Caveats on the equivalent water thickness and surface mascon solutions derived from the GRACE satellite-observed time-variable gravity

The equivalent water thickness (EWT, including mascon) solutions derived from the GRACE time-variable gravity (TVG) data are gaining recognition. We examine the physics of this practice from first principle in light of the non-uniqueness of 3-D gravitational inversion. We raise caveats on the indiscriminate utilization of the EWT solutions, because a surface EWT solution cannot represent an internal process in a physically meaningful way. In practice, EWT is often a good-enough representation of the reality as the predominant TVG signals do originate from surficial processes such as the water cycle, but it should be recognized that all internal geophysical processes leave signatures to different extent in the TVG observations. Treating all TVG as EWT will render physical quantities in general not directly resolvable by gravity to be misinterpreted. As the TVG observations span longer and improve in precision, the gravity itself, rather than EWT, should still be the quantity of choice. This is not just a problem of model uncertainties or numerical errors, but one in the understanding and treatment in the interest of the rigor of physics.