Assessment of the effectiveness of GRACE observations in monitoring groundwater in Poland

Satellite observations of Earth's gravity field from the Gravity Recovery and Climate Experiment (GRACE) mission offer a unique dataset for analysing terrestrial water storage and effectively closing the continental water balance. Comparing terrestrial water storage (TWS) anomalies from GRACE with in-situ groundwater level measurements is crucial for understanding how groundwater contributes to the overall water retention. In this study, monthly changes in TWS anomalies (TWS) from GRACE are compared with changes in groundwater storage (GWS) obtained from in-situ measurements. The analyses are performed for various aquifer systems and different hydrodynamic zones across Poland.The analysis of correlations between GRACE-based TWS and in-situ GWS indicates that, while the magnitude of TWS is greater than that of in-situ GWS, a strong relationship exists between these two quantities in alluvial aquifers and in systems with rapid water exchange, such as fractured and karst aquifers. In contrast, the porous reservoirs in postglacial formations with a thick vadose zone - typical for a large part of the area of Poland - exhibit a weak correlation between TWS and in-situ GWS. This indicates that the standard method of calculating GWS as the difference between TWS-GRACE and TWS from the GLDAS (Global Land Data Assimilation System) model should be revised to account for the complexities of these aquifer systems.Since SWS-GLDAS (Soil Water Storage) effectively captures changes in water content in the vadose zone (VZ), and VZ is a major component of GWS in alluvial aquifers, calculating GWS in these areas as a difference between TWS-GRACE and SWS-GLDAS might be inaccurate. Our results indicate that GWS for alluvial systems should be estimated based solely on TWS-GRACE data. In turn, for aquifer systems with a thick vadose zone (above approximately 2 meters), the GWS should be estimated using the difference between TWS-GRACE and SWS-GLDAS.We also use a balance approach to determine TWS, employing precipitation data (P) from the European E-OBS database, evapotranspiration (ET) from the Simplified Surface Energy Balance database, and river discharge (Q) from in-situ measurements, by calculating TWS = P ET Q. The results demonstrate that from 2009 to 2023, there has been a general decrease in GWS in Poland, with an average rate of -1.4 mm/year. The highest downward trend is observed in the lowland area in central Poland and in the southeast part of the country, with a rate of -1.7 to -2.2 mm/year. In the coastal zone and parts of northeastern Poland, the GWS decline is smallest (from 0 to -1 mm/year). The results of this study suggest that for deeper aquifer systems, GWS accounts for about 76% of the total TWS, whereas for alluvial systems, it is nearly 100%.Satellite gravimetry complements in-situ observations and model data by independently measuring changes in GWS and offering a continuous spatial view of these variations. Combining remote sensing data with the water balance method promises high-resolution estimates of GWS changes, essential for effective groundwater resource management.Key words: GRACE, groundwater, terrestrial water storage