spatial patterns and similarities among all sites.

This study is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project-ID 434617780 – SFB 1464