Evaluation of the impact of the biomass on GRACE-borne total water storage estimates in the Amazon Basin

The Amazon Basin plays a central role in regulating global carbon and water cycles, yet the interaction between forest biomass changes and terrestrial water storage (TWS) remains insufficiently constrained. Monthly GRACE and GRACE-FO surface mass anomalies were determined (2002–2025, using monthly solutions provided by JPL and CSR), and they were compared to aboveground biomass (AGB) variations in order to quantify the contribution of biomass to long-term basin-scale TWS trends. Spatial patterns reveal that TWS anomalies are governed primarily by large-scale hydrological processes, while biomass changes are more localized. Regardless of the differences in spatial distribution, biomass exerts a basin-wide influence via evapotranspiration and water balance feedbacks; thus, the investigation is performed integrally for the entire basin. Linear trends of the spatially integrated time series were calculated, resulting in −0.0016 cm/year for the biomass variations, −0.0165 cm/year for the JPL, and −0.0081 cm/year for the CSR TWS time series. The results indicate that TWS time series contain signals from biomass variation, accounting for approximately 10%–20% of the observed surface mass anomaly. Thus the GRACE-borne TWS variation might overestimate the real water mass variations to this extent.