The combination of GRACE and in--situ data, a numerical hydrologic model, and the mathematical technique of 'data assimilation' permits to separate the total water content observed by GRACE into the various vertical components. See the Earth Observatory Summary for additional explanation.
More details in the papers by Rodell et al (2006) and Zaitchik et al (2008).
Rodell, M., J. Chen, H. Kato, J. Famiglietti, J. Nigro, and C. Wilson (2006). Estimating ground water storage changes in the Mississippi River basin (USA) using GRACE. Hydrogeology Journal, doi:10.1007/s10040-006-0103-7.
Zaitchik, BF; Rodell, M; Reichle, RH (2008): Assimilation of GRACE terrestrial water storage data into a Land Surface Model: Results for the Mississippi River basin. JOURNAL OF HYDROMETEOROLOGY 9 535 - 548