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.

Gravity recovery and climate experiment (GRACE)-derived temporal gravity variations can be resolved within the µgal (10 −8 m/s 2 ) range, if we restrict the spatial resolution to a half-wavelength of about 1,500 km and the temporal resolution to 1 month. For independent validations, a comparison with ground gravity measurements is of fundamental interest. For this purpose, data from selected superconducting gravimeter (SG) stations forming the Global Geodynamics Project (GGP) network are used. For comparison, GRACE and SG data sets are reduced for the same known gravity effects due to Earth and ocean tides, pole tide and atmosphere. In contrast to GRACE, the SG also measures gravity changes due to load-induced height variations, whereas the satellite-derived models do not contain this effect. For a solid spherical harmonic decomposition of the gravity field, this load effect can be modelled using degreedependent load Love numbers, and this effect is added to the P. Schwintzer has deceased. satellite-derived models. After reduction of the known gravity effects from both data sets, the remaining part can mainly be assumed to represent mass changes in terrestrial water storage. Therefore, gravity variations derived from global hydrological models are applied to verify the SG and GRACE results. Conversely, the hydrology models can be checked by gravity variations determined from GRACE and SG observations. Such a comparison shows quite a good agreement between gravity variation derived from SG, GRACE and hydrology models, which lie within their estimated error limits for most of the studied SG locations. It is shown that the SG gravity variations (point measurements) are representative for a large area within the µgal accuracy, if local gravity effects are removed. The individual discrepancies between SG, GRACE and hydrology models may give hints for further investigations of each data series.

If we restrict the spatial resolution to a half-wavelength of about 1500 km and the temporal resolution to 1 month, GRACE-derived temporal gravity variations can be resolved within the μgal (10 −8 m/s 2 ) range. A comparison with ground gravity measurements from selected Superconducting Gravimeter (SG) stations forming the Global Geodynamics Project (GGP) provides an independent validation. For this study, five European SGstations were selected that both cover a large test field and allow closely located SG-stations to be studied. Prior to this comparison, GRACE and SG data sets have to be reduced for the same known gravity effects due to Earth and ocean tides, pole tide, and atmosphere. After these reductions, the remaining part can be mainly attributed to mass changes in terrestrial water storage. For this reason, gravity variations derived from global hydrological models are included in the comparison of SG and GRACE results. Conversely, the hydrology models can be checked by gravity variations determined from GRACE and SG observations. For most of the SG locations investigated here, the comparison based primarily on computed correlations shows quite a good agreement among the gravity variation derived from the three different kinds of data sets: SG, GRACE, and hydrology models. The variations in SG gravity (point measurements) prove to be representative for a large area within the μgal accuracy range, if local gravity effects are removed correctly. Additionally, a methodology for an analysis of dominant common features based on the EOF-technique is proposed and illustrated. The first principal component shows strong periodicity, and the search for arbitrary periods confirms a strong common annual component, which reduces the total signal content considerably. The first eigenvector reveals common features and differences between distinct SG stations. Discrepancies between SG, GRACE, and hydrology models at individual SG stations, detected by both methods, may provide valuable hints for further investigations of respective data series.

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.

[1] In a previous study (Hasan et al., 2006) we applied time series analysis and distributed hydrological modeling techniques to investigate the effect of hydrological processes on observed terrestrial gravity residuals. In this study we apply terrestrial gravity observations (measured in one location) to constrain simple hydrological models in a catchment around the gravimeter. A superconducting gravimeter observes with high frequency (1 Hz) the temporal variations in the gravity field with high accuracy (sub nm s À2 for hourly variation) near Moxa, Germany since 1999. Hourly gravity residuals are derived by filtering and reducing for Earth tides, polar motion, barometric pressure variations, and instrumental drift. These gravity residuals show significant response to hydrological processes (precipitation, evaporation, surface and subsurface flow) in the catchment surrounding the observatory. We can thus consider the observed gravity change as an integrator of catchment-scale hydrological response (similar in nature as discharge measurements), and therefore use it to constrain catchment-scale hydrologic models. We test a set of simple water balance models against measured discharge, and employ observed gravity residuals to evaluate model parameters. Results indicate that a lumped water balance model for unsaturated storage and fluxes, coupled with a semidistributed hydraulic groundwater model for saturated storage and fluxes, successfully reproduces both gravity and discharge dynamics.