Assessment of the Effectiveness of GRACE Observations in Monitoring Groundwater Storage in Poland

Abstract The Gravity Recovery and Climate Experiment (GRACE/GRACE‐FO) mission have been providing global data on terrestrial water storage (TWS) for over 20 years. This study aimed to assess the effectiveness of GRACE/GRACE‐FO observations for monitoring groundwater storage (GWS) in Poland. We proposed new method for estimating GWS by incorporating hydrodynamic zoning. Our approach utilized hydrodynamically conditioned relationships between in situ GWS, GRACE/GRACE‐FO‐derived TWS, and modeled TWS to derive spatial patterns for the separate estimation of satellite‐based GWS within distinct hydrodynamic zones. The GWS was evaluated by comparing it with in situ groundwater measurements from national monitoring points. We showed that for a proper evaluation of the satellite‐based GWS using in situ observations, it is necessary to select monitoring points that adequately represent aquifer systems with rapid water exchange. We analyzed the impact of measurement point location on the agreement between satellite‐based and in situ GWS, taking into account the physiographic region, and hydrodynamic zone. We found highest agreement between satellite‐based and in situ estimates of GWS in aquifer systems with rapid water exchange. In discharge zones, satellite‐based GWS changes aligned well with TWS‐GRACE data in terms of the amplitudes and phases of the seasonal signal as well as for long‐term trends. We demonstrated the existence of long‐term negative trends in GWS changes across the entire country (up to −2.45 mm equivalent water height/year in the southeast region of Poland). These trends have been significantly influenced by climate change and the resulting predominance of evapotranspiration over precipitation.