Hydrological signals in gravity — foe or friend?

Although hydrological effects on gravity are known nearly as long as the influence of barometric pressure, they are not as well understood as the latter. The improvement of gravity data quality during the last years adds weight to the importance of understanding the hydrological influence. Moxa observatory is one station at which studies regarding hydrological effects are carried out. From soil moisture, water level and meteorological observations the effects of different hydrological contributors including snow can be modelled and compared to the gravity residuals of the superconducting gravimeter (SG). The total peak-to-peak amplitude amounts to 35 nm/s2. Contributions from the various areas around the observatory partly compensate due to the hilly morphology. The comparison between residuals and computed total hydrological effect yields a good agreement, but also shows that not all hydrological influences have been taken into account. A significant additional hydrological influence is due to the hill flank near the SG. Besides the possibility of giving an additional constraint to water balance computations, gravity observations might become of interest to hydrologists studying interflow processes. A local gravity network was established around Moxa observatory in order to find out whether slow, e.g. season-related hydrological changes or large-scale fluctuations as caused by snow melt can be detected by repeated gravity measurements. From the six campaigns carried out so far a trend becomes visible: Wet conditions lead to a decrease in the gravity differences between observation points at the foot of the hill and on the upper part of the hill flank. Dry conditions result in increased gravity differences. The changes in the differences which are in the range of several ten nm/s2 can be explained by variations in the amount of water stored in the hill flank. [ABSTRACT FROM AUTHOR]