Advancing hydrological monitoring: Terrestrial gravimetry surveys in the Selke Catchment, Germany

In this study, spatial and temporal variations of soil moisture and water storage were monitored with two complementary methods: terrestrial gravimetry and cosmic ray neutron sensing (CRNS). CRNS monitors near-surface soil moisture by measuring low-energy neutron abundance in the near-surface atmosphere, which inversely correlates with soil moisture in the top decimeters of the soil. Terrestrial gravimetry monitors water storage variations in an integrative way over the entire unsaturated zone and the groundwater. Both methods allow for non-invasive spatially integrated field-scale monitoring around the instruments.

The study area is the Selke catchment in Central Germany with an area of 456 km². It has notable variations in topography, land use, and meteorology from the lowlands to the low mountain ranges. A combined approach was applied for terrestrial gravimetry: continuous stationary and time-lapse network surveys. We deployed a gPhone in a container-based housing (SolarCube) which serves as a base station for the relative gravity campaigns. Using two CG-6 gravimeters, the campaigns were conducted at six sites within the catchment area with co-located CRNS installations. In total five relative gravity surveys were carried out from July to October 2023. Each of them consisted of a two-day campaign where each survey point was visited three times by the two gravimeters. In order to ensure a high quality of the gravity data, capable of resolving a signal in the magnitude typical for hydrological processes in the area, a network adjustment of the repeated survey data was carried out. This included device-specific drift estimations. The results are combined with the continuous time series of the gPhone and analyzed jointly in a spatio-temporal approach with CRNS and in-situ soil moisture observations. Temporal dynamics of storage dynamics are assessed and spatial differences between the upland and the lowland areas are analyzed.