Wahr, J. M., Molenaar, M. ., & Bryan, F. . (1998). Time variability of the Earth\textquoterights gravity field: Hydrological and oceanic effects and their possible detection using GRACE. Journal of Geophysical Research, 103, 30205–30230. http://doi.org/10.1029/98JB02844 (Original work published December 1998)