Signals of mass redistribution observed at the South African Geodynamic Observatory Sutherland

Since the year 2000 a superconducting gravimeter (SG) has been operating at the South African Geodynamic Ob- servatory Sutherland (SAGOS). It is one of the few instruments installed in the southern hemisphere and presently the only one of its kind on the African continent. Time-dependent gravity observations carried out with supercon- ducting gravimeters are long-term stable and of high resolution (~ 1 nm/s² in the time domain) have thus the poten- tial to provide constraints for hydrological models since they completely contain the changes in all water storage components in contrast to hydrological point observations. Esp. in combination with satellite-derived temporal gravity field variations as from the GRACE mission they provide a powerful tool for the investigation of changes in continental water storage. SAGOS is located on a hill in the Karoo, a semi-arid area with an average annual precipitation in the range of 200 to 400 mm. A seasonal effect on gravity related to local hydrological variations is clearly seen in the SG record. It is in the or- der of magnitude of several nm/s². Depending on the hydrological model the large-scale hydrological impact on gravity amounts to 10 - 20 nm/s² with or without a pronounced seasonal variation. Substantial differences exist between both, the amplitudes and the features of the gravity changes derived from global hydrological models. The same applies to the observed hydrology-related gravity changes and the model-based gravity variations. The results of the recently initiated investigations will contribute to an improved local and regional quantification of hydro- logical dynamics and thus establish additional means for the management of the valuable resource water.