Terrestrial water storage monitoring in Bulgaria using Global Navigation Satellite System (GNSS) displacements and comparison with GLDAS, GLWS and GRACE products

The study investigates Terrestrial Water Storage (TWS) variation across Bulgaria using vertical displacements during 2004–2025 derived from 32 operating Global Navigation Satellite System (GNSS) stations. The GNSS vertical displacement time series were analyzed using Singular Spectrum Analysis (SSA) and then inverted into Equivalent Water Height (EWH) anomalies using an elastic Green's function approach combined with Tikhonov regularization. The estimated GNSS-based TWS variations were evaluated against independent hydrological and gravimetric datasets, including Global Land Data Assimilation System (GLDAS)-Noah, Global Land Water Storage (GLWS), and Gravity Recovery and Climate Experiment (GRACE) products. The results show that annual TWS amplitudes over Bulgaria generally range from 10 to 30 mm, with localized maxima exceeding 40 mm. The timing of maximum annual storage typically occurs during June–September, consistent with seasonal recharge mechanisms. Monthly climatology indicates maximum storage during late winter and spring (February-April) and minimum storage during late summer (August-October). Comparison analysis shows strong agreement among the GLDAS, GLWS, and GRACE datasets, with correlations up to 0.927, whereas GNSS-derived estimates exhibit weaker correlations, likely due to station distribution limitations and non-hydrological deformation effects. In general, the study concludes that GNSS provides valuable complementary constraints for monitoring hydrological loading and water storage variability in Bulgaria.