Abstract. Recently, a new application of timedependent gravity observations is emerging: the study of natural hydrological mass changes and their underlying processes. Complementary to GRACE data and continuous recordings with superconducting gravimeters, repeated observations with relative instruments on a local network may contribute to gain additional information on spatial changes in hydrology. The questions that need to be addressed are whether the results of these repeated measurements will be of sufficiently high resolution and accuracy, as well as how unique the information obtained will be. To examine this, a local gravity network with maximum point distances of 65 m was established in a hilly area around the Geodynamic Observatory Moxa, Germany. Using three to five LaCoste & Romberg relative gravimeters repeated measurements were carried out in a seasonal rhythm as well as at particular events like snowmelt or dryness in 17 campaigns between November 2004 and April 2007. The standard deviations obtained by least squares adjustment range from ±9 nm/s 2 to ±14 nm/s 2 for a gravity difference of one campaign, thus for gravity changes between two campaigns from ±13 nm/s 2 to ±20 nm/s 2 . Between the points of the network, spatial gravity changes of up to 171 nm/s 2 (139 nm/s 2 between two successive campaigns) could be proven significantly. They correlate with changes in the local hydrological situation. Particularly, a steep slope next to the observatory is identified as a gravimetrically significant hydrological compartment. The results obtained contribute to an improved reduction of the local hydrological signal in continuous gravity recordings and provide constraints to hydrological models.

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SUMMARY An approach for the evaluation of local hydrological modelling is presented: the deployment of temporal terrestrial gravity measurements and gravimetric 3‐D modelling in addition to hydrological point observations. Of particular interest is to what extent such information can be used to improve the understanding of hydrological process dynamics and to evaluate hydrological models. Because temporal gravity data contain integral information about hydrological mass changes they can be considered as a valuable augmentation to traditional hydrological observations. On the other hand, hydrological effects need to be eliminated from high‐quality gravity time‐series because they interfere with small geodynamic signals. In areas with hilly topography and/or inhomogeneous subsoil, a simple reduction based on hydrological point measurements is usually not sufficient. For such situations, the underlying hydrological processes in the soil and the disaggregated bedrock need to be considered in their spatial and temporal dynamics to allow the development of a more sophisticated reduction. Regarding these issues interdisciplinary research has been carried out in the surroundings of the Geodynamic Observatory Moxa, Germany. At Moxa, hydrologically induced gravity variations of several 10 nm s−2 are observed by the stationarily operating superconducting gravimeter and by spatially distributed and repeated high‐precision measurements with transportable relative instruments. In addition, hydrological parameters are monitored which serve as input for a local hydrological catchment model for the area of about 2 km2 around the observatory. From this model, spatial hydrological variations are gained in hourly time steps and included as density changes of the subsoil in a well‐constrained gravimetric 3‐D model to derive temporal modelled gravity variations. The gravity variations obtained from this combined modelling correspond very well to the observed hydrological gravity changes for both, short period and seasonal signals. From the modelling the amplitude of the impact on gravity of hydrological changes occurring in different distances to the gravimeter location can be inferred. Possible modifications on the local hydrological model are discussed to further improve the quality of the model. Furthermore, a successful reduction of local hydrological effects in the superconducting gravimeter data is developed. After this reduction global seasonal fluctuations are unmasked which are in correspondence to GRACE observations and to global hydrological models.

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/s 2. 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. seasonrelated 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/s 2 can be explained by variations in the amount of water stored in the hill flank.

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The network of superconducting gravimeters (SG) of the 'Global Geodynamics Project' (GGP) offers the unique opportunity to supplement and validate the gravity field variations derived from the GRACE satellite mission. Because of the different spatial and temporal resolution of the gravity data a combination of all data sets can be used to retrieve a maximum of information regarding mass transfers especially related to hydrology which is deployable as constraint for hydrological modelling.For a consistent combination of the data sets the gap between terrestrial data of superconducting and absolute gravimeters (AG) and from satellite data has to be bridged.A successful combination of SG and AG data could be realized for several stations which resulted in time series of highest accuracy and long-term stability.In principle, the same reductions applied to GRACE data have to be taken into account for the terrestrial data. The separation of local hydrological effects in SG observations is crucial for the comparison with satellite-derived gravity data. It is shown that even for stations with a hydrological challenging situation such as Moxa/Germany local hydrology-induced effects can be successfully modelled.* corresponding author, phone: +49-3641-948609, fax: +49-3641-948662, email:Adelheid.Weise@uni-jena.dePage 2 of 21A c c e p t e d M a n u s c r i p tCurrently, the study focuses on Europe with its dense and long-term observation network. Regarding the consistency of the SG gravity variations they are representative for a larger region. From a comparison with GRACE-derived gravity field changes, and the variations due to hydrological models a principle good agreement emerges.

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Please cite this article as: Kroner, C., Thomas, M., Dobslaw, H., Abe, M., Weise, A., Seasonal effects of non-tidal oceanic mass shifts in observations with superconducting gravimeters, Journal of Geodynamics (2008Geodynamics ( ), doi:10.1016Geodynamics ( /j.jog.2009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.Page 1 of 20 A c c e p t e d M a n u s c r i p t AbstractIn order to achieve a consistent combination of terrestrial and satellite-derived (GRACE) gravity field variations reductions of systematic perturbations must be applied to both data sets. At the same time evidence needs to be provided that these reductions are both necessary and sufficient. Based on the OMCT and the ECCO model the gravity effect of non-tidal oceanic mass shifts is computed for various sites equipped with a superconducting gravimeter (SG) and esp. the long-periodic contributions are studied.With these oceanic models the dynamic ocean response to atmospheric pressure loading is automatically computed, and thus goes beyond the more simplistic concepts of an inverted barometer, or alternately a rigid ocean, which is a clear advantage.The findings so far are ambiguous: For instance the systematic seasonal change of about 10 nm/s ¾ in gravity for mid-European stations is presently not found in the observed gravity variations. Generally, the order of magnitude of the total effect of 22 to 27 nm/s ¾ is surprisingly large for inland stations. In some data sections the reduction leads to the removal of some of the larger residuals. The results obtained for the South-African station Sutherland differ. Here the modelled seasonal variation caused by the non-tidal oceanic mass redistribution and gravity residuals generally correlate, and thus by the reduction an improvement of the signal-to-noise ratio in the gravity observations is achieved. proves to be correct universally, this reduction has to be applied.