Signals of mass redistribution at the South African gravimeter site SAGOS

The superconducting gravimeter (SG) operating at the South African Geodynamic Observatory Sutherland (SAGOS) is one of the few instruments installed in the southern hemisphere and presently still the only one of its kind on the African continent. SAGOS is located in the Karoo, a semi-arid area with an average annual precipitation of 200-400 mm. The distance to the ocean is approx. 220 km. A local hydrology-related seasonal effect on gravity is clearly seen in the SG record. Its general order of magnitude is estimated to be about 4-10 nm/s². A large-scale hydrological influence in a similar order of magnitude or even larger (up to 60 nm/s² peak-to-peak) is inferred from global hydrological models for the years 2003-2007. Significant contributions are found for the southern coast, the central Cape region, and the basin of the Orange river. Contributing basins with larger distance comprise the areas of Okavango/Sambesi, Congo, and eastern Africa. Between SG data, temporal GRACE gravity field solutions, and the gravity effect derived from global hydrological models clear differences exist. Among others, the deviations between the hydrological models can be traced to deviations in the gravity effect originating from the Okavango basin and the central Cape region. Gravity residuals reduced for changes in continental water storage are compared to the gravity effect caused by non-tidal oceanic mass changes. A rudimentary correlation between observed variations and modeled effect is found. The peak-to-peak amplitude of the modeled effects amounts to 15 nm/s² for the years 2001-2008. After reducing the SG data for this oceanic effect the variation of the residuals decreases by 9%. The present findings indicate the suitability of the SG observations at Sutherland for studies on mass transport phenomena in the South African region.