A comparison of GRACE/-FO and altimetry derived regional hydrological mass balance: example of the Caspian Sea

Monitoring the evolution of freshwater water resources worldwide is challenging yet essential to prevent severe water scarcity. In that regard, the Gravity Recovery and Climate Experiment and Follow-On satellite missions (GRACE/-FO) have successfully proven to complement in-situ hydrological observations by giving access to variations in continental water storage through measurements of the Earth’s gravity field since 2002.However, the measurement noise and errors inherent to the GRACE/-FO missions design and data processing cause estimation bias and spatial leakage that limit applications to large-scale hydrological basins and complicate geophysical interpretation. Moreover, missing observations during and between the missions, and in particular the 11-month gap between missions make if difficult to monitor long-term mass variations. To overcome these issues, we build a procedure, based on a spectral analysis by Multichannel – Singular Spectrum Analysis (M-SSA), that uses spatio-temporal correlations of the GRACE/-FO time series to fill data gaps and reduce a significant portion of the distinctive noise pattern while maintaining the best possible spatial resolution. This processing reveals hydrological signals that are less well or not resolved by other processing strategies.
We seek validation of the GRACE/-FO M-SSA solution by comparison with independent estimates of regional hydrological mass balance. We first develop a new forward modeling approach to derive regional hydrological balance that accounts for potential bias caused by necessary filtering of the GRACE/-FO data. In particular, we account for signal leakage of a known source while estimating regional mass balance.
The Caspian Sea, due to its relative isolation, large size and significant mass variations over the past decades,  offers an interesting example to validate mass balance derived from GRACE/-FO measurements. We compare space-gravity derived hydrological mass balance with independent altimetry data, accounting for thermosteric effects using in-situ data and ground deformation caused by mass redistribution, and show consistent results on the long term variations. However, we also discuss potential causes of a significant discrepancy in seasonal amplitude.