Integrated gravimetry–altimetry analysis of Caspian Sea level variability (2003–2024)

The Caspian Sea represents a major component of continental water storage and a key target for geodetic monitoring of mass redistribution. This contribution analyses Caspian Sea level (CSL) variability over 2003–2024 using a combination of satellite gravimetry (GRACE/GRACE-FO) and satellite altimetry.

GRACE-derived water mass estimates were corrected for signal leakage and steric effects, improving consistency with independent altimetric observations. The resulting time series was analysed using wavelet decomposition to separate periodic components and extract the underlying trend. Change-point analysis applied to the reconstructed trend reveals four successive linear phases: a rise of 6.66 cm yr^-1 (Jan 2003–Feb 2006), a long decline of −9.99 cm yr^-1 (Feb 2006–Aug 2016), a weaker decline of −3.32 cm yr^-1 (Aug 2016–May 2019), and a strongly accelerated drop of −23.18 cm yr^-1 (May 2019–Dec 2024).

The relationship between CSL and hydroclimatic forcing was examined using difference integral curves and wavelet coherence with precipitation, evaporation and river runoff. At the annual scale, CSL and runoff are nearly in phase. At interannual scales, the dominant control varies over time. From 2003 to 2016, runoff variability closely follows CSL changes. Between 2016 and 2019, increased runoff partly offset decreasing precipitation and increasing evaporation, consistent with the temporary slowdown in CSL decline. Since 2019, the combined effect of reduced precipitation, intensified evaporation and declining runoff explains the recent acceleration in CSL drop.

Acknowledgements: This work was primarily supported by the Spanish national project PID2021-122142OB-I00 (MCIN/AEI/10.13039/501100011033), and additionally by the ThinkInAzul Programme (Generalitat Valenciana, GVA-THINKINAZUL/2021/035) and the EU Horizon Europe project SEA4FUTURE (Grant Agreement No. 101212647).