Evidence of daily hydrological loading in GPS time series over Europe

Loading deformations from atmospheric, oceanic, and hydrological mass changes mask geophysical processes such as land subsidence and tectonic or volcanic deformation. While it is known that hydrological loading plays a role at seasonal time scales, here we demonstrate evidence that also fast water storage changes contribute to daily Global Positioning System (GPS) height time series. So far, no clear strategy, i.e., no single conventional hydrological model, has been proposed for removing hydrological deformation from daily GPS height time series. Hydrological model predictions of total water storage anomalies tend to diverge and (substantially) deviate from Gravity Recovery and Climate Experiment (GRACE) observations, which however have a limited spatial and temporal resolution. Here, we suggest to overcome these limitations by assimilating GRACE data into a high-resolution (12.5 km) hydrological model. We tested this approach over Europe, and we found that accounting for daily hydrological mass changes reduces the root mean square scatter of GPS height time series almost by a factor of two when compared to monthly hydrological mass changes. We suggest that a GRACE-assimilating hydrological model would provide a promising option for removing hydrology-induced vertical deformation from GPS time series also at the global scale.