Repeated absolute gravity (AG) measurements have emerged as a major tool in studies of eodynamical phenomena. Gravity rates are also able to provide information on vertical motion ndependently of reference frame uncertainties and technique issues of geometric methods like NSS. They thus can be used as a consistency check of station vertical velocities, and it has been ven envisaged that global repeated absolute gravity could help to control and stabilize the global eference frame. And the ratio of gravity rate and vertical velocity yields information on the underlying geophysical process, as it is different e.g. for the Glacial Isostatic Adjustment GIA, and or elastic loading. owever, gravity is strongly influenced by near-field density variation close to the sensor, e.g. due o variation in subsurface water storage. The gravity effect of such density variation may overshadow the gravity effect associated with vertical motion or large scale processes under study. ven if the same models are used to correct both station position and gravity for, say, the load ffects of geophysical fluid motion, the near-field direct attraction typically cannot be well approximated by such models. Thus they must be modelled and corrected for separately. Or, to take nother application, the modelling of local effects is indispensable if point observations of timevariable ravity are to be compared with regional variation in gravity from e.g. the GRACE mission. Most insights into hydrological effects in point measurements of terrestrial gravity have been ained by using records of superconducting gravimeters (SG) together with observations of local ydrological and meteorological parameters like groundwater level, soil moisture, precipitation, etc. This will be true in the future as well, and one of the challenges is how these insights can be put to ork at AG sites, which typically have only sporadic gravimeter occupations and few or no local ydrological measurements. hile variation in terrestrial water storage is the omnipresent and perhaps most important local ffect in gravity, there are others: cryosphere (attraction of local snow and glaciers) in polar ravimetry, local atmospheric attraction (e.g. the scale height variation), variable attraction of sea ater at near-coastline sites (non-tidal and tidal), etc.
 

The AG subgroup in working group WG1 of the COST ES0701 Action “Improved Constraints on Models of Glacial Isostatic Adjustment» (see http://www.cost-es0701.gcparks.com/ and http://w3.cost.esf.org/index.php?id=205&action_number=ES0701 ) organizes a workshop on local gravity effects at the Royal Observatory of Belgium in Brussels, March 16–17, 2009. The workshop is open to all interested and financial support can be provided to a limited number of participants. The COST ES0701 is oriented towards the applications in AG, but as the purpose of the workshop is to increase our understanding of the subject, all gravity sensors are of equal interest. Some of the questions that we want to address are:
• what local observations and models do we need to model local (hydrological) gravity?
• how to verify the modelling on stations without a continuous gravity record (i.e., SG)?
• can regional or global hydrological models be used to account for the local effects as well, using auxiliary data like topography and local geology?
• how well can we do in the best of cases?
• how well can we manage with incomplete information?
• is it worth our while to do this at AG stations at all, or can we just rely on statistics (long or/and dense datasets) to iron out the effects in the trends?
• if all else fails, can we at least estimate uncertainty? How?