Inconsistencies in GRACE-Based Groundwater Storage Estimation—A Call for a Proper Use of Land Surface Models

Estimating groundwater storage (GWS) anomalies by subtracting model-derived components from Gravity Recovery and Climate Experiment (GRACE) terrestrial water storage (TWS) anomalies is a common practice in hydrology. Typically, land surface model (LSM) simulated soil moisture (SM), snow water equivalent, and canopy water content are removed from GRACE-derived TWS, and the residual is interpreted as GWS. However, this method implicitly assumes that LSMs account for all non-groundwater storages within distinct, physically meaningful compartments. In this comment, we examine the assumptions, semantic and structural challenges embedded in this approach, suggesting that users consider the consequences of model simplifications and conventions on the results. We encourage careful interpretation and advocate for more sophisticated methods, including making use of data assimilation and models that represent physical hydrological processes more completely.