Estimation of hydrological loading effects to correct VLBI analysis

Geodetic observations such as VLBI are sensitive to changes in terrestrial water storage. The few hydrological models currently available show differences in their spatial distribution, globally and also between models. Periodic seasonal variations in the models with amplitudes of up to a few tens of centimeters primarily affect the vertical component of the geodetic observation. Notably, many stations obtain agreement between the Earth response to hydrological loading and the seasonal fit of the VLBI station heights. However, correcting VLBI station heights and baseline lengths for hydrological loading effects using IVS-R1 and IVS-R4 sessions improves the root mean square deviations only marginally. One reason for the small improvement is the relatively large scatter around station heights and hence the hydrological signal is hidden. Nevertheless, it is incorporated in the geodetic signal and needs to be accounted for. It was not possible to clearly identify the best suitable numerical model and therefore a different approach is applied. GPS and GRACE derived hydrological variations are used to correct the VLBI analysis. Although this technique is not free of assumptions, it has turned out to be a very good method to correct geodetic VLBI for changes in terrestrial water storage while avoiding the highly variable numerical models.