Limits to GRACE-based groundwater storage monitoring in a 23 000 km ² coastal basin: evidence from the Lower Kutai Basin, Indonesia

Abstract. Groundwater is considered a climate-resilient source of freshwater yet its long-term response to climate variability remains poorly understood in environments with limited ground-based monitoring networks. In the Lower Kutai Basin where Indonesia's new capital (Nusantara) is under development, we examine the limitations to Gravity Recovery and Climate Experiment (GRACE) satellite data to estimate groundwater storage changes (ΔGWS) over the last two decades using evidence of other water storage changes from global-scale models. We identify potential ocean signal leakage, inferred from residual correlations (r up to 0.68) between paired land-ocean grids, which can propagate errors into ΔGWS estimates and result in physically implausible ΔGWS values. GRACE-derived terrestrial water storage anomalies (ΔTWS) exhibit strong seasonal and interannual variability that is consistent across different spatial scales (r=0.78-1) and are dominated by changes in root-zone soil moisture storage (ΔSMS). Across 54 realizations, only 21 %–60 % (mean: 42 %) of ΔGWS estimates per realization are physically plausible. Validation of plausible ΔGWS values critically relies on robust storage coefficients. Correlations between GRACE-derived ΔGWS and groundwater-level anomalies (ΔGWL) are generally weak and reflect discrepancies between GRACE's basin-scale signals and localized aquifer dynamics influenced by heterogeneity and groundwater abstraction. Statistical analyses show weak-to-moderate coupling of ΔTWS and ΔSMS with ENSO indices (r=-0.4 to −0.6) whereas ΔGWS is less responsive. Drought conditions associated with the 2015–2016 El Niño are a notable exception as ΔTWS deficits (−2.4 to -4.6cmmonth-1) correspond with plausible ΔGWS declines (-1.1cmmonth-1). High-frequency (hourly) groundwater-level observations indicate that episodic, high-intensity rainfall events (>90th percentile) disproportionately contribute to groundwater recharge. These findings demonstrate that only a subset of ΔGWS values can be plausibly estimated from GRACE so that, without expanded in situ monitoring, ΔGWS estimates in this small coastal basin will remain highly uncertain.